There is a correlation between infidelity and the size of a male's testicles, researchers have found.
A study by scientists at the University of Oslo found that primates with bigger testicles were more likely to be unfaithful.
Petter Bøckman, Assistant Professor, said: “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.
"If the male will only fertilise one female and has no competitors, he only needs sufficient sperm to reach the egg. 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."
Prof Bøckman said bonobos have particularly large testicles and mate in large groups whereas gorillas have small testicles.
He said: "There is an abundant flow of semen. Those who leave the greatest amount of sperm have the largest chance of fathering offspring.
"In gorilla troops there is only one male. Even though the gorilla has a small harem, he has no need for large testicles – his balls are tiny.”
Large testicles can increase the risk of testicular cancer, the study found.
“Animals with short lifespans may have enormously large testicles. In one type of grasshopper the testicles occupy half their body mass,” said Prof Bøckman.
"The testicles are even larger in sea urchins. They spawn directly into the ocean. To increase the chance of fertilising an egg, the sea urchin is a huge testicle with a little shell around it."
The testicles of humans are one and a half times larger than those of gorillas.
Prof Bøckman said: “This testifies with abundant clarity to life in our flock. We can pledge our fidelity until we are blue in the face, but this is evidence that our females are cheating.
"We are not like chimpanzees, where the female has four or five sexual partners every time she is in heat, but there is always a certain likelihood that the neighbouring male has dropped by.”
The testicles are also large in animals that have sex with many females.
Prof Bøckman said: “Male lions have huge balls. All the females in the pride must have sex at the same time. When the female lions in the pride are in heat, he must mate with all the females every half-hour for three days.”
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