Infanticide, the intentional killing of young offspring by a mature animal of the same species, is a widespread phenomenon observed across the animal kingdom. This behavior is documented in a diverse array of species, including mammals, insects, fish, birds, and amphibians. It is understood not as a pathology but as an evolved, adaptive strategy that increases the killer’s lifetime reproductive success or the survival prospects of related kin. The drivers behind this lethal behavior fall into distinct categories related to sexual competition, resource management, or environmental stress.
Reproductive Strategy: Infanticide by New Dominant Males
The most widely studied and documented form of infanticide is performed by males unrelated to the victims, a strategy driven by intense sexual competition. This behavior primarily occurs in species where a few males monopolize breeding access to a group of females, and a new male or coalition of males takes over the social unit. By eliminating the existing dependent young, the male is able to hasten the female’s return to reproductive availability, thereby advancing his own opportunity to pass on his genes.
This strategy is highly effective because, in many mammalian species, a female does not resume ovulation as long as she is nursing a dependent infant, a period known as lactational amenorrhea. The death of the young prematurely ends this period, causing a hormonal shift that makes the female receptive to mating much sooner than if she had raised the infant to independence. The selective pressure for this behavior is immense, especially for males whose tenure as a dominant breeder may be relatively short.
Lion prides provide a classic example, where a new coalition of males taking over a pride systematically targets cubs under approximately nine months old. These cubs were fathered by the previous male coalition, making them direct genetic competition for the usurpers. Up to a quarter of all lion cub deaths are estimated to be due to this male-driven infanticide.
A similar strategy is observed in primates, such as Hanuman langurs, where DNA analyses confirm the attacking males are unrelated to the infants they kill. The infanticidal male is often the father of the female’s subsequent offspring, demonstrating the tactic’s reproductive advantage. Females attempt to counteract this threat by aggressively defending their young or through heightened sexual activity with the new male to confuse paternity.
Environmental Pressures and Parental Culling
In contrast to sexually motivated killings, a parent may commit infanticide against its own offspring, known as filial infanticide. This is a resource management strategy designed to maximize the success of the remaining young. The parent essentially “cuts its losses” when faced with immediate environmental or physical constraints.
Parental culling is often triggered by resource scarcity, such as famine or drought, making it impossible to sustain an entire litter or brood. For example, white storks practice “brood reduction” by killing or ejecting the weakest chick when food is limited. By sacrificing the least viable individual, the parent reallocates limited food and energy to ensure the survival and health of the stronger offspring.
Rodents frequently exhibit maternal infanticide, particularly when a mother gives birth to a litter that is too large or is under intense stress. A mother mouse or pig may consume her young to recoup the energy and nutrients invested in the pregnancy, preserving her own body condition for future, more favorable breeding opportunities.
Lionesses, who fiercely defend their cubs from outside males, may still abandon or reject a cub if it is weak, ill, or if she only has a single cub. Female lions increase their lifetime reproductive success by abandoning a single cub to conserve energy for a larger, more viable litter in the future.
Stress, Inexperience, and Accidental Mortality
While much infanticide is an adaptive evolutionary strategy, some instances are maladaptive, resulting from high stress, inexperience, or accidents. These cases represent a breakdown in typical parental behavior rather than an intentional culling. High levels of environmental stress can disrupt the hormonal and neural pathways that govern parental care.
In controlled environments like zoos or laboratories, stress from crowding, noise, or human interference can trigger infanticide in species like giant otters and polar bears. This stress causes the parent to exhibit aggressive or neglectful behavior toward the infant. First-time mothers may also display inexperience, failing to properly clean or nurse their young, or mishandling them in ways that lead to fatal injury.
Sometimes, the death is purely accidental, particularly in large mammals with poor spatial awareness. A sow may inadvertently crush a piglet, or a large herbivore may step on its newborn in the nesting area. A mother may also accidentally cause fatal injuries while attempting to move her young, such as a tigress damaging a cub’s windpipe or skull while carrying it to safety.