Infanticide occurs across a remarkably wide range of the animal kingdom, from lions and primates to fish and beetles, and the reasons behind it are just as varied. Far from being a rare aberration, killing young is a documented reproductive strategy in hundreds of species. The motivations break down into a handful of recurring patterns: gaining mating access, eliminating competition for resources, adjusting brood size to match food supply, and even restarting a stalled reproductive cycle. What looks like senseless cruelty through a human lens turns out, in most cases, to follow a grim but consistent evolutionary logic.
When a New Male Takes Over
One of the most well-known forms of animal infanticide happens when a new male displaces a resident male and kills the existing young. Lions are the textbook example. When a coalition of males ousts the previous pride males, the newcomers routinely kill cubs sired by their predecessors. The logic is straightforward from an evolutionary standpoint: a female nursing cubs is not fertile. By killing the cubs, the new male accelerates the mother’s return to estrus, giving him the chance to sire his own offspring during his limited tenure as pride male.
This pattern is not unique to lions. It shows up in langur monkeys, gorillas, bears, dolphins, and a range of other mammals where male turnover is common. The unifying feature is that dependent offspring delay the mother’s next reproductive cycle, so a male who is not the father has a reproductive incentive to eliminate them. Researchers have spent decades debating whether this behavior is truly adaptive or simply a byproduct of male aggression, but the weight of evidence across species strongly supports the adaptive explanation: males who commit infanticide leave more descendants than those who do not, at least under conditions of frequent male turnover.
Females That Kill Other Females’ Offspring
Infanticide is not exclusively a male behavior. In many mammal species, females kill the young of other females, and the reasons center on direct competition for resources. A large comparative study of female infanticide across mammals found that it occurs where the proximity of another female’s offspring threatens the killer’s own reproductive success by limiting access to food, shelter, parental care, or social position.1PubMed Central. The evolution of infanticide by females in mammals Interestingly, this happens even among closely related females. The fitness rewards of securing resources for one’s own young apparently outweigh the costs of killing a relative’s offspring.
A striking example comes from dingoes. In a captive dingo pack, all females became pregnant, but the alpha female killed all the pups born to every other female in the group, despite being closely related to them.2Ethology. Social Dynamics of a Captive Dingo Pack: Population Regulation by Dominant Female Infanticide This kind of reproductive suppression through infanticide ensures that the dominant female’s pups get the lion’s share of the group’s resources. Similar dynamics show up in meerkats, wild dogs, and some primates, where dominant females actively prevent subordinates from successfully raising young.
Parents Killing Their Own Young
Perhaps the most counterintuitive form of infanticide is when parents kill their own offspring. Yet it happens routinely in birds and fish, and the reasons tend to fall into two categories: adjusting brood size to match available resources, and cutting losses on a reproductive attempt that is not going well.
In birds, parental infanticide serves as a way to save energy. By eliminating the offspring with the least chance of survival, parents improve the prospects of the remaining young.3European Zoological Journal. Brood reduction This is especially common in larger species like storks, eagles, and pelicans, where raising a full brood demands enormous energy. The victims are nearly always the youngest, weakest, or smallest in the brood, and the behavior is typically triggered by environmental stress such as food shortages during the breeding season.
Fish take a different approach. In species where males guard nests of eggs, fathers sometimes eat their entire clutch in a behavior called filial cannibalism. In one remarkable case, researchers found that male blenniid fish eat their own eggs not because they are hungry but because egg presence in the nest suppresses the hormones they need to court new mates. By consuming or removing all the eggs, a male can restart his courtship behavior and attract a new female.4PubMed. Filial Cannibalism by Male Fish as an Infanticide to Restart Courtship by Self-Regulating Androgen Levels Males sometimes even spat eggs out without eating them, suggesting the goal is clearing the nest rather than gaining nutrition.
Paternity Doubt and Egg Rejection
Males across species are more willing to invest in offspring they are confident are their own, and when that confidence drops, infanticide can follow. In three-spined sticklebacks, males guard and fan eggs laid in their nests by visiting females, but when a higher proportion of eggs in the clutch were fathered by other males, the guarding male was more likely to eat the entire clutch. Males could assess their paternity using cues from the eggs alone, and clutches with more foreign eggs were especially likely to be cannibalized early in the breeding season, when the male still had time to start over.5PubMed Central. To eat or not to eat: egg-based assessment of paternity triggers fine-tuned decisions about filial cannibalism
Birds show parallel behavior. In an experiment with blue-footed boobies, researchers manipulated mating situations so that some males had reason to doubt their paternity. About 44% of males in the experimental group expelled their partner’s first egg from the nest without incubating it, while none of the control males did so. The infanticidal males acted within minutes of their first incubation shift.6Behavioral Ecology. Male boobies expel eggs when paternity is in doubt The speed and decisiveness of the response suggests that males are primed to respond to paternity cues and that egg rejection is a deliberate strategy rather than random aggression.
