One more thing about the Sex at Dawn book:
The core of the book's argument is that the kind of strong social bonds that were needed for our success as a species would not have been possible if mating competition took place at the level of the body and not the level of the microscopic. Sperm competition, the book argues, is where male competition really takes place. Meaning, women in early human societies had sex with multiple partners, and the male with the best sperm became a father. Some of the truth for this hypothesis is found in the human balls.
Human males have larger testes than male gorillas, whose mating competition happens at the physical level of the body. This form of competition, it is believed, lead to the extreme sexual dimorphism of that species—males are twice the size of females—and also to smaller testes. (A gorilla situation has one alpha male and two or three females, who normally only have sex with the alpha male—meaning, once the sex done, the male gorilla does not worry about the female immediately having sex with another male.) Bonobo males, however, have bigger balls than humans, and this is attributed their high promiscuity—sex is at the center of bonobo societies. (I have read somewhere that male chimps, which also live in multi-male, multi-female communities, have balls that are not much smaller than their brains.)
In America, sperm competition raises to the mind the negative image of "sloppy seconds." The minds of some Southern African societies, however, see it differently: sperm competition is called "brotherhood." The Southern African name, unlike the American one, retains the pro-social aspects of this practice.