Main Why Is Sex Fun?: The Evolution Of Human Sexuality

Why Is Sex Fun?: The Evolution Of Human Sexuality

Why are humans one of the few species to have sex in private? Why are human females the only mammals to go through menopause? Why is the human penis so unnecessarily large? There is no more knowledgeable authority than the award-winning author of THE THIRD CHIMPANZEE to answer these intriguing questions. Here is a delightfully entertaining and enlightening look at the unique sex lives of humans.
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Why is Sex Fun
Jared Diamond

"Why is Sex Fun? is the best book on the subject I've read. This lively
exploration of our sexual heritage offers fascinating reading for
anyone curious about why lovers do what they do."
-Diane Ackerman, author of A Natural History of the Senses
"I am so jealous of Jared Diamond, for he writes with such an elegant
simplicity! Here, he takes a loot at the endlessly fascinating topic
of human sexuality His convincing arguments should persuade xm that
there are very special reasons why we evolved to use sex for recreation
as well as for procreatim whereas most other mammals are denied that
pleasure.... It is a great little book, by one of the worlds foremost
biological philosophers."
-ROGER Shohl
Professor of Physiology Monash University Australia
"Once again Jared Diamond provides us with answers to questions we may
never have stopped to ask, but wish we had. In this long essay Diamond
explains that recreational sex, while not unique to humans, is a rare
behavior in the animal world. Above all, we learn, sexual activity
divorced fron procreation is not only part of what it is to be human,
but the very crux of our evolutionary success."
-Bettyaxn Kevles. author of Naked to the Bonn Medical Imaging in the
Twentieth Centnty

The Science Masters Series is a global publishing vonture consisting
of original science books written by leading scientists and published
by a worldwide team of twenty-six publishers assembled by John Brockman.
The series was conceived by Anthony Cheetham of Orion Publishers and
John Brockman of Brockman Inc., a New York literary agency, and developed
in coordination with Basic Books.

The Science Masters name and marks are owned by and licensed to the
publisher by Brockman Inc.

Copyright © 1997 by Jared Diamond.

Published by Basic Books, A Member of the Perseus Books Group.

All rights reserved. Printed in the United States of America. No part
of this book may be used in any manner whatsoever without written
permission except ; in the case of brief quotations embodied in critical
articles and reviews. For information, address Basic Books, 10 East
53rd Street, New York, NY 10022-5299.

Library of Congress Cataloging-in-Publication Data Diamond, Jared M.

Why is sex fun? : the evolution of human sexuality / by Jared Diamond.
1st ed. p. cm.
Includes index.
ISBN 0-465-03127-7 (cloth)
ISBN 0-465-03126-9 (paper)

To Marie, My best friend, coparent, lover, and wife

The Animal with the Weirdest Sex Life
The Battle of the Sexes
Why Don't Men Breast-feed Their Babies? The Non-Evolution of
Male Lactation
Wrong Time for Love: The Evolution of Recreational Sex
What Are Men Good For? The Evolution of Men's Roles
Making More by Making Less: The Evolution of Female Menopause
Truth in Advertising: The Evolution of Body Signals

The subject of sex preoccupies us. It's the source of our most intense
pleasures. Often it's also the cause of misery, much of which arises
from built-in conflicts between the evolved roles of women and men.
This book is a speculative account of how human sexuality came to be
the way it now is. Most of us don't realize how unusual human sexual
practices are, compared to those of all other living animals. Scientists
infer that the sex life of even our recent apelike ancestors was very
different from ours today. Some distinctive evolutionary forces must
have operated on our ancestors to make us different. What were those
forces, and what really is so bizarre about us?
Understanding how our sexuality evolved is fascinating not only in its
own right but also in order to understand our other distinctively human
features. Those features include our culture, speech, parent-child
relations, and mastery of complex tools. While paleontologists usually
attribute the evolution of these features to our attainment of large
brains and upright posture, I argue that our bizarre sexuality was
equally essential for their evolution.
Among the unusual aspects of human sexuality that I discuss are female
menopause, the role of men in human
societies, having sex in private, often having sex for fun

rather than for procreation, and the expansion of women's breasts even
before use in lactation. To the layperson, these features all seem almost
too natural to require explanation. On reflection, though, they prove
surprisingly difficult to account for. I'll also discuss the function
of men's penises and the reasons women but not men nurse their babies.
The answers to these two questions seem utterly obvious. Within even
these questions, though, lurk baffling unsolved problems.
Reading this book will not teach you new positions for enjoying
intercourse, nor will it help you reduce the discomfort of menstrual
cramps or menopause. It will not abolish the pain of discovering that
your spouse is having an affair, neglecting your joint child, or
neglecting you in favor of your child. But this book may help you
understand why your body feels the way it does, and why your beloved
is behaving the way he or she is. Perhaps, too, if you understand why
you feel driven to some self-destructive sexual behavior, that
understanding may help you to gain distance from your instincts and
to deal more intelligently with them.
Earlier versions of material in some chapters appeared as articles in
Discover and Natural History magazines. It is a pleasure to acknowledge
my debt to many scientist colleagues for discussions and comments, to
Roger Short and Nancy Wayne for their scrutiny of the whole manuscript,
to Ellen Modecki for the illustrations, and to John Brock-man for the
invitation to write this book.

If your dog had your brain and could speak, and if you asked it what
it thought of your sex life, you might be surprised by its response.
It would be something like this:
Those disgusting humans have sex any day of the month! Barbara proposes
sex even when she knows perfectly well that she isn't fertile—like just
after her period. John is eager for sex all the time, without caring
whether his efforts could result in a baby or not. But if you want to
hear something really gross—Barbara and John kept on having sex while
she was pregnant! That's as bad as all the times when John's parents
come for a visit, and I can hear them too having sex, although John's
mother went through this thing they call menopause years ago. Now she
can't have babies anymore, but she still wants sex, and John's father
obliges her. What a waste of effort! Here's the weirdest thing of all:
Barbara and John, and John's parents, close the bedroom door and have
sex in private, instead of doing it in front of their friends like any
self-respecting dog!
To understand where your dog is coming from, you need to free yourself
from your human-based perspective
on what constitutes normal sexual behavior. Increasingly today, we
consider it narrow-minded and despicably prejudiced to denigrate those
who do not conform to our own standards. Each such form of
narrow-mindedness is associated with a despicable "ism"—for instance,
racism, sexism, Eurocentrism, and phallocentrism. To that list of
modern "ism" sins, defenders of animal rights are now adding the sin
of species-ism. Our standards of sexual conduct are especially warped,
species-ist, and human-centric because human sexuality is so abnormal
by the standards of the world's thirty million other animal species.
It's also abnormal by the standards of the world's millions of species
of plants, fungi, and microbes, but I'll ignore that broader perspective
because I haven't yet worked through my own zoo-centrism. This book
confines itself to the insights that we can gain into our sexuality
merely by broadening our perspective to encompass other animal species.
As a beginning, let's consider normal sexuality by the standards of
the world's approximately 4,300 species of mammals, of which we humans
are just one. Most mammals do not live as a nuclear family of a mated
adult male and adult female, caring jointly for their offspring. Instead,
in many mammal species both adult males and adult females are solitary,
at least during the breeding season, and meet only to copulate. Hence,
males do not provide paternal care; their sperm is their sole
contribution to their offspring and to their temporary mate.

Even most social mammal species, such as lions, wolves, chimpanzees,
and many hoofed mammals, are not paired off within the
herd/pride/pack/band into male/ female couples. Within such a
herd/pride/et cetera, each adult male shows no signs of recognizing
specific infants as his offspring by devoting himself to them at the
expense of other infants in the herd. Indeed, it is only within the
last few years that scientists studying lions, wolves, and chimpanzees
have begun to figure out, with the help of
DNA testing, which male sired which infant. However, like all
generalizations, these admit exceptions. Among the minority of adult
male mammals that do offer their offspring paternal care are polygynous
male zebras and gorillas with harems of females, male gibbons paired
off with females as solitary couples, and saddleback tamarin monkeys,
of which two adult males are kept as a harem by one polyan-drous adult
Sex in social mammals is generally carried out in public, before the
gazes of other members of the troop. For instance, a female Barbary
macaque in estrus copulates with every adult male in her troop and makes
no effort to conceal each copulation from other males. The
best-documented exception to this pattern of public sex is in chimpanzee
troops, where an adult male and estrous female may go off by themselves
for a few days on what human observers term a "consortship." However,
the same female chimpanzee that has private sex with a consort may also
have public sex with other adult male chimpanzees within the same estrus
Adult females of most mammal species use various means of conspicuously
advertising the brief phase of their reproductive cycle when they are
ovulating and can be fertilized. The advertisement may be visual (for
instance, the area around the vagina turning bright red), olfactory
(releasing a distinctive smell), auditory (making noises), or
behavioral (crouching in front of an adult male and displaying the
vagina). Females solicit sex only during those fertile days, are
sexually unattractive or less attractive to males on other days because
they lack the arousing signals, and rebuff the advances of any male
that is nevertheless interested on other days. Thus, sex is emphatically
not just for fun and is rarely divorced from its function of
fertilization. This generalization too admits exceptions: sex is
flagrantly separated from reproduction in a few species, including
bonobos (pygmy chimpanzees) and dolphins.
Finally, the existence of menopause as a regular phenomenon is not well

