• 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?
• In the face of this 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.

• 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.
• 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.

• 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 gonad waits until the thirteenth week to develop as an ovary.

• 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.. Not only do males of those species have no need to lactate; they also don’t have to bring food, defend a family territory, defend or teach their offspring.
• Only for those 10 percent of mammal species in which male parental care is necessary does the question of male lactation even deserve consideration.

Those minority species include lions, wolves, gibbons, marmosets— and humans. But even in those species requiring male parenting, lactation isn’t necessarily the most valuable form that the father’s contribution can take. What a big lion really must do is to drive off hyenas and other big lions bent on killing his cubs.

• Human milk is more dilute in nutrients, sugary, and low in fat.
• Breast-fed babies acquire stronger immune defenses and are less susceptible to numerous diseases, including diarrhea, ear infections, early-age-onset diabetes, influenza, necrotizing enterocolitis, and SIDS (Sudden Infant Death Syndrome).
• The helpless condition of human infants makes lots of parental care necessary for many years.

Why do most animals consider sex fun only when the female can be fertilized?

• There’s a simple reason why most other animals are sensibly stingy about copulatory effort: sex is costly in energy, time, and risk of injury or death.
• Natural selection favors those individuals whose behavior lets them pass their genes to the most babies.

• For the cavewoman, the male’s departure would expose her eventual child to the likelihood of starvation or murder.
• What can she do to keep that man? Her brilliant solution: remain sexually receptive even after ovulating! Keep him satisfied by copulating whenever he wants! In that way, he’ll hang around, have no need to look for new sex partners, and will even share his daily hunting bag of meat.
• Recreational sex is thus supposed to function as the glue holding a human couple together while they cooperate in rearing their helpless baby.

It arose at a time when our ancestors were still promiscuous or living in harems. At such times, concealed ovulation let the ancestral ape-woman distribute her sexual favors to many males, none of which could swear that he was the father of her child but each of which knew that he might be. As a result, none of those potentially murderous males wanted to harm the ape-woman’s baby, and some may actually have protected or helped feed it.

• By the criterion of services offered to mates and children, males of most mammal species are good for nothing except injecting sperm.
• The males of many territorial animal species, such as songbirds, lions, and chimpanzees, spend much time patrolling their territories.

• The show-offs also gains some advantages apart from adultery, such as prestige in his tribe’s eyes. Others in the tribe want him as a neighbor because of his gifts of meat, and they may reward him with their daughters as mates. For the same reason, the tribe is likely to give favored treatment to the show-off’s children.
• A woman is best off married to a provider, but a man is not best off being a provider.

• Menopause - a woman’s egg supply gets depleted or impaired by around the time she is fifty years old. The challenge is to understand why we evolved that seemingly self-defeating detail of reproductive physiology.
• That one cruel fact underlying human female menopause: the birth of each child immediately jeopardizes a mother’s previous children because of the mother’s risk of death in childbirth.

• Even in affluent, twentieth-century Western societies, the risk of dying in childbirth is seven times higher for a mother over the age of forty than for a twenty-year-old mother.
• But each new child puts the mother’s life at risk not only because of the immediate risk of death in childbirth but also because of the delayed risk of death related to exhaustion by lactation, carrying a young child.

• Animals whose lifestyles carry a high risk of accidental death are evolutionarily programmed to stint on repair and to age rapidly.
• Turtles, protected in the wild by a shell, are programmed to age more slowly than other reptiles, while porcupines, protected by quills, age more slowly than mammals comparable in size.

• We too rely on signals as arbitrary as a widowbird’s tail and a bowerbird’s crest. Our signals include faces, smells, hair color, men’s beards, and women’s breasts.
• Human females signal reproductive maturity by expansion of the breasts. Later in life, we signal our waning fertility and attainment of wise elder status by the whitening of our hair.

• Muscles are a truthful signal of male quality, like a deer’s antlers.
• One might reason that beauty says nothing about good genes, parenting qualities, or food-gathering skills. However, the face is the part of the body most sensitive to the ravages of age, disease, and injury.
• Especially in traditional societies, individuals with scarred or misshapen faces may thereby be advertising their proneness to disfiguring infections, inability to take care of themselves, or burden of parasitic worms.

• Our remaining candidate for a truthful signal is women’s body fat.
• Lactation and child care are a big energy drain on a mother, and lactation tends to fail in an undernourished mother.
• Hence a woman’s body fat would be a truthful signal to a man that she was capable of rearing his child.
• Women of all populations tend to accumulate fat in the breasts and hips, to a degree that varies geographically.
• Men throughout the world tend to be interested in women’s breasts, hips, and buttocks.

• Scientists have suggested that large fatty breasts are not only an honest signal of good overall nutrition but also a deceptive specific signal of high milk-producing ability (deceptive because milk is actually secreted by breast glandular tissue rather than by breast fat).
• Similarly, it has been suggested that fat deposition in the hips of women is also both an honest signal of good health and a deceptive specific signal suggesting a wide birth canal (deceptive because a truly wide birth canal would minimize the risk of birth traumas but mere fat hips would not).

• The length of the erect penis is only about 1 ¼ inches in gorillas and 1 ½ inches in orangutans but 5 inches in humans, even though males of the two apes have much bigger bodies than men.
• The size of the penis that we evolved was unfortunately limited by the length of a woman’s vagina.
• Hence a large penis’s cost should be regarded as a lost-opportunity cost: the energy squandered on one structure comes at the expense of energy potentially available for another structure.

• Almost anyone would assert that the functions of the penis are to eject urine, inject sperm, and stimulate women physically during intercourse.
• But the comparative approach teaches us that those functions are accomplished elsewhere in the animal world by a relatively much smaller structure than the one with which we encumber ourselves.
• Thus, even the most familiar and seemingly most transparent piece of human sexual equipment surprises us with unsolved evolutionary questions.