Size may not matter when it comes to your penis, but in the area of testicles, it means a lot when it comes to infidelity.
Researchers have found that there is a correlation between the size of a male's testicles and the infidelity his female counterpart, reported The Telegraph.
Primates with bigger testicles were more likely to be unfaithful, according to a study by scientists at the University of Oslo.
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."
The study found bonobos, who have particularly large testicles, mate in large groups, whereas gorillas, who have smaller testicles, have a smaller number of partners.
"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.
"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.” Bøckman said.
Humans have testicles that are one and a half times larger than those of gorillas. The researchers claim that this can be used to show that we are an inherently unfaithful species.
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“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.” Bøckman said.
Animals that have sex with many females are also likely to have larger testicles.
Prof Bøckman said: "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.
“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.”
The researchers also studied the animals with larger testicles, finding that bigger testicles can also increase the risk of testicular cancer.
“Animals with short lifespans may have enormously large testicles. In one type of grasshopper the testicles occupy half their body mass,” said Bøckman.