Some male mammals are known to kill babies of the same species. The only trait that really explains infanticide is the possibility for females to breed in any season, according to a study published in Science this week. The one successful defense against infanticide is female promiscuity, which creates confusion about the paternity of the offspring. The result of this battle of the sexes is increased sperm competition and, of course, larger testes. Turns out, infanticide is a consequence—and not a cause—of intense sexual conflict in social systems where males compete to reproduce.

Observations in the wild of males taking control over a group females and killing off juveniles have accumulated over the last 50 years. With Chacma baboons (pictured below), for example, outsiders fight dominant males for access to females, and when rivals take over a group, they’ll kill the infants of previously dominant males to render the females sexually receptive again. Some males kill more infants than disease or predators. It’s so surprisingly common—from the house mouse to the king of the jungle—yet we’re still unclear about why it develops in some mammals and not others.

For a comparative analysis, University of Cambridge's Dieter Lukas and Elise Huchard looked at 260 mammal species with a variety of mating systems: 119 species that exhibit infanticide, 141 that don’t. Infanticide, they found, evolves in social species where sexual conflict is the most intense, and reproduction is monopolized by a minority of males.

The only life history trait that significantly explains male infanticide is the possibility for females to breed any time. By speeding up the female’s return to her reproductive state, the infanticidal males redirect her investment from unrelated offspring to his own. Especially because they’re only in charge for a short time before getting deposed themselves. 

This brutal tactic, however, can be successfully thwarted by female sexual promiscuity. This paternity dilution strategy is simple: Mate with multiple males in a short amount of time to make it hard for any one male to know that he is (or isn’t) the dad. 

In mouse lemurs, for example, reproductive competition shifts to after copulation, and not before. The most successful male has sperm that outcompetes that of others. This leads to males producing ever larger quantities of sperm, which then leads to increases in testicle size (pictured above). "In species in which infanticide occurs, testis size increases over generations, suggesting that females are more and more promiscuous to confuse paternity," Lukas says in a news release. "Once sperm competition has become so intense that no male can be certain of his own paternity, infanticide disappears."

And it works. Infanticide has been lost in some species where balls have gotten huge. Male chimps commit infanticide, but male bonobos don’t and their testes are 15 percent larger. Huchard adds: "Male infanticide appears and disappears over evolutionary times according to the state of the evolutionary arms race between the sexes.”

Images: Cornelia Kraus (top), Elise Huchard (middle)