established for most wild mammal populations. By menopause is meant
a complete cessation of fertility within a time span that is much briefer
than the previous fertile career, and that is followed by an infertile
life span of significant length. Instead, wild mammals either are still
fertile at the time of death or else exhibit gradually diminishing
fertility with advancing age.
Now contrast what I have just said about normal mammalian sexuality
with human sexuality. The following human attributes are among those
that we take for granted as normal:
1: Most men and women in most human societies end up in a long-term
pair relationship ("marriage") that other members of the society
recognize as a contract involving mutual obligations. The couple has
sex repeatedly, and mainly or exclusively with each other.
2: In addition to being a sexual union, marriage is a partnership for
joint rearing of the resulting babies. In particular, human males as
well as females commonly provide parental care.
3: Despite forming a couple (or occasionally a harem), a husband and
wife (or wives) do not live (like gibbons) as a solitary couple in an
exclusive territory that they defend against other couples, but instead
they live embedded in a society of other couples with whom they cooperate
economically and share access to communal territory.
4: Marriage partners usually have sex in private, rather than being
indifferent to the presence of other humans.
5: Human ovulation is concealed rather than advertised. That is, women's
brief period of fertility around the time of ovulation is difficult
to detect for their potential sex partners as well as for most women
themselves. A woman's sexual receptivity extends beyond the time of
fertility to encompass most or all of the menstrual cycle. Hence, most
human copulations occur at a time unsuitable for conception. That is,
human sex is mostly for fun, not for insemination.
6: All women who live past the age of forty or fifty undergo menopause,
a complete shutdown of fertility. Men in general do not undergo menopause:
while individual men may develop fertility problems at any age, there
is no age-clumping of infertility or universal shutdown.
Norms imply violation of norms: we call something a "norm" merely because
it is more frequent than its opposite (the "violation of the norm").
That's as true for human sexual norms as for other norms. Readers of
the last two pages will surely have been thinking of exceptions to the
supposed generalizations that I have been describing, but they still
stand as generalizations. For example, even in societies that recognize
monogamy by law or custom there is much extramarital and premarital
sex, and much sex that is not part of a long-term relationship. Humans

do engage in one-night stands. On the other hand, most humans also engage
in many-year or many-decade stands, whereas tigers and orangutans
engage in nothing except one-night stands. The genetically based
paternity tests developed over the last half-century have shown that
the majority of American, British, and Italian babies are indeed sired
by the husband (or steady boyfriend) of the baby's mother.
Readers may also bristle at hearing human societies described as
monogamous; the term "harem," which zoologists apply to zebras and
gorillas, is taken from the Arabic
word for a human institution. Yes, many humans practice sequential
monogamy. Yes, polygyny (long-term simultaneous unions between one man
and multiple wives) is legal in some countries today, and polyandry
(long-term simultaneous unions between one woman and multiple husbands)
is legal in a few societies. In fact, polygyny was accepted in the great
majority of traditional human societies before the rise of state
institutions. However, even in officially polygynous societies most
men have only one wife at a time, and only especially wealthy men can
acquire and maintain a few wives simultaneously. The large harems that
spring to mind at the mention of the word polygamy, such as those of
recent Arabian and Indian royalty, are possible only in the state-level
societies that arose very late in human evolution and that permitted
a few men to concentrate great wealth. Hence the generalization stands:
most adults in most human societies are at any given moment involved
in a long-term pair bond that is often monogamous in practice as well
as legally.
Still another cause for bristling may have been my description of human
marriage as a partnership for the joint rearing of the resulting babies.
Most children receive more parental care from their mothers than from
their fathers. Unwed mothers form a significant proportion of the adult
population in some modern societies, though it has been much harder
for unwed mothers to rear children successfully in traditional
societies. But the generalization again holds: most human children
receive some parental care from their father, in the form of child care,
teaching, protection, and provision of food, housing, and money.
All these features of human sexuality—long-term sexual pnrtnerships,
coparenting, proximity to the sexual partnerships of others, private
sex, concealed ovulation, extended female receptivity, sex for fun,
and female menopause— constitute what we humans assume is normal
sexuality. It titillates, amuses, or disgusts us to read of the sexual


of elephant seals, marsupial mice, or orangutans, whose lives are so
different from ours. Their lives seem to us bizarre. But that proves
to be a species-ist interpretation. By the standards of the world's
4,300 other species of mammals, and even by the standards of our own
closest relatives, the great apes (the chimpanzee, bonobo, gorilla,
and orangutan), we are the ones who are bizarre.
However, I am still being worse than zoo-centric. I am falling into
the even narrower trap of mammalo-centrism. Do we become more normal
when judged by the standards of nonmammalian animals? Other animals
do exhibit a wider range of sexual and social systems than do mammals
alone. Whereas the young of most mammal species receive maternal care
but no paternal care, the reverse is true for some species of birds,
frogs, and fish in which the father is the sole caretaker for his
offspring. The male is a parasitic appendage fused to the female's body
in some species of deep-sea fish; he is eaten by the female immediately
after copulation in some species of spiders and insects. While humans
and most other mammal species breed repeatedly, salmon, octopus, and
many other animal species practice what is termed big-bang reproduction,
or semelparity: a single reproductive effort, followed by preprogrammed
death. The mating system of some species of birds, frogs, fish, and
insects (as well as some bats and antelope) resembles a singles bar—at
a traditional site, termed a "lek," many males maintain stations and
compete for the attention of visiting females, each of which chooses
a mate (often the same preferred male chosen by many other females),
copulates with him, and then goes off to rear the resulting offspring
without his assistance.
Among other animal species, it is possible to point out some whose
sexuality resembles ours in particular respects. Most European and
North American bird species form pair bonds that last for at least one
breeding season (in some cases for life), and the father as well as
the mother
cares for the young. While most such bird species differ from us in
that pairs occupy mutually exclusive territories, most species of sea
birds resemble us further in that mated pairs breed colonially in close
proximity to each other. However, all these bird species differ from
us in that ovula-tion is advertised, female receptivity and the sex
act are mostly confined to the fertile period around ovulation, sex
is not recreational, and economic cooperation between pairs is slight
or nonexistent. Bonobos (pygmy chimpanzees) resemble or approach us
in many of these latter respects: female receptivity is extended through
several weeks of the estrus cycle, sex is mainly recreational, and there
is some economic cooperation between many members of the band. However,

bonobos still lack our pair-bonded couples, our well-concealed
ovulation, and our paternal recognition of and care for offspring. Most
or all of these species differ from us in lacking a well-defined female
Thus, even a non-mammalo-centric view reinforces our dog's
interpretation: we are the ones who are bizarre. We marvel at what seems
to us the weird behavior of peacocks and big-bang marsupial mice, but
those species actually fall securely within the range of animal
variation, and in fact we are the weirdest of them all. Species-ist
zoologists theorize about why hammer-headed fruit bats evolved their
lek mating system, yet the mating system that cries out for explanation
is our own. Why did we evolve to be so different?
This question becomes even more acute when we compare ourselves with
our closest relatives among the world's mammal species, the great apes
(as distinguished from the gibbons or little apes). Closest of all are
Africa's chimpanzee and bonobo, from which we differ in only about 1.6
percent of our nuclear genetic material (DNA). Nearly as
close are the gorilla (2.3 percent genetic difference from us) and the
orangutan of Southeast Asia (3.6 percent different). Our ancestors
diverged "only" about seven million years ago from the ancestors of
chimpanzees and bonobos, nine million years ago from the ancestors of
gorillas, and fourteen million years ago from the ancestors of
That sounds like an enormous amount of time in comparison to an
individual human lifetime, but it's a mere eye-blink on the evolutionary
time scale. Life has existed on Earth for more than three billion years,
and hard-shelled, complex large animals exploded in diversity more than
half a billion years ago. Within that relatively short period during
which our ancestors and the ancestors of our great ape relatives have
been evolving separately, we have diverged in only a few significant
respects and to a modest degree, even though some of those modest
differences— especially our upright posture and larger brains—have had
enormous consequences for our behavioral differences.
Along with posture and brain size, sexuality completes the trinity of
the decisive respects in which the ancestors of humans and great apes
diverged. Orangutans are often solitary, males and females associate
just to copulate, and males provide no paternal care; a gorilla male
gathers a harem of a few females, with each of which he has sex at
intervals of several years (after the female weans her most recent
offspring and resumes menstrual cycling and before she becomes pregnant
again); and chimpanzees and bonobos live in troops with no lasting
male-female pair bonds or specific father-offspring bonds. It is clear

how our large brain and upright posture played a decisive role in what
is termed our humanity—in the fact that we now use language, read books,
watch TV, buy or grow most of our food, occupy all continents and oceans,
keep members of our own and other species in cages, and are exterminating
most other animal and plant species, while the great apes still
speechlessly gather wild fruit in the jungle, occupy
small ranges in the Old World tropics, cage no animal, and threaten
the existence of no other species. What role did our weird sexuality
play in our achieving these hallmarks of humanity?
Could our sexual distinctiveness be related to our other distinctions
from the great apes? In addition to (and probably ultimately as a product
of) our upright posture and large brains, those distinctions include
our relative hair-lessness, dependence on tools, command of fire, and
development of language, art, and writing. If any of these distinctions
predisposed us toward evolving our sexual distinctions, the links are
certainly unclear. For example, it is not obvious why our loss of body
hair should have made recreational sex more appealing, nor why our
command of fire should have favored menopause. Instead, I shall argue
the reverse: recreational sex and menopause were as important for our
development of fire, language, art, and writing as were our upright
posture and large brains.
The key to understanding human sexuality is to recognize that it is
a problem in evolutionary biology. When Darwin recognized the
phenomenon of biological evolution in his great book On the Origin of
Species, most of his evidence was drawn from anatomy. He inferred that
most plant and animal structures evolve—that is, they tend to change
from generation to generation. He also inferred that the major force
behind evolutionary change is natural selection. By that term, Darwin
meant that plants and animals vary in their anatomical adaptations,
that certain adaptations enable individuals bearing them to survive
and reproduce more successfully than other individuals, and that those
particular adaptations therefore increase in frequency in a population
from generation to generation. Later biologists showed that Darwin's
reasoning about anatomy also applies to physiology and biochemistry:
animal's or plant's physiological and biochemical characteristics also
adapt it to certain lifestyles and evolve in response to environmental
More recently, evolutionary biologists have shown that animal social
systems also evolve and adapt. Even among closely related animal species,

