By Kashmira Gander From The Independent

“As a result, some males have testicles which are up to 10 times larger than normal in breeding season”

Some female primates are promiscuous in order to prevent males from killing their offspring.

It is common for male mammals to kill infants in species where the females live in social groups dominated by one of a few males.

And when a rival male takes over a group, he will kill the group’s young to ensure he can produce his own offspring, researchers from the University of Cambridge said.

But researchers have discovered that the females of some species, like the mouse lemur, will mate as much as possible in a short time to make it unclear who the father is, as males fear the risk of killing their own children.

In turn, this tactic means males of this species focus on producing larger quantities of sperm to ensure they can reproduce.

As a result, in breeding season the male mouse lemur and some other species have testicles which are up to 10 times larger than normal.

Dr Dieter Lukas, from the university’s department of zoology, said: “In species in which infanticide occurs, testis size increases over generations, suggesting that females are more and more promiscuous to confuse paternity.

“Once sperm competition has become so intense that no male can be certain of his own paternity, infanticide disappears – since males face the risk of killing their own offspring, and might not get the benefit of siring the next offspring,” she added.

Closely related species that differ in infanticide and testes size include chimpanzees, where the males commit infanticide, versus bonobos, where males have not been observed to kill offspring. Bonobos have testes that are roughly 15 percent larger than those of chimpanzees.

Dr Lukas added that male Canadian Townsend voles do not commit infanticide, and have 50 per cent larger testes compared to infanticidal males of close relatives the North American meadow voles.

Additional reporting by PA

For more on this story go to: http://www.independent.co.uk/news/science/female-primates-are-promiscuous-to-stop-males-from-killing-their-offspring-9860030.html