Time to evolve a new strategy (Image: Ingo Arndt/NaturePL)

Great balls of furriness! Among mammals, large testicles are a sign of a species with a history of males that have no qualms about killing the babies of their competitors.

A study of more than 200 mammals from mice to lions reveals that in species where infanticide is frequent, females make it hard for males to know which baby to kill by mating with lots of different males during the same season. The study also finds that the ancestor to all great apes – including humans – probably committed infanticide.

Infanticide is widespread among mammals. Lions do it, chimps do it, many adorable-looking lemurs do it. Why? A leading theory is that males kill infants sired by other males because it frees up females to have their own offspring, perpetuating their own genes rather than those of their competitors.


To test this in a large evolutionary model, Dieter Lukas of the University of Cambridge and Elise Huchard of the University of Montpellier in France drew up a huge database of behaviours found in over 200 species of mammals and mapped them onto the mammalian family tree.

The pair confirmed that infanticide was most frequent in species that lived in groups where a few dominant males monopolise the right to mate with the clan’s females, and their tenure as top sperm donor is short.

“The males don’t manage to stay dominant for very long, so when they can mate with the females, they need to do it as quickly as possible,” explains Huchard. “It’s not in their interest to wait for the females to finish rearing infants.” Killing babies, in this instance, is an efficient way to fast-track nursing females back to fertility.

“The study confirms that infanticide isn’t some curious thing caused by humans encroaching on animal territory, it is a male tactic to improve their mating opportunities,” says Kit Opie of University College London.

The tree also revealed that infanticide was probably prevalent in the common ancestor of all great apes. The behaviour lives on in chimps and gorillas, although bonobos, orang-utans and – thankfully – humans have lost the trait.

Promiscuous response

No animal likes having its baby killed, so what’s the defence? A recent study of primates found that they evolved monogamy early as a response to infanticide.

But in the larger mammalian tree, Lukas and Huchard found hints of another trick, in the form of a very strong link between infanticide and testicle size. Males belonging to species that commit infanticide frequently evolve large balls.

“It has long been known that testes size reflects the number of sex partners that females have,” says Huchard. The large testicular size of males in species that commit infanticide suggests that females are more promiscuous, mating with multiple males before giving birth to their offspring.

What’s more, by studying their evolutionary tree, the team found that large testes tended to evolve after infanticide had appeared, and continued to become larger over time. This suggests that when male mammals begin to kill infants, females respond by mating with lots of different males during the same season, forcing males into a sperm competition. Males grow larger testes to produce more sperm, but also don’t know which infant is theirs. As a result, infanticide becomes counterproductive – there is always a risk they might wipe out their own gene pool.

And it’s a response that works. Huchard and Lukas found that infanticide can be lost in species where the testes have grown large. Bonobos, for instance, appear to have lost infanticide since they diverged from chimps.

Journal reference: Science, DOI: 10.1126/science.1257226