Infanticide, the intentional killing of dependent young by an adult member of the same species, is a widespread phenomenon observed in over 50 primate species, including Hanuman langurs, chimpanzees, gorillas, and baboons. This behavior represents a severe form of sexual conflict and is overwhelmingly perpetrated by males against infants they did not sire. The systematic nature of these attacks, which target vulnerable, unweaned infants, suggests the behavior follows a specific evolutionary logic driven by reproductive advantage.
The Reproductive Acceleration Hypothesis
The primary evolutionary driver for male infanticide is the sexual selection hypothesis, which argues that males act to maximize their reproductive fitness. A female who is nursing an infant is typically in a state of lactational amenorrhea, meaning hormonal signals associated with milk production suppress her ovulation and prevent her from becoming pregnant again. By terminating the life of the infant, the male effectively terminates the female’s reproductive investment in a rival male’s offspring.
The cessation of lactation causes the female’s reproductive hormones to shift, quickly bringing her back into estrus and making her sexually receptive much sooner. This shortens the female’s interbirth interval, allowing the infanticidal male to mate with her and sire his own offspring in a condensed timeframe. Since a male’s tenure as a dominant breeder can be limited, this strategy ensures he redirects the mother’s future reproductive effort toward his own lineage.
Social Context and Environmental Triggers
Infanticide is highly concentrated around specific social disruptions within a group. The most common trigger is the arrival of a new adult male who has recently taken over a social group or ascended to the dominant position. Since the new male is highly unlikely to have sired any of the group’s existing infants, those unweaned young become the primary targets.
The former dominant male was typically a protector of the infants, but his ousting leaves the young vulnerable. The infanticidal male must be confident that the infant is not his own, as mistakenly killing his offspring would severely reduce his fitness. While the reproductive motive is the main driver, external environmental factors like high population density or resource scarcity can exacerbate these tense social situations. These pressures intensify competition for mates, making the time-saving strategy of infanticide even more advantageous for the new male.
Female Counter-Strategies Against Infanticide
In response to this severe reproductive threat, female primates have evolved behavioral adaptations to combat or confuse infanticidal males.
Paternity Confusion
One widespread strategy is paternity confusion, where females mate promiscuously with multiple males, including the new dominant male, during their fertile periods. By mating with several potential sires, the female obscures the true paternity of her infant, which makes any male less likely to commit infanticide for fear of inadvertently killing his own child.
Protector Males
Females also form social alliances with specific males who act as “protector males.” These males associate closely with the mother and infant, providing a physical defense against other, potentially infanticidal males.
Pseudo-Estrus
In some species, females will also engage in exaggerated sexual signaling or “pseudo-estrus” while pregnant or lactating. This behavior confuses the male about the female’s reproductive status, further complicating his ability to determine which infants are worth attacking.
Alternative Hypotheses and Misinterpretations
While the reproductive acceleration hypothesis is the most supported explanation, other ideas have been proposed. The resource competition hypothesis suggests that males kill infants to reduce the overall number of competitors for food, water, or space. However, the reproductive gain of siring new offspring far outweighs the small energetic benefit of eliminating a single infant competitor.
A less-supported view is that infanticide is pathological behavior, caused by high stress, illness, or overcrowding. While stress can certainly increase aggression, the highly specific context of infanticide—targeting only unrelated, unweaned young—strongly suggests an adaptive, strategic basis rather than random violence. The idea that infanticide serves as a form of population control is also discounted, as the act is a selfish strategy that benefits the individual male’s genes, not the overall welfare of the group.