In langur troops, a core of related females and their offspring associate for up to a few years with unrelated immigrant males.  If the newcomers win, they drive the other males away and systematically kill infants less than six months old.  The mothers come into estrus again much sooner than if the infants had survived and continued to nurse, and they are impregnated by the new males.

From Melvin Konner’s The Evolution of Childhood, the best book on evolution I’ve read since Graham Bell’s Selection: The Mechanism of EvolutionThis I promise: Once we uncover a massive immigrant conspiracy to drive off the American male population, murder our young, and enslave American womanhood, I’ll be the first to eat crow.