Matching Brood Size to Resources
Some species overproduce offspring deliberately, then cull the extras to match what the environment can support. Burying beetles are one of the clearest examples. These insects lay their eggs on small animal carcasses, which serve as food for the developing larvae. The beetle Nicrophorus vespilloides typically lays more eggs than the carcass can support, then kills first-instar larvae through partial filial cannibalism during brood care.7PubMed Central. Experimental evolution of a more restrained clutch size when filial cannibalism is prevented in burying beetles Nicrophorus vespilloides When researchers prevented this culling behavior over multiple generations, the beetles evolved to lay fewer eggs in the first place, confirming that overproduction-plus-cannibalism is a deliberate reproductive strategy rather than a mistake.
The flexibility here is key. On larger carcasses, where more food is available, females from both experimental and control populations laid similar numbers of eggs. It was only on small carcasses, where resources were tight, that the difference in egg-laying strategy appeared. The beetles are, in effect, using infanticide as a fine-tuning mechanism to match offspring number to food supply in real time.
Sibling-on-Sibling Killing
Not all infanticide comes from parents or unrelated adults. In many bird species, siblings do the killing. Obligate siblicide, where one chick always kills its nestmate, occurs in species like the masked booby and some eagles. The second egg functions as an insurance policy: if the first egg fails to hatch or the first chick dies, the second chick survives. But if both hatch, the older, larger chick almost invariably kills the younger one within days. Parents rarely intervene.
Facultative siblicide is more common and less predictable. In raptors like black kites and in many heron species, the older chick may or may not kill its siblings depending on food availability. When prey is abundant, all chicks can survive. When food is scarce, the size advantage of the first-hatched chick, which got a head start because eggs hatch asynchronously, becomes lethal. The older chick monopolizes food deliveries and may peck its siblings to death or push them from the nest. From the parents’ perspective, hatching eggs at intervals is itself a strategy that facilitates brood reduction when conditions demand it, without requiring the parents to make the kill themselves.
How Females Defend Against Infanticidal Males
Given how widespread male infanticide is, females across many species have evolved counter-strategies. These span a wide range of approaches, from sexual behavior to physiology to social alliances.8PubMed Central. Infanticide as sexual conflict: coevolution of male strategies and female counterstrategies
One of the most common is mating with multiple males. If a female mates with several males, each one has some probability of being the father, which reduces the incentive for any individual male to kill her offspring. This pattern shows up clearly in primates, rodents, and carnivores. In some species, females actively seek extra-pair matings specifically around the time of male group takeovers, apparently to confuse paternity and protect existing or future young.
Females also employ direct aggression. Mothers in many primate species fiercely defend their infants against infanticidal males, sometimes forming coalitions with other females to drive off attackers. In species with strong female social bonds, like bonobos, male infanticide is virtually nonexistent, partly because females collectively outpower individual males.
The Bruce Effect
One of the more dramatic physiological counter-strategies is spontaneous pregnancy termination, known as the Bruce effect. When a new male appears and the old male disappears, pregnant females in some species abort their fetuses. This was first documented in lab mice, but it also occurs in the wild. In gelada monkeys, females terminated roughly 80% of pregnancies in the weeks following the replacement of a dominant male.9PubMed. A Bruce effect in wild geladas Data on interbirth intervals suggested this offered real fitness benefits: females who aborted and then mated with the new male avoided losing an infant to infanticide and produced their next offspring sooner than they otherwise would have.
The logic is bleak but consistent. If a new male is likely to kill any infant he did not sire, a pregnant female saves months of gestation and nursing by aborting early and starting over with the new male’s offspring. The energy savings from ending a doomed pregnancy early can be substantial, especially for species with long gestation periods.10PubMed Central. The Bruce effect revisited: is pregnancy termination in female rodents an adaptation to ensure breeding success after male turnover in low densities?
