The research was conducted by a team from University of Oslo, and its findings were published in The Telegraph recently. Petter Bøckman, who led the study, is quoted as saying: “We can determine the degree of fidelity in the female by looking at the size of the male’s testicles. The less faithful the female, the larger the male’s testicles.” According to the researchers, if the female has few sexual partners, the male does not require large testicles, as it will have sufficient sperm to reach and fertilise the egg. But if competition is intense and the females of the species are known to mate a number of males, then the male members tend to have larger testicles. Bøckman told the newspaper, “If the female mates on the side, it is smart to have as many cars as possible in the race. Then, the male must have testicles that are as large as possible.”
The researchers found that bonobos have particularly large testicles because they mate in large groups. Gorillas, in comparison, have tiny testicles as they are known to have fewer partners. The researchers say that lions also have large testicles because lionesses are known to have sex with multiple partners when in heat.
Humans in comparison have testicles that are at least one-and-a- half times larger than those of gorillas. Bøckman argues that this indicates that humans are an inherently unfaithful species. A previous study, published in Proceedings of the National Academy of Sciences, however, found that the size of a man’s testicles has a correlation with his parenting abilities. Fathers were probed on details of their parenting and MRI scans were used to see how their brains’ reward centres reacted on being shown pictures of their children. Men with smaller testicles were found to be better parents. The researchers argued that this was the result of an evolutionary trade off—to focus either on mating or parenting.