Moderate body-size dimorphism isn’t the only anatomical suggestion of promiscuity in our species. The ratio of testicular volume to overall body mass can be used to read the degree of sperm competition in any species. Jared Diamond considers the theory of testis size to be “one of the triumphs of modern physical anthropology.” Like most great ideas, the theory of testis size is simple: species that copulate more often need larger testes, and species in which several males routinely copulate with one ovulating female need even bigger testes.
If a species has cojones grandes, you can bet that males have frequent ejaculations with females that sleep around. Where the females save it for Mr. Right, the males have smaller testes, relative to their overall body mass. The correlation of slutty females with big balled males appears to apply not only to humans and other primates, but to many other mammals, as well as to birds, butterflies, reptiles, and fish.
In gorillas’, winner-take-all approach to mating, males compete to see who gets all the booty, as it were. So, although an adult silverback gorilla weighs in at about four hundred pounds, his penis is just over an inch long, at full mast, and his testicles are the size of kidney beans, though you’d have trouble finding them, as they’re safely tucked up inside his body. A one-hundred-pound bonobo has a penis three times as long as the gorilla’s and testicles the size of chicken eggs. The extra-large, AAA type. In bonobos, since everybody gets some sugar, the competition takes place on the level of the sperm cell, not at the level of the individual male. Still, although almost all bonobos are having sex, given the realities of biological reproduction, each baby bonobo, still has only one biological father.
So the game’s still the same—getting one’s genes into the future—but the field of play is different. With harem-based polygynous systems like the gorilla’s, individual males fight it out before any sex takes place. In sperm competition, the cells fight in there so males don’t have to fight out here. Instead, males can relax around one another, allowing larger group sizes, enhancing cooperation, and avoiding disruption of the social dynamic. This helps explain why no primate living in multimale social groups is monogamous. It just would not work.
As always, natural selection targets the relevant organs and systems for adaptation. Through the generations, male gorillas evolved impressive muscles for their reproductive struggle, while their relatively unimportant genitals dwindled down to the bare minimum needed for uncontested fertilization. Conversely, male chimps, bonobos, and humans had less need for oversized muscles for fighting but evolved larger, more powerful testicles and, in the case of humans, a much more interesting penis.
We can almost hear some of our readers thinking, “But my testicles aren’t the size of chicken eggs!” No, they’re not. But we’re guessing they’re not tiny kidney beans tucked up inside your abdomen, either. Humans fall in the middle ground between gorillas and bonobos on the testicular volume/body-mass scale. Those who argue that our species has been sexually monogamous for millions of years point out that human testicles are smaller than those of chimps and bonobos. Those who challenge this narrative (like us, for example) note that human testicular ratios are far beyond those of the polygynous gorilla or the monogamous gibbon.
So, is the scrotum half-empty or half-full?
Sex at Dawn
Ryan and Jethá 2010