some are solitary, others live in small groups, and still others live
in large groups. But social behavior has consequences for survival and
reproduction. Depending, for example, on whether a species' food supply
is clumped or spread out, and on whether a species faces high risk of
attack by predators, either solitary living or group living may be better
for promoting survival and reproduction.
Similar considerations apply to sexuality. Some sexual characteristics
may be more advantageous for survival and reproduction than others,
depending on each species' food supply, exposure to predators, and other
biological characteristics. At this point I shall mention just one
example, a behavior that at first seems diametrically opposed to
evolutionary logic: sexual cannibalism. The male of some species of
spiders and mantises is routinely eaten by his mate just after or even
while he is copulating with her. This cannibalism clearly involves the
male's consent, because the male of these species approaches the female,
makes no attempt to escape, and may even bend his head and thorax toward
the female's mouth so that she may munch her way through most of his
body while his abdomen remains to complete the job of injecting sperm
into her.
If one thinks of natural selection as the maximization of survival,
such cannibalistic suicide makes no sense. Actually, natural selection
maximizes the transmission of genes, and survival is in most cases just
one strategy that provides repeated opportunities to transmit genes.
Suppose that opportunities to transmit genes arise unpre-dictably and
infrequently, and that the number of offspring
produced by such opportunities increases with the female's nutritional
condition. That's the case for some species of spiders and mantises
living at low population densities. A male is lucky to encounter a female
at all, and such luck is unlikely to strike twice. The male's best
strategy is to produce as many offspring bearing his genes as possible
out of his lucky find. The larger a female's nutritional reserves, the
more calories and protein she has available to transform into eggs.
If the male departed after mating, he would probably not find another
female and his continued survival would thus be useless. Instead, by
encouraging the female to eat him, he enables her to produce more eggs
bearing his genes. In addition, a female spider whose mouth is distracted
by munching a male's body allows copulation with the male's genitalia
to proceed for a longer time, resulting in more sperm transferred and
more eggs fertilized. The male spider's evolutionary logic is
impeccable and seems bizarre to us only because other aspects of human
biology make sexual cannibalism disadvantageous. Most men have more
than one lifetime opportunity to copulate; even well-nourished women

usually give birth to only a single baby at a time, or at most twins;
and a woman could not consume enough of a man's body at one sitting
to improve significantly the nutritional basis for her pregnancy.
This example illustrates the dependence of evolved sexual strategies
on both ecological parameters and the parameters of a species' biology,
both of which vary among species. Sexual cannibalism in spiders and
mantises is favored by the ecological variables of low population
densities and low encounter rates, and by the biological variables of
a female's capacity to digest relatively large meals and to increase
her egg output considerably when well nourished. Ecological parameters
can change overnight if an individual colonizes a new type of habitat,
but the colonizing individual carries with it a baggage of inherited
attributes that can change only slowly, through natural selection.
Hence it is not enough to consider a species' habitat and lifestyle,
design on paper a set of sexual characteristics that would be well
matched to that habitat and lifestyle, and then be surprised that those
supposedly optimal sexual characteristics do not evolve. Instead,
sexual evolution is severely constrained by inherited commitments and
prior evolutionary history.
For example, in most fish species a female lays eggs and a male fertilizes
those eggs outside the female's body, but in all placental mammal species
and marsupials a female gives birth to live young rather than to eggs,
and all mammal species practice internal fertilization (male sperm
injected into the female's body). Live birth and internal fertilization
involve so many biological adaptations and so many genes that all
placental mammals and marsupials have been firmly committed to those
attributes for tens of millions of years. As we shall see, these
inherited commitments help explain why there is no mammal species in
which parental care is provided solely by the male, even in habitats
where mammals live alongside fish and frog species whose males are the
sole providers of parental care.
We can thus redefine the problem posed by our strange sexuality. Within
the last seven million years, our sexual anatomy diverged somewhat,
our sexual physiology further, and our sexual behavior even more, from
those of our closest relatives, the chimpanzees. Those divergences must
reflect a divergence between humans and chimpanzees in environment and
lifestyle. But those divergences were also limited by inherited
constraints. What were the lifestyle changes and inherited constraints
that molded the evolution of our weird sexuality?

In the preceding chapter we saw that our effort to understand human
sexuality must begin by our distancing ourselves from our warped human
perspective. We're exceptional animals in that our fathers and mothers
often remain together after copulating and are both involved in rearing
the resulting child. No one could claim that men's and women's parental
contributions are equal: they tend to be grossly unequal in most
marriages and societies. But most fathers make some contribution to
their children, even if it's just food or defense or land rights. We
take such contributions so much for granted that they're written into
law: divorced fathers owe child support, and even an unwed mother can
sue a man for child support if genetic testing proves that he is her
child's father.
But that's our warped human perspective. Alas for sexual equality, we're
aberrations in the animal world, and especially among mammals. If
orangutans, giraffes, and most other mammal species could express their
opinion, they would declare our child support laws absurd. Most male
mammals have no involvement with either their offspring or their
offspring's mother after inseminating her; they are too busy seeking
other females to inseminate. Male animals in general, not just male
mammals, provide much less parental care (if any) than do females.
Yet there are quite a few exceptions to this chauvinist pattern. In
some bird species, such as phalaropes and Spotted Sandpipers, it's the
male that does the work of incubating the eggs and rearing the chicks,
while the female goes in search of another male to inseminate her again
and to rear her next clutch. Males of some fish species (like seahorses
and sticklebacks) and some amphibian males (like midwife toads) care
for the eggs in a nest or in their mouth, pouch, or back. How can we
explain simultaneously this general pattern of female parental care
and also its numerous exceptions?
The answer comes from the realization that genes for behavior, as well
as for malaria resistance and teeth, are subject to natural selection.
A behavior pattern that helps individuals of one animal species pass
on their genes won't necessarily be helpful in another species. In
particular, a male and female that have just copulated to produce a
fertilized egg face a "choice" of subsequent behaviors. Should that
male and female both leave the egg to fend for itself and set to work
on producing another fertilized egg, copulating either with the same
partner or with a different partner? On the one hand, a time-out from
sex for the purpose of parental care might improve the chances of the
first egg surviving. If so, that choice leads to further choices: both

the mother and the father could choose to provide the parental care,
or just the mother could choose to do so, or just the father could.
On the other hand, if the egg has a one-in-ten chance of surviving even
with no parental care, and if the time you'd devote to tending it would
alternatively let you produce 1,000 more fertilized eggs, you'd be host
off leaving that first egg to fend for itself and going on to produce
more fertilized eggs.
I've referred to these alternatives as "choices." That word may seem
to suggest that animals operate like human (Incision-makers,
consciously evaluating alternatives and finally choosing the
particular alternative that seems most
likely to advance the animal's self-interest. Of course, that's not
what happens. Many of the so-called choices actually are programmed
into an animal's anatomy and physiology. For example, female kangaroos
have "chosen" to have a pouch that can accommodate their young, but
male kangaroos have not. Most or all of the remaining choices are ones
that would be anatomically possible for either sex, but animals have
programmed instincts that lead them to provide (or not to provide)
parental care, and this instinctive "choice" of behavior can differ
between sexes of the same species. For example, among parent birds,
both male and female albatrosses, male but not female ostriches, females
but not males of most hummingbird species, and no brush turkeys of either
sex are instinctively programmed to bring food to their chicks, although
both sexes of all of these species are physically and anatomically
perfectly capable of doing so.
The anatomy, physiology, and instincts underlying parental care are
all programmed genetically by natural selection. Collectively, they
constitute part of what biologists term a reproductive strategy. That
is, genetic mutations or recombinations in a parent bird could
strengthen or weaken the instinct to bring food to the chicks and could
do so differently in the two sexes of the same species. Those instincts
are likely to have a big effect on the number of chicks that survive
to carry on the parent's genes. It's obvious that a chick to which a
parent brings food is more likely to survive, but we shall also see
that a parent that forgoes bringing food to its chicks thereby gains
other increased chances to pass on its genes. Hence the net effect of
a gene that causes a parent bird instinctively to bring food to its
chicks could be either to increase or to decrease the number of chicks
carrying on the parent's genes, depending on ecological and biological
factors that we shall discuss.
Genes that specify the particular anatomical structures

or instincts most likely to ensure the survival of offspring bearing
the genes will tend to increase in frequency. This statement can be
rephrased: anatomical structures and instincts that promote survival
and reproductive success tend to become established (genetically
programmed) by natural selection. But the need to make wordy statements
such as these arises very often in any discussion of evolutionary biology.
Hence biologists routinely resort to anthropomorphic language to
condense such statements—for example, they say that an animal "chooses"
to do something or pursues a certain strategy. This shorthand vocabulary
should not be misconstrued as implying that animals make conscious
For a long time, evolutionary biologists thought of natural selection
as somehow promoting "the good of the species." In fact, natural
selection operates initially on individual animals and plants. Natural
selection is not just a struggle between species (entire populations),
nor is it just a struggle between individuals of different species,
nor just between conspecific individuals of the same age and sex. Natural
selection can also be a struggle between parents and their offspring
or a struggle between mates, because the self-interests of parents and
their offspring, or of father and mother, may not coincide. What makes
individuals of one age and sex successful at transmitting their genes
may not increase the success of other classes of individuals.
In particular, while natural selection favors both males and females
that leave many offspring, the best strategy for doing so may be
different for fathers and mothers. That generates a built-in conflict
between the parents, a conclusion that all too many humans don't need
scientists to reveal to them. We make jokes about the battle of the
sexes, but the battle is neither a joke nor an aberrant accident of
how individual father or mothers behave on particular occasions. It
is indeed perfectly true that behavior that is in a male's genetic
interests may not necessarily be in the interests of his female co-parent,
and vice versa. That cruel fact is one of the fundamental causes of
human misery.
Consider again the case of the male and female that have just copulated
to produce a fertilized egg and now face the "choice" of what to do
next. If the egg has some chance of surviving unassisted, and if both
the mother and the father could produce many more fertilized eggs in
the time that they would devote to tending that first fertilized egg,
then the interests of the mother and father coincide in deserting the
egg. But now suppose that the newly fertilized, laid, or hatched egg
or newborn offspring has absolutely zero chance of surviving unless

