Grizzly bears (Ursus arctos horribilis) are powerful apex predators and symbols of wilderness. While known for their fierce protectiveness, a complex behavior exists within their social structure: the killing of cubs by adult males. This behavior, though brutal, is a natural phenomenon rooted in reproductive competition.
Infanticide: The Confirmed Reality
Decades of wildlife research confirm that male grizzlies kill cubs. This act, known as infanticide, is the killing of dependent young by another member of the same species (conspecific). Infanticide is well-documented in brown bear populations, including the North American grizzly bear subspecies. The males responsible are typically unrelated to the cubs they target, which is a key characteristic of this behavior.
Infanticide is a significant cause of cub mortality, especially in populations with high densities of adult males. Biologists have recorded instances where large male bears, or boars, attack litters, sometimes killing all of a female’s offspring. Evidence strongly suggests the primary motivation is reproductive, not nutritional, ruling out confusion with predation for food.
The Evolutionary Basis for the Behavior
The underlying reason for male infanticide is a reproductive strategy known as sexually selected infanticide. This behavior provides the male with an immediate opportunity to breed, maximizing his genetic contribution. A female grizzly bear (sow) nursing cubs will not typically enter estrus (heat) for two to three years. This delay is caused by lactational amenorrhea, where nursing suppresses the reproductive cycle.
Killing the litter eliminates the suckling stimulus, quickly halting the hormonal suppression in the female. The sow may then re-enter estrus within days or weeks, making her available to mate during the current breeding season, which peaks in June. The infanticidal male attempts to mate with the mother, ensuring the next litter carries his genes. This strategy shortens the wait time for a mating opportunity, providing a significant advantage in a polygamous system.
This reproductive motivation is supported by the fact that infanticidal males generally do not kill their own offspring. The ability to induce estrus in a female who would otherwise be unavailable for years is a strong selective pressure favoring this lethal behavior. However, the infanticidal male is not guaranteed to mate with the female, as a more dominant male may intervene. The risk of infanticide is highest during the breeding season (May through early July) when males actively search for receptive females.
Maternal Defense and Cub Survival
Female grizzlies have developed intense defensive strategies to protect their young from infanticidal males. The protective instinct of a sow with cubs is legendary and results in fierce encounters. A primary survival strategy is “home range avoidance,” where the sow guides her family unit away from areas frequented by large males.
This avoidance behavior means sows often choose habitats with less optimal foraging opportunities, trading nutrition for increased safety. Females with cubs have been observed utilizing areas closer to human activity, such as roads or settlements, which adult males tend to avoid. The presence of humans acts as an unintended “human shield” for the sow and her offspring during the high-risk mating season.
Despite the sow’s fierce defense, infanticide remains a major cause of mortality for grizzly bear cubs. Cubs stay with their mother for two to three years, and the mother’s defense is continuous until they are weaned. The constant need to avoid infanticidal males imposes a significant energetic cost on the female, impacting her body condition and future reproductive success.