The co-author of a new study estimates that 25% of our planet's mammals practice infanticide — and she thinks she knows why

Elise Huchard and Dieter Lukas studied 260 species; in about 45% of them, babies are killed by male adults. And it all boils down to sex, says Huchard. In species where a few males tend to prevail over others as mates, adult males kill the babies — though not their own — so the mother is available for mating. In the case of Chacma baboons, for example, as many as half the infants are killed by adult males, notes Lukas. Infanticide is most commonly seen in species that live in groups, versus those that are solitary. Among its practitioners: mice, squirrels, lions, horses, hippos, and gorillas. But the mother has a trick up her sleeve. The females of some species, such as the mouse lemur, essentially sleep around. By mating with a lot of males in a short period, they breed confusion about the baby's paternity. After all, "males stop killing offspring if there is a risk that the offspring might be their own," Lukas says. Instead, males try to produce larger amounts of sperm to guarantee they reproduce; mouse lemur testicles can balloon to 10 times their normal size around breeding time. Another nuance: The researchers found that the practice wasn't observed in mammals that reproduced seasonally, as the males would need to hold off on mating until the breeding season arrived to mate. Females commit infanticide, too. Huchard and Lukas will be studying them next.