it is cared for by one parent. Then there is indeed a conflict of interest.
Should one parent succeed in foisting the obligation of parental care
onto the other parent and then going off in search of a new sex partner,
then the foister will have advanced her or his genetic interests at
the expense of the abandoned parent. The foister will really promote
his or her selfish evolutionary goals by deserting his or her mate and
In such cases when care by one parent is essential for offspring survival,
child-rearing can be thought of as a cold-blooded race between mother
and father to be the first to desert the other and their mutual offspring
and to get on with the business of producing more babies. Whether it
actually pays you to desert depends on whether you can count on your
old mate to finish rearing the kids, and whether you are then likely
to find a receptive new mate. It's as if, at the moment of fertilization,
the mother and father play a game of chicken, stare at each other, and
simultaneously say, "I am going to walk off and find a new partner,
and you can care for this embryo if you want to, but even if you don't,
I won't!" If both partners call each
other's bluff in that race to desert their embryo, then the embryo dies
and both parents lose the game of chicken. Which parent is more likely
to back down?
The answer depends on such considerations as which parent has more
invested in the fertilized egg, and which parent has hotter alternative
prospects. As I said before, neither parent makes a conscious
calculation; the actions of each parent are instead programmed
genetically by natural selection into the anatomy and instincts of their
sex. In many animal species the female backs down and becomes sole parent
while the male deserts, but in other species the male assumes
responsibility and the female deserts, and in still other species both
parents assume shared responsibility. Those varying outcomes depend
on three interrelated sets of factors whose differences between the
sexes vary among species: investment in the already fertilized embryo
or egg; alternative opportunities that would be foreclosed by further
care of the already fertilized embryo or egg; and confidence in the
paternity or maternity of the embryo or egg.
All of us know from experience that we are much more reluctant to walk
away from an ongoing enterprise in which we have invested a lot than
from one in which we have invested only a little. That's true of our
investments in human relationships, in business projects, or in the
stock market. It's true regardless of whether our investment is in the
form of money, time, or effort. We lightly end a relationship that turns
bad on the first date, and we stop trying to construct from parts a

cheap toy when we hit a snag within a few minutes. But we agonize over
ending a twenty-five-year marriage or an expensive house remodeling.
The same principle applies to parental investment in potential
offspring. Even at the moment when an egg is fertilized by a sperm,
the resulting fertilized embryo generally
represents a greater investment for the female than for the male, because
in most animal species the egg is much larger than the sperm. While
both eggs and sperm contain chromosomes, the egg in addition must contain
enough nutrients and metabolic machinery to support the embryo's
further development for some time, at least until the embryo can start
feeding itself. Sperm, in contrast, need contain only a flagellar motor
and sufficient energy to drive that motor and support swimming for at
most a few days. As a result, a mature human egg has roughly one million
times the mass of the sperm that fertilizes it; the corresponding factor
for kiwis is one million billion. Hence a fertilized embryo, viewed
simply as an early-stage construction project, represents an utterly
trivial investment of its father's body mass compared to its mother's.
But that doesn't mean that the female has automatically lost the game
of chicken before the moment of conception. Along with the one sperm
that fertilized the egg, the male may have produced several hundred
million other sperm in the ejaculate, so that his total investment may
be not dissimilar to the female's.
The act of fertilizing an egg is described as either internal or external,
depending on whether it takes place inside or outside the female's body.
External fertilization characterizes most species of fish and amphibia.
For example, in most fish species a female and a nearby male
simultaneously discharge their eggs and sperm into the water, where
fertilization occurs. With external fertilization, the female's
obligate investment ends at the moment she extrudes the eggs. The embryos
may then be left to float away and fend for themselves without parental
care, or they may receive care from one parent, depending on the species.
More familiar to humans is internal fertilization, the male's injection
of sperm (for example, via an intromittive penis) into the female's
body. What happens next in most species is that the female does not
immediately extrude
the embryos but retains them in her body for a period of development
until they are closer to the stage when they can survive by themselves.
The offspring may eventually be packaged for release within a protective
eggshell, together with an energy supply in the form of yolk—as in all
birds, many reptiles, and monotreme mammals (the platypus and echidnas

of Australia and New Guinea). Alternatively, the embryo may continue
to grow within the mother until the embryo is "born" without an eggshell
instead of being "laid" as an egg. That alternative, termed vi-vipary
(Latin for "live birth"), characterizes us and all other mammals except
monotremes, plus some fish, reptiles, and amphibia. Vivipary requires
specialized internal structures—of which the mammalian placenta is the
most complex—for the transfer of nutrients from the mother to her
developing embryo and the transfer of wastes from embryo to mother.
Internal fertilization thus obligates the mother to further investment
in the embryo beyond the investment that she has already made in
producing the egg until it is fertilized. Either she uses calcium and
nutrients from her own body to make an eggshell and yolk, or else she
uses her nutrients to make the embryo's body itself. Besides that
investment of nutrients, the mother is also obligated to invest the
time required for pregnancy. The result is that the investment of an
internally fertilized mother at the time of hatching or birth, relative
to the father's, is likely to be much greater that that of an externally
fertilized mother at the time of unfertilized egg extrusion. For
instance, by the end of a nine-month pregnancy a human mother's
expenditure of time and energy is colossal in comparison with her
husband's or boyfriend's pathetically slight investment during the few
minutes it took him to copulate and extrude his one milliliter of sperm.
As a result of that unequal investment of mothers and fathers in
internally fertilized embryos, it becomes harder
for the mother to bluff her way out of post-hatching or post-birth
parental care, if any is required. That care takes many forms: for
instance, lactation by female mammals guarding the eggs by female
alligators, and brooding the eggs by female pythons. Nevertheless, as
we shall see, there are other circumstances that may induce the father
to stop bluffing and to start assuming shared or even sole responsibility
for his offspring.
I mentioned that three related sets of factors influence the "choice"
of parent to be caretaker, and that relative size of investment in the
young is only one of those factors. A second factor is foreclosed
opportunity. Picture yourself as an animal parent contemplating your
newborn offspring and coldly calculating your genetic self-interest
as you debate what you should now do with your time. That offspring
bears your genes, and its chance of surviving to perpetuate your genes
would undoubtedly be improved if you hung around to protect and feed
it. If there is nothing else you could do with your time to perpetuate
your genes, your interests would be best served by caring for that
offspring and not trying to bluff your mate into being sole parent.

On the other hand, if you can think of ways to spread your genes to
many more offspring in the same time, you should certainly do so and
desert your current mate and offspring.
Now consider a mother and father animal both doing that calculation
the moment after they have mated to produce some fertilized embryos.
If fertilization is external, neither mother nor father is
automatically committed to anything further, and both are theoretically
free to seek another partner with whom to produce more fertilized embryos.
Yes, their just-fertilized embryos may need some care, but mother and
father are equally able to try to bluff the other into providing that
care. But if fertilization is
internal, the female is now pregnant and committed to nourishing the
fertilized embryos until birth or laying. If she is a mammal, she is
committed for even longer, through the period of lactation. During that
period it does her no genetic good to copulate with another male, because
she cannot thereby produce more babies. That is, she loses nothing by
devoting herself to child care.
But the male who has just discharged his sperm sample into one female
is available a moment later to discharge another sperm sample into
another female, and thereby potentially to pass his genes to more
offspring. A man, for example, produces about two hundred million sperm
in one ejaculate—or at least a few tens of millions, even if reports
of a decline in human sperm count in recent decades are correct. By
ejaculating once every 28 days during his recent partner's 280-day
pregnancy—a frequency of ejaculation easily within the reach of most
men—he would broadcast enough sperm to fertilize every one of the world's
approximately two billion reproductively mature women, if he could only
succeed in arranging for each of them to receive one of his sperm. That's
the evolutionary logic that induces so many men to desert a woman
immediately after impregnating her and to move on to the next woman.
A man who devotes himself to child care potentially forecloses many
alternative opportunities. Similar logic applies to males and females
of most other internally fertilized animals. Those alternative
opportunities available to males contribute to the predominant pattern
of females providing child care in the animal world.
The remaining factor is confidence of parenthood. If you are going to
invest time, effort, and nutrients in raising a fertilized egg or embryo,
you'd better make damn sure first that it's your own offspring. If it
turns out to be somebody else's offspring, you've lost the evolutionary
race. You'll have knocked yourself out in order to pass on a rival's

For women and other female animals practicing inter-nal fertilization,
doubt about maternity never arises. Into the mother's body, containing
her eggs, goes sperm. Out of her body sometime later comes a baby. There's
no way that the baby could have been switched with some other mother's
baby inside of her. It's a safe evolutionary bet for the mother to care
for that baby.
But males of mammals and other internally fertilized animals have no
corresponding confidence in their paternity. Yes, the male knows that
his sperm went into a female's body. Sometime later, out of that female's
body, comes a baby. How does the male know whether the female copulated
with other males while he wasn't looking? How does he know whether his
sperm or some other male's sperm was the one that fertilized the egg?
In the face of this inevitable uncertainty, the evolutionary conclusion
reached by most male mammals is to walk off the job immediately after
copulation, seek more females to impregnate, and leave those females
to rear their offspring— hoping that one or more of the females with
which he copulated will actually have been impregnated by him and will
succeed in rearing his offspring unassisted. Male parental care would
be a bad evolutionary gamble.
Yet we know, from our own experience, that some species constitute
exceptions to that general pattern of male post-copulatory desertion.
The exceptions are of three types. One type is those species whose eggs
are fertilized externally. The female ejects her not yet fertilized
eggs; the male, hovering nearby or already grasping the female, spreads
his sperm on the eggs; he immediately scoops up the eggs, before any
other males have a chance to cloud the picture with their sperm; and
he proceeds to care for the eggs, completely confident in his paternity.
This is the evolutionary logic that programs some male fish and frogs
play the role of sole parent after fertilization. For example, the male
midwife toad guards the eggs by wrapping them around his hind legs;
the male glass frog stands watch over eggs in vegetation over a stream
into which the hatched tadpoles can drop; and the male stickleback builds
a nest in which to protect the eggs against predators.
A second type of exception to the predominant pattern of male
post-copulatory desertion involves a remarkable phenomenon with a long
name: sex-role-reversal polyandry. As the name implies, this behavior
is the opposite of the common polygynous breeding systems in which big
males compete fiercely with each other to acquire a harem of females.
Instead, big females compete fiercely to acquire a harem of smaller
males, for each of which in turn the female lays a clutch of eggs, and

