Bears killing and sometimes consuming their own young, known as infanticide and cannibalism, is a complex biological behavior driven by evolutionary and ecological pressures. While these actions may appear shocking, they are recognized as a suite of strategies rooted in survival, reproductive competition, and nutritional needs. Understanding this behavior requires analyzing the scientific drivers that shape the reproductive and foraging decisions of these large mammals. Most documented instances are not random acts of aggression but result from pressures to maximize genetic fitness or secure immediate survival.
Male Infanticide and Reproductive Advantage
The most widely studied cause of cub mortality is sexually selected infanticide (SSI), a reproductive strategy employed predominantly by adult male bears. This behavior is common in species with a polygamous mating system, extended maternal care, and significant size differences between the sexes. The male bear, often called a boar, targets cubs that are not his own.
A female bear, or sow, remains with her cubs for one to three years, during which she is inhibited from entering estrus, the period of readiness to mate. This reproductive suppression is linked to nursing. By killing the dependent young, the male prematurely ends the mother’s lactation cycle.
The cessation of nursing forces the female to cycle back into estrus much sooner than she would otherwise, creating an immediate mating opportunity for the male. This process effectively shortens the female’s interbirth interval, allowing her to reproduce again sooner.
For this strategy to be advantageous, the male must not be the father of the cubs he kills and must have a high probability of siring the subsequent litter. This behavior eliminates the genes of a rival male while advancing the infanticidal male’s own reproductive timeline. Females have developed counter-strategies, such as mating with multiple males to confuse paternity or utilizing habitats where males are less likely to be present.
Cannibalism as a Response to Resource Scarcity
Cannibalism, the act of consuming a member of the same species, often follows infanticide but can also occur independently as a direct survival mechanism. Bears are opportunistic omnivores, and in situations of extreme food scarcity, a conspecific carcass represents a concentrated source of high-quality protein and fat. This nutritional response is straightforward.
Cannibalism is sometimes recorded where an unrelated bear kills a cub purely for caloric gain. This is especially true for the most carnivorous bear species, such as the Polar bear, which may view a young conspecific as viable prey when primary food sources are unavailable. Increased cannibalism in Polar bears is linked to nutritional stress from longer ice-free seasons.
Maternal cannibalism, or filicide, is a less common but documented behavior where a mother consumes her own offspring. This is generally considered a form of reproductive triage. The female may consume a stillborn or severely weakened cub to recover lost energy and nutrients. By reabsorbing the caloric investment made in a doomed offspring, the mother conserves resources to improve her own condition or increase the chances of survival for healthier cubs.
Differences Across Bear Species and Habitats
The frequency and motivation behind infanticide and cannibalism vary significantly across the eight global bear species and their diverse habitats. Brown bears, including grizzlies, and Polar bears exhibit the most documented cases of sexually selected infanticide (SSI). In some Scandinavian brown bear populations, up to 80% of cub mortality occurs during the mating season, strongly suggesting SSI as the driving force.
Polar bears account for the majority of documented cannibalism events. Their highly carnivorous diet and increasing environmental stress from climate change contribute to this higher rate, as they are more prone to viewing other bears as potential prey.
American black bears, which are generally smaller and more omnivorous, exhibit lower rates of documented infanticide compared to Brown bears. In some populations, the killing of cubs by males may be driven more by a desire to reduce competition for resources than a reproductive advantage. The availability of alternative food sources, such as salmon runs, can modulate the intensity of these behaviors.
The Influence of Disease and Social Stress
Beyond the primary drivers of reproduction and nutrition, environmental and health factors contribute to cub mortality. A female bear may abandon or cull a cub that is diseased or seriously injured to conserve her own energy and maximize the reproductive output of future litters. This triage behavior ensures that maternal resources are not wasted on an offspring with a low probability of long-term survival.
The stress induced by human presence and habitat fragmentation also increases cub vulnerability. Bears living near human-dominated areas often experience higher levels of physiological stress, indicated by elevated cortisol levels in their hair and feces. This chronic stress can lead to decreased immune function, making them more susceptible to infectious diseases and parasites, which are documented causes of cub mortality.
Increased human development can force bears to live at unnaturally high densities, leading to more frequent and aggressive interactions between individuals. Higher social stress and proximity increase the likelihood of traumatic encounters, including fights between adults that result in the death of a cub.