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Once people believed her careful documentation, there was an easy answer—since babies are cute and inhibit aggression, something pathological must be happening. Maybe the Abu langur population density was too high and everyone was starving, or male aggression was overflowing, or infanticidal males were zombies. Something certifiably abnormal.
Hrdy eliminated these explanations and showed a telling pattern to the infanticide. Female langurs live in groups with a single resident breeding male. Elsewhere are all-male groups that intermittently drive out the resident male; after infighting, one male then drives out the rest. Here’s his new domain, consisting of females with the babies of the previous male. And crucially, the average tenure of a breeding male (about twenty-seven months) is shorter than the average interbirth interval. No females are ovulating, because they’re nursing infants; thus this new stud will be booted out himself before any females wean their kids and resume ovulating. All for nothing, none of his genes passed on.
What, logically, should he do? Kill the infants. This decreases the reproductive success of the previous male and, thanks to the females ceasing to nurse, they start ovulating.
That’s the male perspective. What about the females? They’re also into maximizing copies of genes passed on. They fight the new male, protecting their infants. Females have also evolved the strategy of going into “pseudoestrus”—falsely appearing to be in heat. They mate with the male. And since males know squat about female langur biology, they fall for it—“Hey, I mated with her this morning and now she’s got an infant; I am one major stud.” They’ll often cease their infanticidal attacks.
Despite initial skepticism, competitive infanticide has been documented in similar circumstances in 119 species, including lions, hippos, and chimps.
A variant occurs in hamsters; because males are nomadic, any infant a male encounters is unlikely to be his, and thus he attempts to kill it (remember that rule about never putting a pet male hamster in a cage with babies?). Another version occurs among wild horses and gelada baboons; a new male harasses pregnant females into miscarrying. Or suppose you’re a pregnant mouse and a new, infanticidal male has arrived. Once you give birth, your infants will be killed, wasting all the energy of pregnancy. Logical response? Cut your losses with the “Bruce effect,” where pregnant females miscarry if they smell a new male.
Thus competitive infanticide occurs in numerous species (including among female chimps, who sometimes kill infants of unrelated females). None of this makes sense outside of gene-based individual selection.
Individual selection is shown with heartbreaking clarity by mountain gorillas, my favorite primate. They’re highly endangered, hanging on in pockets of high-altitude rain forest on the borders of Uganda, Rwanda, and the Democratic Republic of the Congo. There are only about a thousand gorillas left, because of habitat degradation, disease caught from nearby humans, poaching, and spasms of warfare rolling across those borders. And also because mountain gorillas practice competitive infanticide. Logical for an individual intent on maximizing the copies of his genes in the next generation, but simultaneously pushing these wondrous animals toward extinction. This isn’t behaving for the good of the species.
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Robert M. Sapolsky