Nearly half of mammal species surveyed in a new study practice infanticide. Of the 260 species studied, 119 kill their own young, researchers report online today in Science. Infanticide was most likely to occur in species in which the female was capable of breeding at any point in the year. The findings suggest that a male kills unrelated offspring in order to shorten a female’s postpartum infertility phase, so he can mate with her sooner—a hypothesis confirmed by the observation that males will often mate with the mothers of children they’ve killed. (Human hunting may also play a role, as appears to be the case in bears.) However, females can use sexual promiscuity to create confusion about the paternity of offspring and discourage infanticide. Oddly enough, scientists believe this leads to larger testicles in males: When females are promiscuous, sperm competition increases, and evolution selects for males with the largest testicles capable of producing the most sperm. Sure enough, the team found larger testes in species that have been practicing infanticide for longer. That may explain the absurd size of the gonads of the male mouse lemur, seen above.