According to lead researcher Petter Bøckman, in primates, it’s possible to determine how faithful a female will be by looking at the size of a male’s testicles. In the majority of cases they found that the larger a male’s testicles were, the more likely it was that the female would be unfaithful.
‘If the male has no competitors and fertilises one female egg, he needs only sufficient sperm to reach that egg.’ states Bøckman. ‘If the female is likely to mate on the side it’s sensible to have as many cars as possible in the race, this maxim is true within all primate species, including humans.’
According to Bøckman, the testicles of bonobo monkeys, who mate in large groups, are particularly large, whereas gorillas have relatively small testicles.
‘Those who leave the largest volume of sperm have the best chance of fathering offspring.’ says Bøckman. ‘Gorilla troupes only include one male, so he doesn’t need big testicles, his balls are tiny.’
In addition, the survey found that the larger the testicles, the greater the risk of testicular cancer, whilst animals with short lifespans can have enormous testicles. According to Bøckman, testicles take up half the body mass of one particular species of grasshopper whilst sea urchins have even larger assets. They spawn into the ocean directly which means they need to increase their chances of fertilisation as much as possible, so in effect a sea urchin is one giant testicle wrapped in a shell.
Human testicles are one and a half times bigger than gorilla testicles and Bøckman claims that this proves our females cheat. ‘We can claim fidelity till we’re blue in the face, but science shows that our females are programmed to sleep with several partners.’
But, says Bøckman, we’re not quite as bad as out chimp cousins, who mate with four or five sexual partners when they’re in heat. However, there is a likelihood that women will cheat with at least one or two people during the course of a long term relationship.
Human men don’t get off scot free, the Oslo University study also found that male animals with large testicles are also more likely to have multiple partners. Lions top the poll as the biggest balled mammals because pride leaders must mate with all the female members at the same time. When female lions are in heat the male needs to mate with them all on a half-hourly basis for three days straight, which means they need a lot of sperm in reserve.
Bøckman believes that the human male’s relatively large testicle size also shows that men are instinctively programmed to sew their wild oats in order to engender the highest chance of having offspring. The study was published in the Apollon journal in January and coincides with a new exhibition at the University of Oslo’s Natural History Museum entitled ‘Sexus’.