Women’s fidelity has come under fire at the University of Chicago. Some claim that women are naturally endowed with cheating hearts, and they are pointing to the size of men’s testicles as proof.

A Chicago research team, led by Dr. Bruce Lahn, found that the intensity of sexual competition in a variety of primate species is directly related to the size of males’ testicles. After obtaining DNA sequences from the SEM2 gene from 12 species of primates, including humans, the team examined the evolution of these primates’ sperm.

“We found that the rate of evolution is much higher—that is, the gene has undergone much more dynamic changes—in primate species with promiscuous females than in primate species with monogamous females,” Lahn said. “In other words, when males have to compete more in the game of fertilization, their semen protein has to make more innovations along the way.”

Female chimpanzees are notorious for their unwillingness to settle down. Males in the species must not only compete for a mate, but their sperm must also win a struggle against sperm from other males once copulation is complete. The sperm from various partners battle to fertilize their mate’s egg. Consequently, male chimps boast testicles and sperm counts disproportionately large for their body size.

On the opposite end of the fidelity spectrum are female gorillas, which mate exclusively with a dominant male. This lucky alpha male accumulates an entourage of female mates that lasts indefinitely. His reign as pimp daddy ends only when he is beaten in a fight by an up-and-coming alpha male. With this kind of sexual security, the alpha male need not be armed with the virility of a chimpanzee. Since he does not need to produce as much competitive semen, the gorilla’s testicles are nothing more than a hiccup in relation to his body size.

If a primate is not equipped with enormous semen-producing testicles, he can boost his chances of fertilizing the egg by producing sticky semen that coagulates in his mate’s vagina, creating a barrier. This virtual “chastity belt” prevents competing semen from getting to the egg.

Human testicles fall somewhere in the middle of chimp and gorilla testicles in size. This suggests that women’s fidelity patterns likewise fall somewhere in between these two species, so that women are neither as faithful as gorillas nor as naughty as chimps.

The research team plans to continue studying sex-related genes. “As far as the SEMG2 gene itself is concerned, we can study how evolutionary changes in the gene alter the biochemical properties of its protein products,” Lahn said.