Grizzly bears, powerful symbols of the wild, are known to engage in deadly conflict with one another, which is a significant source of natural mortality within their populations. These intraspecific killings are a recognized part of bear ecology, though they are often difficult for researchers to document fully in remote habitats where carcasses are quickly scavenged. The aggressive encounters are a natural consequence of the species’ solitary nature and evolutionary pressures related to survival and reproduction.
Intraspecific Mortality: The Scientific Reality
Intraspecific killing, the death of an animal by a member of its own species, is a documented phenomenon in grizzly bear populations. While human-related causes, such as hunting, control kills, and vehicle collisions, frequently account for the majority of known adult grizzly bear deaths in many regions, aggression between bears remains a consistent natural factor. Natural mortality rates for adult females have been shown to be higher than for males in some studies, with a portion of these deaths attributed to conspecifics. For younger bears, intraspecific aggression is a more frequent cause of natural mortality compared to adults, alongside accidents and nutrition-related issues.
Primary Motivations for Fatal Aggression
Fatal aggression between adult grizzly bears is driven by two main ecological factors: reproductive competition and the defense of valuable resources. During the breeding season in late spring and early summer, large adult males engage in intense, often deadly, fights for access to receptive females. These dominance battles are a form of sexual selection, ensuring the largest and most powerful males secure mating opportunities. Conflicts also arise over concentrated, high-value food sources, such as large animal carcasses or dense patches of salmon streams. Adult males often dominate these prime feeding locations and will aggressively exclude or kill rivals to protect their energy intake.
Dynamics of Infanticide: Victims and Aggressors
The most commonly observed and lethal form of intraspecific killing in grizzlies is infanticide, the killing of young by an adult bear. Adult males are the primary aggressors, with cubs and sub-adults being the most frequent victims. This behavior is often viewed through the lens of sexually selected infanticide (SSI), a reproductive strategy where the male aims to bring the female back into estrus sooner. Female grizzly bears typically care for their young for two to three years, during which they cannot become pregnant again. By killing the cubs, the male shortens the female’s reproductive cycle, providing an earlier opportunity to mate and sire his own offspring.
To avoid this threat, females with cubs often exhibit avoidance strategies, such as selecting den sites at higher elevations or avoiding habitats frequented by large males, such as salmon streams. Despite these efforts, studies suggest that approximately 80% of infanticidal attacks in brown bears are perpetrated by males. This intense threat is a major reason mother bears are highly protective and aggressive toward any intruding adult male.