each of which proceeds to do most or all of the work of incubating the
eggs and rearing the young. The best known of these female sultans are
the shore birds called jacanas (alias lily-trotters), Spotted
Sandpipers, and Wilson's Phalaropes. For instance, flocks of up to ten
female phalaropes may pursue a male for miles. The victorious female
then stands guard over her prize to ensure that only she gets to have
sex with him, and that he becomes one of the males rearing her chicks.
Clearly, sex-role-reversal polyandry represents for the successful
female the fulfillment of an evolutionary dream. She wins the battle
of the sexes by passing on her genes to far more clutches of young than
she could rear, alone or with one male's help. She can utilize nearly
her full egg-laying potential, limited only by her ability to defeat
other females in the quest for males willing to take over parental care.
But how did this strategy evolve? Why did males of some shorebird species
end up seemingly defeated in the battle of the sexes, as polyandrous
co-"husbands," when males of almost all other bird species avoided that
fate or even reversed it to become polygynists?
The explanation depends on shorebirds' unusual reTHE BATTLE OF THE SEXES
productive biology. They lay only four eggs at a time, and the young
are precocial, meaning that they hatch already covered with down, with
their eyes open, and able to run and find food for themselves. The parent
doesn't have to feed the chicks but only has to protect them and keep
them warm. That's something a single parent can handle, whereas it takes
two parents to feed the young of most other bird species.
But a chick that can run around as soon as it hatches has undergone
more development inside the egg than the usual helpless chick. That
requires an exceptionally large egg. (Take a look sometime at a pigeon's
typically small eggs, which produce the usual helpless chicks, to
understand why egg farmers prefer to rear chickens with big eggs and
precocial chicks.) In Spotted Sandpipers, each egg weighs fully
one-fifth as much as its mother; the whole four-egg clutch weighs an
astonishing 80 percent of her weight. Although even monogamous
shorebird females have evolved to be slightly larger than their mates,
the effort of producing those huge eggs is still exhausting. That
maternal effort gives the male both a short-term and a long-term
advantage if he takes over the not too onerous responsibility of rearing
the precocial chicks alone, thereby leaving his mate free to fatten
herself up again.
His short-term advantage is that his mate thereby becomes capable of
producing another clutch of eggs for him quickly, in case the first
clutch is destroyed by a predator. That's a big advantage, because
shorebirds nest on the ground and suffer horrendous losses of eggs and

chicks. For example, in 1975 a single mink destroyed every nest in a
population of Spotted Sandpipers that the ornithologist Lewis Oring
was studying in Minnesota. A study of jacanas in Panama found that
forty-four out of fifty-two nests failed.
Sparing his mate may also bring the male a long-term advantage. If she
does not become exhausted in one breed28 WHY IS SEX FUN?
ing season, she is more likely to survive to the next season, when he
can mate with her again. Like human couples, experienced bird couples
that have worked out a harmonious relationship are more successful at
raising young than are bird newly weds.
But generosity in anticipation of later repayment carries a risk, for
male shorebirds as for humans. Once the male assumes sole parental
responsibility, the road is clear for his mate to uso her free time
in whatever way she chooses. Perhaps she'll choose to reciprocate and
remain available to her mate, on the chance that her first clutch might
be destroyed and he would require a replacement clutch. But she might
also choose to pursue her own interests, seeking out some other male
available immediately to receive her second clutch. If her first clutch
survives and continues to occupy her former mate, her polyandrous
strategy has thereby doubled her genetic output.
Naturally, other females will have the same idea, and all of them will
find themselves in competition for a dwindling supply of males. As the
breeding season progresses, most males become tied up with their first
clutch and unable to accept further parental responsibilities. Although
the numbers of adult males and females may be equal, the ratio of sexually
available females to males rises as high as seven-to-one among breeding
Spotted Sandpipers and Wilson's Phalaropes. Those cruel numbers are
what drive sex-role reversal even further toward an extreme. Though
females already had to be slightly larger than males in order to produce
large eggs, they have evolved to become still larger in order to win
the fights with other females. The female reduces her own parental care
contribution further and woos the male rather than vice versa.
Thus, the distinctive features of shorebird biology— especially their
precocial young, clutches of few but large eggs, ground-nesting habits,
and severe losses from preda-tion—predispose them to male uniparental
male emancipation or desertion. Granted, females of most shorebird
species can't exploit those opportunities for polyandry. That's true,
for instance, of most sandpipers of the high Arctic, where the very
short breeding season leaves no time for a second clutch to be reared.

Only among a minority of species, such as the tropical jacanas and
southerly populations of Spotted Sandpipers, is polyandry frequent or
routine. Though seemingly remote from human sexuality, shorebird
sexuality is instructive because it illustrates the main message of
this book: a species' sexuality is molded by other aspects of the
species' biology. It's easier for us to acknowledge this conclusion
about shorebirds, to which we don't apply moral standards, than about
The remaining type of exception to the predominant pattern of male
desertion occurs in species in which, like us, fertilization is internal
but it's hard or impossible for a single parent to rear the young
unassisted. A second parent may be required to gather food for the
coparent or the young, tend the young while the coparent is off gathering
food, defend a territory, or teach the young. In such species the female
alone would not be able to feed and defend the young without the male's
help. Deserting a fertilized mate to pursue other females would bring
no evolutionary gain to a male if his offspring thereby died of
starvation. Thus, self-interest may force the male to remain with his
fertilized spouse, and vice versa.
That's the case with most of our familiar North American and European
birds: males and females are monogamous, and they share in caring for
the young. It's also approximately true for humans, as we know so well.
Human single-parenthood is difficult enough, even in these days of
supermarket shopping and babysitters for hire. In ancient
hunter-gatherer days, a child orphaned by either its
mother's or its father's death faced reduced chances of survival. The
father as well as the mother desirous of passing on genes finds it a
matter of self-interest to care for the child. Hence most men have
provided food, protection, and housing for their spouse and kids. The
result is our human social system of nominally monogamous married
couples, or occasionally of harems of women committed to one affluent
man. Essentially the same considerations apply to gorillas, gibbons,
and the other minority mammals practicing male parental care.
Yet that familiar arrangement of coparenthood does not end the battle
of the sexes. It does not necessarily dissolve the tension between the
mother's and father's interests, arising from their unequal investments
before birth. Even among those mammal and bird species that provide
paternal care, males try to see how little care they can get away with
and still have the offspring survive owing mainly to the mother's efforts.
Males also try to impregnate other males' mates, leaving the unfortunate
cuckolded male to care unknowingly for the cuckolder's offspring. Males
become justifiably paranoid about their mates' behavior.

An intensively studied and fairly typical example of those built-in
tensions of coparenthood is the European bird species known as the Pied
Flycatcher. Most flycatcher males are nominally monogamous, but many
try to be polygynous, and quite a few succeed. Again, it is instructive
to devote a few pages of this book on human sexuality to another example
involving birds, because (as we'll see) the behavior of some birds is
strikingly like that of humans but does not arouse the same moral
indignation in us.
Here is how polygyny works for Pied Flycatchers. In the spring a male
finds a good nest hole, stakes out his territory around it, woos a female,
and copulates with her. When this female (termed his primary female)
lays her first egg, the male feels confident that he has fertilized
her, that she'll be busy incubating his eggs, and that she won't be
terested in other males and is temporarily sterile anyway. Hence the
male finds another nest hole nearby, courts another female (termed his
secondary female), and copulates with her.
When that secondary female begins laying, the male feels confident that
he has fertilized her as well. Around that same time, the eggs of his
primary female are starting to hatch. The male returns to her, devotes
most of his energy to feeding her chicks and devotes less or no energy
to feeding the chicks of his secondary female. Numbers tell the cruel
story: the male averages fourteen deliveries of food per hour to the
primary female's nest but only seven deliveries of food per hour to
the secondary female's nest. If enough nest holes are available, most
mated males try to acquire a secondary female, and up to 39 percent
Obviously, this system produces both winners and losers. Since the
numbers of male and female flycatchers are roughly equal, and since
each female has one mate, for every bigamous male there must be one
unfortunate male with no mate. The big winners are the polygynous males,
who sire on the average 8.1 flycatcher chicks each year (adding up the
contributions of both mates), compared to only 5.5 chicks sired by
monogamous males. Polygynous males tend to be older and bigger than
unmated males, and they succeed in staking out the best territories
and best nest holes in the best habitats. As a result, their chicks
end up 10 percent heavier than the chicks of other males, and those
big chicks have a better chance of surviving than do smaller chicks.
The biggest losers are the unfortunate unmated males, who fail to acquire
any mates and sire no offspring at all (at least in theory—more on that
later). The other losers are the secondary females, who have to work
much harder than primary females to feed their young. The former end

