The Web of Life (121-3-6) Infanticide


Dominant animals are worse off when subordinates in their group try to breed – explaining why they brutally suppress others much of the time. ~ English zoologist Matthew Bell

Infanticide – killing and often eating infants, either one’s own or others nearby – is practiced by microscopic rotifers, insects, fish, amphibians, snakes, rodents, colonial birds and several birds of prey, lions, and primates, including humans. Ovicide is the analogous destruction of eggs.

The root of infanticide is typically sexual conflict: a male taking a new partner after killing her offspring by another. As an effective evolutionary device, certain conditions must be met. It occurs in species where estrus is year-round; otherwise infanticide does not necessarily free up a female for breeding.

There’s no sense for a male to kill the offspring in the previous year, because he has to wait anyway. ~ English ethologist Dieter Lukas

Infanticide emerges in species with group living where females greatly outnumber males. This situation favors a male dominance hierarchy for mating rights.

In species where infanticide appears, females gradually mate with more males. This evolutionary adaptation confuses patrimony, and so lessens the incentive for baby killing.

Whenever promiscuity is high enough, it does not pay for males to commit infanticide. ~ Carel van Schaik

Conversely, infanticide also drives monogamy. This both makes patrimony clearer and provides for offspring protection by the male parent. Extra-pair coupling in females may still occur, but usually as a deceptive practice. Humans are exemplary.

Langurs are a social Old World monkey living in India. Each group is sexually dominated by a single male. In a violent overthrow of the alpha male, infants of the vanquished are killed, though only for a short time after the takeover.

The same dynamic applies to a pride of lions when a new male takes over. The domineering newcomer will try to kill any cubs 9 months or younger. As in other species, females defend their cubs viciously.

An intriguing exception to male infanticide occurs in Japanese anemone fish. If a male parent looking after a clutch of eggs is killed or deposed by a new male, the newcomer assumes paternal duties with as much vigor as his predecessor. Genetically this makes no sense. But the female will refuse the new male if he is not a decent foster-parent. Only when he starts caring for the young does she stop beating upon him. The male’s short-term altruism is rewarded with his own clutch next go-round. Such a system can only work when females are atypically aggressive.

Male mice have biorhythms of infanticide. After fertilizing a female, a male kills any pups it comes across for 3 weeks. Then the prolactin kicks in as they become caring for their own offspring. This last for 2 months, whereupon they return to their infanticidal ways. By no coincidence, female gestation is 3 weeks, and it takes 2 months for pups to be weaned and on their way. Thus, this is well-timed infanticide via sexual conflict.

Males can be infant killers even incidentally, or against their own genetic interests. Polar bear cubs must avoid adult males, even dad, who regard the little ones as prey.

Male eared seals (otariids), which includes sea lions and fur seals, form mating harems, with territories on breeding beaches. A pup that strays may be killed by the next-door bull for crossing into his dominion.

Another harem-polygynous species, the colonial black-tailed prairie dog, practices infanticide as part of invasion by males, and as a marauding behavior by females. The point for females is to reduce competition with other females for food and future offspring.

Such resource competition also occurs in meerkats. A female may kill the offspring of her mother, sister, or daughter. Infanticidal raids from neighboring groups also occurs.

Mothers of many ground squirrels, and especially rats, may cannibalize young if extremely stressed. Other mothers may kill younglings when not so mentally rent.

In species where paternal care is the norm, females compete with others by killing their offspring. Wattled jacanas, a tropical wading bird with a polyandrous mating system, practice this infanticide. Wattled jacana males exclusively brood while females defend territory.

A similar dynamic happens in giant water bugs, where males take care of egg masses. Females that cannot find a mate often stab the eggs of a brooding male. This ovicide is rewarded by the female being fertilized by the male, who is in the mood when he has nothing to brood.

Some nonbreeding colonial birds, such as gulls, kill and eat nestlings. Overcrowding and attendant resource competition is typically the inspiration.

 Burying Beetles

Burying beetles have complex parental care practices. Both parents provide diligent nurturing; though, as in birds and humans, females take the lead.

Burying beetles find a carcass, usually a small bird or mouse; a prize worth fighting for. And they do.

Mating pairs pair off as rivals: female against female, male against male. The larger contestants typically triumph.

The winning pair gets the spoiled spoils, which is buried (hence the namesake) to remove it from further competition.

The burying beetle couple then busy themselves building a nursery: dig a hole under the corpse into which the kill collapses; curve the corpse into a circle; remove all the hair; coat the body with antibacterial and antifungal secretions from their saliva; eat enough of the corpse to create a cavity. The preparation process may take 8 hours.

A couple of days later, the female lays her eggs into the cavity. A parent – usually the female – watches over the eggs.

The eggs hatch into larvae, which feed upon the corpse.

Although the larvae can feed themselves, both parents practice progressive provisioning: digesting flesh and regurgitating liquid food for the larvae to feed on. Parental secretions may also benefit the youngsters.

Such hubbub helps grow the grubs. The adult beetles feed and protect the larvae for several days, until they are ready to move into the soil and pupate.

A few larvae get greedy, pestering their parents for more than their share. These younglings are cannibalized for their temerity, engendering more honest communication about apportioning food.

Older male beetles make more diligent parents than younger ones, particularly when paternity is uncertain. More breeding opportunities lay ahead for beetles in their prime. An elder beetle may be tending his last brood, and so takes extra care.

It certainly provides some incentive for females to want to mate with older males. ~ English evolutionary biologist Megen Head

If the carcass is large enough, several beetle pairs will cooperate to bury and breed a communal brood. Conversely, if pickings are slim, parents cull their young early on, to match the number of larvae to the size of the supply carcass, carefully estimating enough food to go around to pupation. Burying beetles are both caring parents and killer calculators.


Acorn woodpecker females nest together or risk egg tossing by rivals. Even early laid eggs in a nest are subject to ovicide. Eventually, the entire group lays on the same day, whereupon cooperation breaks out and the females collectively incubate eggs.

Likewise, colonial birds can be egg tossers as a strategy of clutch coordination. Ovicide occurs until all birds in a common nest are ready for brooding. Hence, early egg layers do not dominate reproduction.

Many adult snakes consider smaller snakes a snack, even their own offspring from eggs laid weeks earlier. The cannibalistic king cobra guards her eggs during incubation but leaves them just before they hatch, thus avoiding her own temptation.

Sometimes infanticide is a disguised form of selective culling, as a brood fitness test. When young bass hatch from a spawn, dad is on guard: circling and herding them together, providing protection from would-be predators. After a few days, most of the brood swim away. Paternal behavior flips like a switch. The stragglers are treated as any other small prey: eaten when in the way.

Siblicide – younglings killing each other – is a lethal hazard for herons, boobies, and several raptors. The youngest chick may succumb to the aggressive attentions of older nest mates.

In parasitic wasps, the strongest larva kills its sibling rivals; survival of the fittest grub. The only known instance of siblicide in mammals is the spotted hyena.