Monkeys and other primates kill infants primarily because it gives the killer a reproductive advantage. Across 38 well-studied wild primate populations, about 15% of all offspring are killed by males, and in some populations that figure reaches as high as 47%. This isn’t random violence. It follows a clear biological pattern that scientists have studied for over 50 years, and the reasons differ depending on whether the killer is a male or a female.
The Main Reason: Males Kill to Breed Sooner
The most common and best-understood form of infant killing in primates is carried out by males who are not the father. The logic is brutally simple: a nursing infant acts as a natural contraceptive for its mother. While she’s breastfeeding, her body suppresses ovulation, a state called lactational amenorrhea. She won’t become fertile again until her infant is weaned, which in many primate species takes months or even years. By killing the infant, a male removes that contraceptive effect and the mother returns to a fertile state much sooner.
This pattern, known as the sexual selection hypothesis, was first proposed by primatologist Sarah Hrdy in the 1970s after observing Hanuman langurs in India. It was controversial at the time because many scientists assumed the behavior was pathological, caused by overcrowding or stress. But decades of field research have confirmed that infanticide follows a consistent and predictable script. It typically happens when a new male takes over a group, replacing the previous dominant male. The new male targets infants he could not have fathered, the mother resumes cycling, and the new male then mates with her. Long-term studies of langurs in Jodhpur, India, found that infanticide shortened the gap between births by an average of 2.1 months, giving the killer a measurable head start on fathering his own offspring.
Group Structure Changes the Risk
Not all primate societies face the same level of infanticide, and the difference comes down to how groups are organized. In species where a single male controls a group of females (called uni-male societies), infant mortality from male infanticide averages 23%. In multi-male, multi-female groups, the rate drops to about 15%. And in species with complex, multilevel societies where multiple family units travel together, it falls to just 3%.
The pattern makes sense. When one male holds exclusive breeding access, a takeover by a rival creates a stark before-and-after: every nursing infant in the group was almost certainly fathered by the previous male, making them targets. In groups with many breeding males, paternity is less certain. A new male can’t be sure which infants aren’t his, which reduces his incentive to kill indiscriminately. In about a third of studied populations, infanticide accounts for more than half of all infant deaths, making it one of the single largest causes of mortality for young primates.
Females Kill for Different Reasons
Male infanticide gets most of the attention, but females kill infants too, and their motivations are distinct. Female-driven infanticide is most common in species that live in stable groups with rigid social hierarchies, where a female’s rank determines her access to food, shelter, and safety.
In these species, every infant born to a rival female represents a future competitor for your own offspring. Killing another female’s baby can weaken a competing family line and protect your own offspring’s social standing. This is especially true in species where daughters inherit their mother’s rank. Each additional daughter strengthens a matriline’s hold on dominance, so eliminating rivals’ offspring is, in evolutionary terms, a way of securing your family’s position for the next generation. Female infanticide is most frequent in group-living species where breeding competition is intense and resources are limited.
How Females Try to Protect Their Infants
Because infanticide is so costly to mothers, female primates have evolved counter-strategies. One of the most widespread is mating with multiple males. By doing so, a female creates uncertainty about who fathered her infant. If several males in the group have mated with her, each has some reason to believe the infant could be his, and less reason to kill it.
Some researchers initially thought that mating during pregnancy (which some primate females do) might serve a similar purpose: confusing incoming males about paternity so they’d spare the infant. But field observations in langurs showed this doesn’t always work. In at least one documented case, an infanticidal male killed infants even after mating with pregnant females, suggesting the strategy has limits. Pregnant females also didn’t distinguish between the father and non-fathers when mating, which undermines the idea that this behavior evolved specifically as a defense against infanticide.
Other protective strategies include forming close bonds with specific males who will act as bodyguards, staying near the center of the group where infants are harder to isolate, and in some species, physically mobbing an attacking male as a coalition of females.
When It’s Not Strategic at All
Not every case of infant killing fits neatly into an evolutionary framework. Some instances appear to be genuinely non-adaptive, or what researchers have called pathological. These may occur during periods of intense social upheaval, extreme stress, or in situations where normal social structures have broken down.
Habitat loss is making this worse in some populations. Southern pig-tailed macaques living in a mosaic of rainforest and oil palm plantation in Malaysia experience 57% infant mortality. Infants whose mothers spent more time than average in plantation areas were three times less likely to survive, facing threats from predators unfamiliar to them, human-wildlife conflict, agricultural chemicals, and heightened aggression within the group. While not all of these deaths are infanticide, the stress and disruption of degraded habitats can amplify the conditions under which lethal aggression toward infants occurs.
Why Evolution Doesn’t Prevent It
It can seem paradoxical that natural selection would favor killing babies, since those babies carry the species’ genes forward. But evolution doesn’t operate at the level of the species. It operates at the level of the individual. A male who kills an unrelated infant and replaces it with his own gains a direct genetic advantage over males who wait patiently. Over thousands of generations, that advantage compounds, which is why infanticide is not a quirk of one or two species but shows up across dozens of primate species, from langurs and gorillas to howler monkeys and chimpanzees.
The same logic applies to females. A mother who eliminates a rival’s offspring and secures better resources or social standing for her own young will, on average, leave more surviving descendants than one who doesn’t. The behavior persists because, in the cold arithmetic of reproductive success, it works.