up making twenty food deliveries per hour to the nest, compared with
only thirteen for the latter. Since the secondary
females thus exhaust themselves, they may die earlier. Despite her
herculean efforts, one hardworking secondary female can't bring as much
food to the nest as a relaxed primary female and a male working together.
Hence some chicks starve, and the secondary females end up with fewer
surviving chicks than do primary females (on the average, 3.4 versus
5.4 chicks). In addition, the surviving chicks of secondary females
are smaller than the chicks of primary females, and hence are less likely
to survive the rigors of winter and migration.
Given these cruel statistics, why should any female accept the fate
of being the "other woman"? Biologists used to speculate that secondary
females choose their fate, reasoning that the neglected second spouse
of a good male is better off than the sole spouse of a lousy male with
a poor territory. (Rich married men have been known to make similar
pitches to prospective mistresses.) It turns out, though, that the
secondary females do not accept their fate knowingly but are tricked
into it.
The key to this deception is the care that polygynous males take to
set up their second household a couple of hundred yards from their first
household, with many other males' territories intervening. It's
striking that polygynous males don't court a second spouse at any of
dozens of potential nest holes near the first nest, even though they
would thereby reduce their commuting time between nests, have more time
available to feed their young, and reduce their risk of being cuckolded
while en route. The conclusion seems inescapable that polygynous males
accept the disadvantage of a remote second household in order to deceive
the prospective secondary mate and conceal from her the existence of
the first household. Life's exigencies make a female Pied Flycatcher
especially vulnerable to being deceived. If she discovers after
egg-laying that her mate is polygynous, it's too late for her to do
anything about it. She's better off staying with those eggs than deTHE BATTLE OF THE SEXES
serting them, seeking a new mate from the males now available (most
of them are would-be bigamists anyway), and hoping the new mate will
prove to be any better than the former one.
The remaining strategy of male Pied Flycatchers has been dressed up
by male biologists in the morally neutral-sounding term "mixed
reproductive strategy" (abbreviated MRS). What this means is that mated
male Pied Flycatchers don't just have a mate: they also sneak around
trying to inseminate the mates of other males. If they find a female

whose mate is temporarily absent, they try to copulate with her and
often succeed. Either they approach her singing loudly or they sneak
up to her quietly; the latter method succeeds more often.
The scale of this activity staggers our human imagination. In act 1
of Mozart's opera Don Giovanni, the Don's servant, Leporello, boasts
to Donna Elvira that Don Giovanni has seduced 1,003 women in Spain alone.
That sounds impressive until you realize how long-lived we humans are.
If Don Giovanni's conquests took place over thirty years, he seduced
only one Spanish woman every eleven days. In contrast, if a male Pied
Flycatcher temporarily leaves his mate (for instance, to find food),
then on the average another male enters his territory in ten minutes
and copulates with his mate in thirty-four minutes. Twenty-nine percent
of all observed copulations prove to be EPCs (extra-pair copulations),
and an estimated 24 percent of all nestlings are "illegitimate." The
intruder-seducer usually proves to be the boy next door (a male from
an adjoining territory).
The big loser is the cuckolded male, for whom EPCs and MRSs are an
evolutionary disaster. He squanders a whole breeding season out of his
short life by feeding chicks that do not pass on his genes. Although
the male perpetrator of an EPC might seem to be the big winner, a little
reflection makes it clear that working out the male's balance sheet
tricky. While you are off philandering, other males have the chance
to philander with your mate. EPC attempts rarely succeed if a female
is within ten yards of her mate, but the chances of success rise steeply
if her mate is more distant than ten yards. That makes MRSs especially
risky for polygynous males, who spend much time in their other territory
or commuting between their two territories. The polygynous males try
to pull off EPCs themselves and on the average make one attempt every
twenty-five minutes, but once every eleven minutes some other male is
sneaking into their own territory to try for an EPC. In half of all
EPC attempts, the cuckolded male flycatcher is off in pursuit of another
female flycatcher at the very moment when his own mate is under siege.
These statistics would seem to make MRSs a strategy of dubious value
to male Pied Flycatchers, but they are clever enough to minimize their
risks. Until they have fertilized their own mate, they stay within two
or three yards of her and guard her diligently. Only when she has been
inseminated do they go off philandering.
Now that we have surveyed the varying outcomes of the battle of the
sexes in animals, let's see how humans fit into this broader picture.
While human sexuality is unique in other respects, it is quite ordinary
when it comes to the battle of the sexes. Human sexuality resembles

that of many other animal species whose offspring are internally
fertilized and require biparental care. It thereby differs from that
of most species whose young are externally fertilized and given only
uniparental care or even no care at all.
In humans, as in all other mammalian and bird species except brush
turkeys, an egg that has just been fertilized is incapable of independent
survival. In fact, the length of time until the offspring can forage
and care for itself is at least as long for humans as for any other
animal species,
and far longer than for the vast majority of animal species. Hence
parental care is indispensable. The only question is, which parent will
provide that care or will both parents provide it?
For animals, we saw that the answer to that question depends on the
relative size of the mother's and father's obligate investment in the
embryo, their other opportunities foreclosed by their choice to provide
parental care, and their confidence in their paternity or maternity.
Looking at the first of those factors, the human mother has a greater
obligate investment than the human father. Already at the time of
fertilization a human egg is much larger than a human sperm, though
that discrepancy disappears or is reversed if the egg is compared to
an entire ejaculate of sperm. After fertilization the human mother is
committed to up to nine months of time and energy expenditure, followed
by a period of lactation that lasted about four years under the
conditions of the hunter-gatherer lifestyle that characterized all
human societies until the rise of agriculture about ten thousand years
ago. As I recall well myself from watching how fast the food disappeared
from our refrigerator when my wife was nursing our sons, human lactation
is energetically very expensive. The daily energy budget of a nursing
mother exceeds that of most men with even a moderately active lifestyle
and is topped among women only by marathon runners in training. Hence
there is no way that a just-fertilized woman can rise from the conjugal
bed, look her spouse or lover in the eye, and tell him, "You'll have
to take care of this embryo if you want it to survive, because I won't!"
Her consort would recognize this for an empty bluff.
The second factor affecting the relative interest of men and women in
child care is their difference in other opportunities thereby
foreclosed. Because of the woman's time commitment to pregnancy and
(under hunter-gatherer conditions) lactation, there is nothing she can
do during that
time that would permit her to produce another offspring. The traditional

nursing pattern was to nurse many times each hour, and the resulting
release of hormones tended to cause lactational amenorrhea (cessation
of menstrual cycles) for up to several years. Hence hunter-gatherer
mothers had children at intervals of several years. In modern society
a woman can conceive again within a few months of delivery, either by
forgoing breast-feeding in favor of bottle-feeding or by nursing the
infant only every few hours (as modern women tend to do for convenience).
Under those conditions the woman soon resumes menstrual cycles.
Nevertheless, even modern women who eschew breast-feeding and
contraception rarely give birth at intervals of less than a year, and
few women give birth to more than a dozen children over the course of
their lives. The record lifetime number of offspring for a woman is
a mere sixty-nine (a nineteenth-century Moscow woman who specialized
in triplets), which sounds stupendous until compared with the numbers
achieved by some men to be mentioned below.
Hence multiple husbands do not help a woman to produce more babies,
and very few human societies regularly practice polyandry. In the only
such society that has received much study, the Tre-ba of Tibet, women
with two husbands have on the average no more children than women with
one husband. The reasons for Tre-ba polyandry are instead related to
the Tre-ba system of land tenure: Tre-ba brothers often marry the same
woman in order to avoid subdividing a small landholding.
Thus, a woman who "chooses" to care for her offspring is not thereby
foreclosing other spectacular reproductive opportunities. In contrast,
a polyandrous female phalarope produces on the average only 1.3 fledged
chicks with one mate, but 2.2 chicks if she can corner two mates, and
3.7 chicks if she can corner three. A woman also differs in that respect
from a man, whose theoretical ability to impregnate
all the women of the world we have already discussed. Unlike the genetic
unprofitability of polyandry for Tre-ba women, polygyny paid off well
for nineteenth-century Mormon men, whose average lifetime output of
children increased from a mere seven children for Mormon men with one
wife to sixteen or twenty children for men with two or three wives,
respectively, and to twenty-five children for Mormon church leaders,
who averaged five wives.
Even these benefits of polygyny are modest compared to the hundreds
of children sired by modern princes able to commandeer the resources
of a centralized society for rearing their offspring without directly
providing child care themselves. A nineteenth-century visitor to the
court of the Nizam of Hyderabad, an Indian prince with an especially
large harem, happened to be present during an eight-day period when
four of the Nizam's wives gave birth, with nine more births anticipated

for the following week. The record for lifetime number of offspring
sired is credited to Morocco's Emperor Ismail the Bloodthirsty, father
of seven hundred sons and an uncounted but presumably comparable number
of daughters. These numbers make it clear that a man who fertilizes
one woman and then devotes himself to child care may by that choice
foreclose enormous alternative opportunities.
The remaining factor tending to make child care genetically less
rewarding for men than for women is the justified paranoia about
paternity that men share with the males of all other internally
fertilized species. A man who opts for child care runs the risk that,
unbeknownst to him, his efforts are transmitting the genes of a rival.
This biological fact is the underlying cause for a host of repulsive
practices by which men of various societies have sought to increase
their confidence in paternity by restricting their wife's opportunity
for sex with other men. Among such practices are high bride prices only
for brides delivered as proven virgin goods; traditional adultery laws
that define
adultery by the marital status only of the participating woman (that
of the participating man being irrelevant); chaperoning or virtual
imprisonment of women; female "circumcision" (clitoridectomy) to
reduce a woman's interest in initiating sex, whether marital or
extramarital; and infibulation (suturing a woman's labia majora nearly
shut so as to make intercourse impossible while the husband is away).
All three factors—sex differences in obligate parental investment,
alternative opportunities foreclosed by child care, and confidence in
parenthood—contribute to making men much more prone than women to desert
a spouse and child. However, a man is not like a male hummingbird, male
tiger, or the male of many other animal species, who can safely fly
or walk away immediately after copulation, secure in the knowledge that
his deserted female sex partner will be able to handle all the ensuing
work of promoting the survival of his genes. Human infants virtually
need biparental care, especially in traditional societies. While we
shall see in chapter 5 that activities represented as male parental
care may actually have more complex functions than meet the eye, many
or most men in traditional societies do undoubtedly provide services
to their children and spouse. Those services include: acquiring and
delivering food; offering protection, not only against predators but
also against other men who are sexually interested in a mother and regard
her offspring (their potential stepchildren) as a competing genetic
nuisance; owning land and making its produce available; building a house,
clearing a garden, and performing other useful labor; and educating
children, especially sons, so as to increase the children's chances

