In the gentle world of nature documentaries, we rarely linger on one of evolution's most disturbing truths. Infanticide is shockingly common across the animal kingdom. Lions, langurs, dolphins, rodents, birds, and even our closest primate relatives routinely kill infants—often their own species' young.

Our instinct is to recoil, to label this behavior as pathological or aberrant. But evolutionary biology tells a different story. Infanticide isn't a malfunction of nature. It's a strategy—one that has evolved independently across hundreds of species because, under certain conditions, it works.

Understanding why animals kill young forces us to confront the amoral logic of natural selection. Evolution doesn't care about suffering. It cares about reproductive success. And sometimes, in the cold arithmetic of gene propagation, killing an infant makes evolutionary sense.

When a male lion takes over a pride, one of his first acts is often to kill all the cubs. This seems monstrous until you understand the mathematics. Female lions suppress ovulation while nursing. A lioness caring for cubs won't become receptive to mating again for up to two years.

The new male faces a problem. His tenure as pride leader is uncertain—other males will eventually challenge him. Every month a female spends nursing another male's offspring is a month she's not producing his cubs. By killing existing young, he resets her reproductive clock. She'll return to estrus within days or weeks.

This pattern repeats across species where male tenure is short and females delay breeding while raising young. Langur monkeys, Hanuman langurs especially, became famous in primatology for male infanticide following takeovers. The incoming male gains immediate reproductive access. His genes win at the expense of the previous male's lineage.

The evolutionary logic is brutal but clear. Genes that promote infanticide in incoming males spread because those males father more offspring. The behavior persists not because it benefits the species—it clearly doesn't—but because it benefits the individual perpetrator's genetic legacy.

Takeaway

Natural selection optimizes for individual reproductive success, not species welfare or moral outcomes. Behaviors that seem cruel often persist because they give perpetrators a genetic advantage.

Evolution never produces a strategy without generating counter-strategies. Females have evolved remarkable defenses against infanticidal males. These range from direct physical protection to sophisticated manipulation of male behavior.

One of the most elegant solutions is paternity confusion. Many primate females mate with multiple males, even when already pregnant. By doing so, they create uncertainty about who fathered their infant. A male who might have sired an offspring is far less likely to kill it. Chimpanzees, baboons, and many other primates show this promiscuous mating pattern precisely when infanticide risk is high.

Some females form coalitions to defend against infanticidal males. Female langurs will band together to attack incoming males, buying time for their infants to grow past the vulnerable stage. In species like house mice, females will nest communally, creating defensive groups that can better protect young.

Physical adaptations have emerged too. In some species, females have evolved concealed ovulation—hiding their fertile periods so males cannot easily time attacks. Others produce larger litters, essentially hedging their bets against infanticide losses. The female genome carries generations of evolutionary wisdom about resisting male violence.

Takeaway

Every evolutionary strategy creates selective pressure for counter-strategies. The resulting arms race shapes both male and female behavior, creating complex social dynamics from simple selective pressures.

Sexual selection explains infanticide by adult males, but it doesn't account for all cases. Infanticide also evolves as a response to resource competition. Here, the killers might be siblings, neighbors, or even mothers themselves.

Siblicide—the killing of siblings—is disturbingly common in birds. Many eagle and heron species lay two eggs as insurance, but the older, larger chick frequently kills its sibling. The parents don't intervene. In years of food scarcity, raising two chicks is impossible anyway. The 'insurance' egg only survives in exceptional years.

Territorial animals sometimes kill neighbors' offspring to eliminate future competitors. Ground squirrels, a seemingly innocent creature, commit infanticide against neighboring litters. Removing those infants means less competition for burrow sites and food when the killer's own offspring grow up.

Most troubling to human sensibilities, mothers themselves sometimes commit infanticide. When resources are desperately scarce or offspring are defective, abandoning or killing young can allow a female to survive and reproduce successfully later. The calculation is grim but mathematically sound: a dead infant now versus the mother's potential future reproductive success.

Takeaway

Resource limitation creates selection pressure for infanticide even outside sexual competition. When survival is zero-sum, eliminating competitors—even relatives—can be adaptive.

Infanticide reveals evolution's fundamental indifference to suffering. These behaviors persist not because they're good for species or ecosystems, but because they increase the reproductive success of individuals carrying genes that promote them.

Understanding this dark logic doesn't require us to accept it in human contexts. We've evolved cultural and moral systems precisely to override our more brutal instincts. But we can't fully understand our own nature without acknowledging what selection has built into biological systems.

The natural world contains beauty and horror in equal measure. Evolutionary biology helps us understand both—not to justify cruelty, but to see clearly the forces that shaped all living things, including ourselves.