Infanticide, the killing of children, is a disturbingly common activity among males in several species of mammals. A new study reveals why it happens, but more importantly how females have learned to stop it: It all has to do with male competition, female promiscuity and who has the biggest testicles.

“In species in which infanticide occurs, testis size increases over generations, suggesting that females are more and more promiscuous to confuse paternity,” said lead author Dr Dieter Lukas, from University of Cambridge’s Department of Zoology.

The scenario goes as follows: Within a group of animals (say, baboons, or lions) one male tends to be the alpha and as such has mating priority with all females. Other males will eventually challenge, either when they reach maturity or the alpha male reaches old age. When they successfully usurp the alpha, they want to mate, but females with offspring typically aren’t capable of immediate reproduction. In order to change that, they’ll kill the former alpha’s offspring, thereby inducing fertility in the females.

That’s a terrible thing, but females develop a simple (and fun sounding) mitigation strategy: Promiscuity.

“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,” said Dr Lukas.

Basically, the females increase the odds of infant survival by mating with as many males as possible. By doing so, paternity becomes a crapshoot, and it isn’t worth it for the new alpha of the group to start killing offspring at the risk of cutting his own bloodline.

Not content to let the females have their way, males have evolved to compensate. Since mating occurs at a higher clip, sperm production significantly increases a given male’s chances of siring offspring. That’s why, in species where this behavior has been observed, the males tend to have larger testicles.

The data supports this: Bonobos and chimpanzees are very closely related species, yet their rates of infanticide are night and day – Male chimpanzees have been observed killing adolescents, where bonobos have not. It’s no surprise, then, that male bonobos’ testicles are on average 15% larger than those of their chimpanzee cousins. The same goes for male Canadian Townsend voles, who don’t commit infanticide and whose testes are a whopping 50% larger than their North American meadow vole relatives.

“While it had previously been suggested that infanticide might be an evolutionary driver in mammalian societies – leading to females allying themselves with other females or forming bonds with a specific male in order to defend their offspring – we’ve now shown that this isn’t the case: male infanticide is a consequence of variation in sociality, most commonly occurring in species where both sexes live together in stable groups,” said Lukas.