of survival.
Sex differences in the genetic value of parental care to the parent
provide a biological basis for the all-too-familiar differing attitudes
of men and women toward extramarital sex. Because a human child virtually
required paternal
care in traditional human societies, extramarital sex is most
profitable for a man if it is with a married woman whose husband will
unknowingly rear the resulting child. Casual sex between a man and a
married woman tends to increase the man's output of children, but not
the woman's. That decisive difference is reflected in men's and women's
differing motivations. Attitude surveys in a wide variety of human
societies around the world have shown that men tend to be more interested
than women in sexual variety, including casual sex and brief
relationships. That attitude is readily understandable because it tends
to maximize transmission of the genes of a man but not of a woman. In
contrast, the motivation of a woman participating in extramarital sex
is more often self-reported as marital dissatisfaction. Such a woman
tends to be searching for a new lasting relationship: either a new
marriage or a lengthy extramarital relationship with a man better able
than her husband to provide resources or good genes.
The Non-Evolution of Male Lactation
Today, we men are expected to share in the care of our children. We
have no excuse not to, because we are perfectly capable of doing for
our kids virtually anything that our wives can do. And so, when my twin
sons were born in 1987,I duly learned to change diapers, clean up vomit,
and perform the other tasks that come with parenthood.
The one task that I felt excused from was nursing my infants. It was
visibly a tiring task for my wife. Friends kidded me that I should get
hormone injections and share the burden. Yet cruel biological facts
seemingly confront those who would bring sexual equality into this last
bastion of female privilege or male cop-out. It appears obvious that
males lack the anatomical equipment, the priming experience of
pregnancy, and the hormones necessary for lactation. Until 1994, not
a single one of the world's 4,300 mammal species was suspected of male
lactation under normal conditions. The nonexistence of male lactation
may thus seem to be a solved problem requiring no further discussion,
and it may seem doubly irrelevant to a book about how the unique aspects
of human sexuality evolved. After all, the problem's solution seems
to depend on facts of physiology rather than on evolutionary reasoning,

exclusively female lactation is apparently a universal mammalian
phenomenon not at all unique to humans.
In reality, the subject of male lactation follows perfectly from our
discussion of the battle of the sexes. It illustrates the failure of
strictly physiological explanations and the importance of evolutionary
reasoning for understanding human sexuality. Yes, it's true that no
male mammal has ever become pregnant, and that the great majority of
male mammals normally don't lactate. But one has to go further and ask
why mammals evolved genes specifying that only females, not males, would
develop the necessary anatomical equipment, the priming experience of
pregnancy, and the necessary hormones. Both male and female pigeons
secrete crop "milk" to nurse their squab; why not men as well as women?
Among seahorses it's the male rather than the female that becomes
pregnant; why is that not also true for humans?
As for the supposed necessity of pregnancy as a primary experience for
lactation, many female mammals, including many (most?) women, can
produce milk without first being primed by pregnancy. Many male mammals,
including some men, undergo breast development and lactate when given
the appropriate hormones. Under certain conditions, a considerable
fraction of men experience breast development and milk production even
without having been treated hormonally. Cases of spontaneous lactation
have long been known in male domestic goats, and the first case of male
lactation in a wild mammal species has been reported recently.
Thus, lactation lies within the physiological potential of men. As we
shall see, lactation would make more evolutionary sense for modern men
than for males of most other mammal species. But the fact remains that
it's not part of our normal repertoire, nor is it known to fall within
the normal repertoire of other mammal species except for that single
case reported recently. Since natural selection eviWHY DON'T MEN BREAST-FEED THEIR BABIES? 43
dently could have made men lactate, why didn't it? That turns out to
be a major question that cannot be answered simply by pointing to the
deficiencies of male equipment. Male lactation beautifully illustrates
all the main themes in the evolution of sexuality: evolutionary
conflicts between males and females, the importance of confidence in
paternity or maternity, differences in reproductive investment between
the sexes, and a species' commitment to its biological inheritance.
As the first step in exploring these themes, I have to overcome your
resistance to even thinking about male lactation, a product of our

unquestioned assumption that it's physiologically impossible. The
genetic differences between males and females, including those that
normally reserve lactation for females, turn out to be slight and labile.
This chapter will convince you of the feasibility of male lactation
and will then explore why that theoretical possibility normally
languishes unrealized.
Our sex is ultimately laid down by our genes, which in humans are bundled
together in each body cell in twenty-three pairs of microscopic packages
called chromosomes. One member of each of our twenty-three pairs was
acquired from our mother, and the other member from our father. The
twenty-three human chromosome pairs can be numbered and distinguished
from each other by consistent differences in appearance. In chromosome
pairs 1 through 22, the two members of each pair appear identical when
viewed through a microscope. Only in the case of chromosome pair 23,
the so-called sex chromosomes, do the two representatives differ, and
even that's true only in men, who have a big chromosome (termed an X
chromosome) paired with a small one (a Y chromosome). Women instead
have two paired X chromosomes.
What do the sex chromosomes do? Many X chromosome
genes specify traits unrelated to sex, such as the ability to distinguish
red and green colors. However, the Y chromosome contains genes
specifying the development of testes. In the fifth week after
fertilization human embryos of either sex develop a "bipotential" gonad
that can become either a testis or an ovary. If a Y chromosome is present,
that bet-hedging gonad begins to commit itself in the seventh week to
becoming a testis, but if there's no Y chromosome, the go-nnd waits
until the thirteenth week to develop as an ovary.
That may seem surprising: one might have expected the second X chromosome
of girls to make ovaries, and the Y chromosome of boys to make testes.
In fact, though, people abnormally endowed with one Y and two X
chromosomes turn out most like males, whereas people endowed with three
or just one X chromosome turn out most like females. Thus, the natural
tendency of our bet-hedging primordial gonad is to develop as an ovary
if nothing intervenes; something extra, a Y chromosome, is required
to change it into a testis.
It's tempting to restate this simple fact in emotionally loaded terms.
As the endocrinologist Alfred Jost put it, "Becoming a male is a
prolonged, uneasy, and risky venture; it is a kind of struggle against
inherent trends towards femaleness." Chauvinists might go further and
hail becoming a man as heroic, and becoming a woman as the easy fallback
position. Conversely, one might regard womanhood as the natural state
of humanity, with men just a pathological aberration that regrettably

must be tolerated as the price for making more women. I prefer merely
to acknowledge that a Y chromosome switches gonad development from the
ovarian path to the testicular path, and to draw no metaphysical
But there's more to a man than testes alone. A penis and prostate gland
are among the many other obvious necessiWHY DON'T MEN BREAST-FEED THEIR BABIES?
ties of manhood, just as women need more than ovaries (for instance,
it helps to have a vagina). It turns out that the embryo is endowed
with other bipotential structures besides the primordial gonad. Unlike
the primordial gonad, though, these other bipolar structures have a
potential that is not directly specified by the Y chromosome. Instead,
secretions produced by the testes themselves are what channel these
other structures toward developing into male organs, while lack of
testicular secretions channels them toward making female organs.
For example, already in the eighth week of gestation the testes begin
producing the steroid hormone testosterone, some of which gets
converted into the closely related steroid dihydrotestosterone. These
steroids (known as an-drogens) convert some all-purpose embryonic
structures into the glans penis, penis shaft, and scrotum; the same
structures would otherwise develop into the clitoris, labia minora,
and labia majora. Embryos also start out bet-hedging with two sets of
ducts, known as the Mullerian ducts and Wolffian ducts. In the absence
of testes, the Wolffian ducts atrophy, while the Mullerian ducts grow
into a female fetus's uterus, fallopian tubes, and interior vagina.
With testes present, the opposite happens: androgens stimulate the
Wolffian ducts to grow into a male fetus's seminal vesicles, vas deferens,
and epididymis. At the same time, a testicular protein called Mullerian
inhibiting hormone does what its name implies: it prevents the Mullerian
ducts from developing into the internal female organs.
Since a Y chromosome specifies testes, and since the presence or absence
of the testes' secretions specifies the remaining male or female
structures, it might seem as if there's no way that a developing human
could end up with ambiguous sexual anatomy. Instead, you might think
that a Y chromosome should guarantee 100 percent male organs, and that
lack of a Y chromosome should guarantee 100 percent female organs.
In fact, a long series of biochemical steps is required to produce all
those other structures besides ovaries or testes. Each step involves
the synthesis of one molecular ingredient, termed an enzyme, specified
by one gene. Any enzyme can be defective or absent if its underlying
gene is altered by a mutation. Thus, an enzyme defect may result in

a male pseudohermaphrodite, defined as someone possessing some female
structures as well as testes. In a male pseudoher-maphrodite with an
enzyme defect, there is normal development of the male structures
dependent on enzymes that act at the steps of the metabolic pathway
before the defective enzyme. However, male structures dependent on the
defective enzyme itself or on subsequent biochemical steps fail to
develop and are replaced either by their female equivalent or by nothing
at all. For example, one type of pseudohermaphrodite looks like a normal
woman. Indeed, "she" conforms to the male ideal of female pulchritude
even more closely than does the average real woman, because "her" breasts
are well developed and "her" legs are long and graceful. Hence cases
have turned up repeatedly of beautiful women fashion models not
realizing that they are actually men with a single mutant gene until
genetically tested as adults.
Since this type of pseudohermaphrodite looks like a normal girl baby
at birth and undergoes externally normal development and puberty, the
problem isn't even likely to be recognized until the adolescent "girl"
consults a doctor over failure to begin menstruating. At that point,
the doctor discovers a simple reason for that failure: the patient has
no uterus, fallopian tubes, or upper vagina. Instead, the vagina ends
blindly after two inches. Further examination reveals testes that
secrete normal testosterone, are programmed by a normal Y chromosome,
and are abnormal only for being buried in the groin or labia. In other
words, the beautiful model is an otherwise normal male who happens to
have a genetically determined biochemical block in his ability to
respond to testosterone.
That block turns out to be in the cell receptor that would normally
bind testosterone and dihydrotestos-terone, thereby enabling those
androgens to trigger the further developmental steps of the normal male.
Since the Y chromosome is normal, the testes themselves form normally
and produce normal Mullerian inhibiting hormone, which acts as in any
man to forestall development of the uterus and fallopian tubes. However,
development of the usual male machinery to respond to testosterone is
interrupted. Hence development of the remaining bipotential embryonic
sex organs follows the female channel by default: female rather than
male external genitalia, and atrophy of the Wolffian ducts and hence
of potential male internal genitalia. In fact, since the testes and
adrenal glands secrete small amounts of estrogen that would normally
be overridden by androgen receptors, the complete lack of those
receptors in functional form (they are present in small numbers in normal
women) makes the male pseudohermaphrodite appear externally

