Human brains are nearly three times larger than those of chimpanzees, but we’ve got nothing on our closest cousins in the testicle department. Whereas human testes top out at about 50 grams, chimpanzees’ routinely reach weights of 150 to 170 grams. Scientists think the difference has to do with each species’ mating habits. Among primates in which females tend to have one mate at a time, like humans and gorillas, testicles are generally smaller. Larger testes—along with more tissue designed for holding semen—are found in species in which females have sex with several males during a fertility cycle, like chimpanzees and rhesus macaques. New research suggests that the forces governing the difference lie in each species’ transcriptome, or the set of RNA molecules that translates genetic codes into actual physical traits. In more polygamous species, the transcriptome directs testicles to continue developing for longer after birth, researchers report this month in a study on the preprint server bioRxiv. In humans, testicle development starts later and ends earlier, leaving men about as well endowed as an adolescent chimpanzee. But hope is not lost, puny humans! Our primate ancestors appear to have switched between mating types—and, therefore, testicle size—at least six times before we came along, suggesting that testicle tissue may respond to evolutionary pressure more rapidly than other body parts do.