The killing of rivals’ offspring represents a violent manifestation of competition, and a significant source of offspring mortality in some mammalian populations. Previous research on such infanticide has focused on males, but a new study by Dieter Lukas from the Max Planck Institute for Evolutionary Anthropology and Elise Huchard from the Institut des Sciences de l’Évolution, Montpellier shows that infanticide by females is also widespread across mammals and that females are likely to gain substantial benefits from it.