Thus, the overall genetic difference between men and women is modest,
despite the big consequences of that modest difference. A small number
of genes on chromosome 23, acting in concert with genes on other
chromosomes, ultimately determine all differences between men and women.
The differences, of course, include not just those in the reproductive
organs themselves but also all other postadolescent sex-linked
differences, such as the differences in beards, body hair, pitch of
voice, and breast development.
The actual effects of testosterone and its chemical derivatives vary
with age, organ, and species. Animal species differ greatly in how the
sexes differ, and not only in mammary gland development. Even among
higher anthropoids— humans and our closest relatives, the apes—there
familiar differences in sexual distinctiveness. We know from zoos and
photos that adult male and female gorillas differ obviously at a long
distance by the male's much greater size (his weight is double the
female's), different shape of head, and silver-haired back. Men also
differ, though much less obviously, from women in being slightly heavier
(by 20 percent on the average), more muscular, and bearded. Even the
degree of that difference varies among human populations: for example,
the difference is less marked among Southeast Asians and Native
Americans, since men of those populations have on the average much less
body hair and beard development than in Europe and Southwest Asia. But
males and females of some gibbon species look so similar that you
couldn't distinguish them unless they permitted you to examine their
In particular, both sexes of placental mammals have mammary glands.
While the glands are less well developed and nonfunctional in males
of most mammal species, that degree of male underdevelopment varies
among species. At the one extreme, in male mice and rats, the mammary
tissue never forms ducts or a nipple and remains invisible from the
outside. At the opposite extreme, in dogs and primates (including humans)
the gland does form ducts and a nipple in both males and females and
scarcely differs between the sexes before puberty.
During adolescence the visible differences between the mammalian sexes
increase under the influence of a mix of hormones from the gonads,
adrenal glands, and pituitary gland. Hormones released in pregnant and
lactating females produce a further mammary growth spurt and start milk
production, which is then reflexly stimulated by nursing. In humans,
milk production is especially under the control of the hormone prolactin,
while the responsible hormones in cows includes somatotropin, alias
"growth hormone" (the hormone behind the current debate over proposed

hormonal stimulation of milk cows).
It should be emphasized that male/female differences in hormones aren't
absolute but a matter of degree: one sex may have higher concentrations
and more receptors for a particular hormone. In particular, becoming
pregnant is not the only way to acquire the hormones necessary for breast
growth and milk production. For instance, normally circulating hormones
stimulate a milk production, termed witch's milk, in newborns of several
mammal species. Direct injection of the hormones estrogen or
progesterone (normally released during pregnancy) triggers breast
growth and milk production in virgin female cows and goats—and also
in steers, male goats, and male guinea pigs. The hor-monally treated
virgin cows produced on the average as much milk as their half-sisters
that were nursing calves to which they had given birth. Granted,
hormonally treated steers produced much less milk than virgin cows;
you shouldn't count on steer's milk in the supermarkets by next Christmas.
But that's not surprising since the steers have previously limited their
options: they haven't developed an udder to accommodate all the mammary
gland tissue that hormonally treated virgin cows can accommodate.
There are numerous conditions under which injected or topically applied
hormones have produced inappropriate breast development and milk
secretion in humans, both in men and in nonpregnant or non-nursing women.
Men and women cancer patients being treated with estrogen proceeded
to secrete milk when injected with prolactin; among such patients was
a sixty-four-year-old man who continued to produce milk for seven years
after hormonal treatment was discontinued. (This observation was made
in the 1940s, long before the regulation of medical research by human
subjects protection committees, which now forbid such experiments).
Inappropriate lactation has been observed in people taking
tranquilizers that influence the hypothalamus (which controls the
pituitary gland, the source of prolactin); it also has been observed
in people
recovering from surgery that stimulated nerves related to the suckling
reflex, as well as in some women on prolonged courses of estrogen and
progesterone birth-control pills. My favorite case is the chauvinist
husband who kept complaining about his wife's "miserable little
breasts," until he was shocked to find his own breasts growing. It turned
out that his wife had been lavishly applying estrogen cream to her
breasts to stimulate the growth craved by her husband, and the cream
had been rubbing off on him.
At this point, you may be starting to wonder whether all these examples

are irrelevant to the possibility of normal male lactation, since they
involve medical interventions such as hormone injections or surgery.
But inappropriate lactation can occur without high-tech medical
procedures: mere repeated mechanical stimulation of the nipples
suffices to trigger milk secretion in virgin females of several mammal
species, including humans. Mechanical stimulation is a natural way of
releasing hormones by means of nerve reflexes connecting the nipples
to hormone-releasing glands via the central nervous system. For
instance, a sexually mature but virgin female marsupial can regularly
be stimulated to lactate just by fostering another mother's young onto
her teats. The "milking" of virgin female goats similarly triggers them
to lactate. That principle might be transferable to men, since manual
stimulation of the nipples causes a prolactin surge in men as well as
in nonlac-tating women. Lactation is a not infrequent result of nipple
self-stimulation in teenage boys.
My favorite human example of this phenomenon comes from a letter to
the widely syndicated newspaper column "Dear Abby." An unmarried woman
about to adopt a newborn infant longed to nurse the infant and asked
Abby whether taking hormones would help her to do so. Abby's reply was:
Preposterous, you'll only make yourself sprout
hair! Several indignant readers then wrote in to describe cases of women
in similar situations who succeeded in nursing an infant by repeatedly
placing it at the breast.
Recent experience of physicians and nurse lactation specialists now
suggest that most adoptive mothers can begin producing some milk within
three or four weeks. The recommended preparation for prospective
adoptive mothers is to use a breast pump every few hours to simulate
sucking, beginning about a month before the expected delivery of the
birth mother. Long before the advent of modern breast pumps, the same
result was achieved by repeatedly putting a puppy or a human infant
to the breast. Such preparation was practiced especially in traditional
societies when a pregnant woman was sickly and her own mother wanted
to be ready to step in and nurse the infant in case the daughter proved
unable to do so. The reported examples include grandmothers up to the
age of seventy-one, as well as Ruth's mother-in-law Naomi in the Old
Testament. (If you don't believe it, open a Bible and turn to the Book
of Ruth, chapter 4, verse 16.)
Breast development occurs commonly, and spontaneous lactation
occasionally, in men recovering from starvation. Thousands of cases
were recorded in prisoners of war released from concentration camps
after World War II; one observer noted five hundred cases in survivors
of one Japanese POW camp alone. The likely explanation is that starvation

inhibits not only the glands that produce hormones but also the liver,
which destroys those hormones. The glands recover much faster than the
liver when normal nutrition is resumed, so that hormone levels soar
unchecked. Again, turn to the Bible to discover how Old Testament
patriarchs anticipated modern physiologists: Job (chapter 21, verse
24) remarked of a well-fed man that "His breasts are full of milk."
It has been known for a long time that many otherwise perfectly normal
male goats, with normal testes and proven
ability to inseminate females, surprise their owners by spontaneously
growing udders and secreting milk. Billy-goat milk is similar in
composition to she-goat milk but has even higher fat and protein content.
Spontaneous lactation has also been observed in a captive monkey, the
stump-tailed macaque of Southeast Asia.
In 1994, spontaneous male lactation was at last reported in males of
a wild animal species, the Dyak fruit bat of Malaysia and adjacent
islands. Eleven adult males captured alive proved to have functional
mammary glands that yielded milk when manually expressed. Some of the
males' mammary glands were distended with milk, suggesting that they
had not been suckled and as a result milk had accumulated. However,
others may have been suckled because they had less distended (but still
functional) glands, as in lactating females. Among three samples of
Dyak fruit bats caught at different places and seasons, two included
lactating males, lactating females, and pregnant females, but adults
of both sexes in the third sample were reproductively inactive. This
suggests that male lactation in these bats may develop along with female
lactation as part of the natural reproductive cycle. Microscopic
examination of the testes revealed apparently normal sperm development
in the lactating males.
Thus, while usually mothers lactate and fathers don't, males of at least
some mammal species have much of the necessary anatomical equipment,
physiological potential, and hormone receptors. Males treated either
with the hormones themselves, or with other agents likely to release
hormones, may undergo breast development and some lactation. There are
several reports of apparently normal adult men nursing babies; one such
man whose milk was analyzed secreted milk sugar, protein, and
electrolytes at levels similar to those of mother's milk. All these
facts suggest that it would have been easy for male lactation to evolve;
perhaps it would have required just a few mutaWHY DON'T MEN BREAST-FEED THEIR BABIES?
tions causing increased release or decreased breakdown of hormones.
Evidently, evolution just didn't design men to utilize that

physiological potential under normal conditions. In computing
terminology, at least some males have the hardware; we merely haven't
been programmed by natural selection to use it. Why not?
To understand why, we need to switch from physiological reasoning, which
we have been using throughout this chapter, back to the evolutionary
reasoning that we were using in chapter 2. In particular, recall how
the evolutionary battle of the sexes has resulted in parental care being
provided by the mother alone in about 90 percent of all mammal species.
For those species, in which offspring will survive with zero paternal
care, it's obvious that the question of male lactatio