The Hormonal Machinery Behind Infanticide
Infanticide is not simply a learned or situational behavior. It has hormonal underpinnings that researchers are still working to understand. In rodents, the hormone progesterone plays a documented role in triggering aggression toward pups. In one experiment, male mice that had not previously cared for pups were treated with progesterone and then exposed to young. About 78% of progesterone-treated males attacked pups, compared to roughly 27% of untreated males.11PubMed Central. Effects of Progesterone on Male-Mediated Infant-Directed Aggression The finding is surprising because progesterone is more commonly associated with maternal behavior in females, but in males that have not been through the experience of fatherhood, it appears to promote aggression toward young instead.
In fish, the hormonal picture is different but equally telling. Male blenniid fish that are guarding eggs experience suppressed androgen levels, which prevents them from courting new females. Filial cannibalism functions as a way to remove the stimulus (the eggs themselves) that keeps androgens low, allowing courtship hormones to rise again.4PubMed. Filial Cannibalism by Male Fish as an Infanticide to Restart Courtship by Self-Regulating Androgen Levels The takeaway is that infanticidal behavior across species is not a single phenomenon driven by a single mechanism. Different hormonal systems mediate the behavior depending on the species and the context, and the same hormone can promote parental care or infanticide depending on the animal’s sex, reproductive history, and social circumstances.
When the Environment Pushes Animals to Extremes
Environmental stress can trigger infanticide and cannibalism even in species where it is not a routine reproductive strategy. Polar bears provide a stark example. A documented case from Svalbard involved an adult male polar bear in poor body condition killing and eating a seven-month-old cub near its mother, who was also in poor condition. Researchers concluded the attack was likely motivated by nutritional need rather than any reproductive strategy.12Polar Record. An incident of polar bear infanticide and cannibalism on Phippsøya, Svalbard As Arctic sea ice declines and polar bears spend longer periods on land with limited access to their primary prey, incidents like this may become more common.
Starvation-driven infanticide is conceptually different from the reproductive strategies described earlier in this article. A starving animal killing a conspecific’s young for food is behaving more like a predator than a strategist. But the line between the two categories can blur. A male bear that kills a cub may gain both a meal and eventual mating access to the mother, and teasing apart nutritional motives from reproductive ones in the field is difficult. The same ambiguity shows up in primates and rodents, where stressed or resource-deprived individuals sometimes kill young that they would tolerate under better conditions.
Brood Parasitism and Killing Across Species Lines
Most infanticide discussed so far happens within a species, but some animals kill the young of other species as a reproductive strategy. Brood parasites like cuckoos and cowbirds lay their eggs in the nests of other bird species, and in many cuckoo species, the parasitic chick hatches first and physically ejects the host’s eggs or chicks from the nest. The host parents then devote all their care to the imposter. This is, functionally, interspecific infanticide carried out by a newborn.
The host species are not passive in this arms race. Many have evolved the ability to recognize and reject foreign eggs, which in turn has driven brood parasites to evolve increasingly convincing egg mimicry. Some host species have abandoned egg recognition entirely and instead mob adult cuckoos near their nests. The coevolutionary dynamic between parasites and hosts is one of the longest-running and most intricate in biology, with each side continually adapting to the other’s latest counter-move.
Why Infanticide Persists
A common question is why natural selection has not simply eliminated infanticide. The answer is that for every individual who loses offspring to infanticide, there is usually another individual who benefits reproductively. A male lion who kills unrelated cubs and sires his own leaves more descendants. A mother stork who sacrifices her weakest chick raises the rest more successfully. A female dingo who kills a rival’s pups secures more food for her own litter. The genes underlying these behaviors persist because they confer a reproductive advantage to the individual performing them, even though they impose severe costs on the victims.
The arms-race dynamic between infanticidal strategies and counter-strategies also keeps the behavior in evolutionary play. As males evolve more effective infanticidal tendencies, females evolve better defenses, from paternity confusion through multiple mating to pregnancy termination to coalitionary defense. Neither side wins permanently, and the result is an ongoing evolutionary tension that shapes mating systems, social structures, and parental behavior across the animal kingdom. Researchers studying wild gelada monkeys, for instance, found that pregnancy termination rates tracked male replacement events closely enough to suggest the two are locked in a tight coevolutionary cycle.9PubMed. A Bruce effect in wild geladas The fact that such costly female responses have evolved and persisted is itself strong evidence for how powerful a selective force infanticide has been over evolutionary time.