Pinker compares the mating habits of various species:

Apes have a wide variety of sexual arrangements. The means, by the way, that there is no such thing as an “ape legacy” that humans are doomed to live by. Gorillas live on the fringes of forests in small groups of one male and several females, and the males fight each other for control over females, the males evolving to be twice the females’ size. Gibbon females are solitary and widely dispersed, and the male finds a female’s territory and acts as a faithful consort. Since other males are off in other territories, they fight no more than females do and are no bigger. Orangutan females are solitary but close enough together that a male can monopolize two or more of their ranges, and the males are about 1.7 times the size of the females. Chimps live in large, unstable groups that no male could dominate. Groups of males live with the females, and the males compete for dominance, which confers more opportunities to copulate. The males are about 1.3 as large as females. With lots of males around, a female has an incentive to mate with many of them so that a male can never be sure that an infant is not his and hence will not murder the infant to make its mother available to bear his own offspring. Bonobo (pygmy chimp) females are almost indiscriminately promiscuous, and the males fight less and are about the same size as females. They compete in a different way: inside the females’ bodies.

Sperm can survive in the vagina for several days, so a promiscuous female can have several males’ sperm competing inside her for a chance at fertilizing the egg. The more sperm a male produces, the greater the chance that one of his will get there first. That explains why chimpanzees have enormous testicles for their body size. Bigger testes make more sperm, which have a better chance inside promiscuous females. A gorilla is four times the weight of a chimpanzee, but his testicles are four times smaller. The females in his harem have no chance to copulate with any other male, so his sperm do not have to compete. Gibbons, who are monogamous, have small testicles too.

In almost all primates (indeed, in almost all mammals), the males are deadbeat dads, contributing nothing to their offspring but DNA. Other species are more fatherly. […] The evolution of male parental investments is helped along by several things. One is external fertilization, found in most fishes, where the female drops her eggs and the male fertilizes them in the water. The male is guaranteed that the fertilized eggs carry his genes […] But in most mammals the cards are stacked against doting fatherhood. The egg is tucked away inside the mother, where some other male can fertilize it, so a male is never certain an offspring is his. He faces the danger of wasting his investment on another male’s genes. […]

When males become devoted fathers, the rules of the mating game change. A female may choose to mate based on his ability and his willingness to invest in their offspring, insofar as she can judge. Females, not just males, compete for mates, though the prizes are different: males compete for fertile females willing to copulate, females compete for flush males willing to invest. Polygamy is no longer a matter of one male beating up all the others, or the females all wanting to be inseminated by the fiercest or prettiest male. When males invest more than females, as we have seen, the species may be polyandrous, with tough females keeping harems of males. (The mammals’ body plan has foreclosed that option.) When one male has much more to invest than others (because, say, he controls a better territory), females may be better off sharing him—polygyny—than each having her own mate, because a fraction of a big resource may be better than the entirety of a small one. When males’ contributions are more equal, the undivided attention of one becomes valuable, and the species settles on monogamy.

[…]In some species of birds, a third of the offspring contain the DNA of a male other than the female’s consort. The male bird is adulterous because he tries to raise the offspring of one female and mate with others, hoping that her offspring will survive on their own, or best of all, be raised by a cuckolded consort. The female bird is adulterous because she has a chance of getting the best of both worlds: the genes of the fittest male and the investment of the most willing male. The victim is worse off than if he had failed to breed at all, because he has devoted his worldly efforts to the genes of a competitor. So in a species whose male invest, the male’s jealousy is directed not only at rival males but at the female. He may guard her, follow her around, copulate repeatedly, and avoid females that show sign of having recently matted.