• 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 treatme...

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 ...

• 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 o...

• 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 Infan...

• 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.
• We're exceptional animals in that our fathers and mothers often remain together after copulating and are both i...

• 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 chil...

• 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 vagin...

• 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...

• 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 a...

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 b...

• 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.

• 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...

• 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...

• 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 a...

• 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 “bipote...

• 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.
...

• 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 eac...

• 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...

• 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 rearin...

• 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 str...