Polar bears sometimes engage in the killing and consumption of their own species, particularly cubs. While this behavior is shocking, it results from a complex interplay of natural evolutionary pressures and rapidly changing environmental conditions in the Arctic. Understanding this intraspecific killing, known as infanticide, requires separating the baseline biological motive from the factors making these incidents more visible today. This rare but documented behavior is driven by survival and reproductive imperatives.
The Biological Context of Infanticide
The primary biological driver for killing cubs is a reproductive strategy employed by large adult male polar bears. Female polar bears will not enter estrus, or breeding condition, during the two to three years they spend nursing their young. This extended maternal care creates a bottleneck for males seeking to pass on their genes.
For a male, eliminating a dependent cub accelerates the mother’s reproductive cycle, forcing her to become available for mating sooner. This behavior, known as sexually selected infanticide, is a form of reproductive competition common in many large mammal species with extended maternal care. The mother bear’s body quickly returns to a hormonal state ready for breeding, often within weeks after the loss of her litter.
In rare cases, a female might kill a cub, but this is usually linked to severe resource deprivation. This contrasts clearly with the male’s mating-driven motive.
Climate Change and Nutritional Stress
While infanticide has a natural biological basis, the increasing visibility of intraspecific predation and cannibalism is linked to the effects of a warming Arctic. Polar bears rely on sea ice to hunt their primary prey, ringed seals, whose blubber provides the high-calorie energy needed to survive long fasting periods. As climate change causes the sea ice to melt earlier and freeze later, bears are forced to spend longer periods fasting on land.
These prolonged fasting periods lead to extreme nutritional deficits and an energetic crisis. A bear under severe nutritional stress is more likely to view another bear, particularly a vulnerable cub, as an opportunistic caloric resource. The resulting starvation increases the likelihood of a desperate bear resorting to any available food source.
This pattern is evident in areas like the Southern Beaufort Sea and Western Hudson Bay, where declining sea ice stability has caused a drop in polar bear body condition. Instances of cannibalism often involve visibly thin adult males, suggesting a motivation of nutritional need rather than reproductive advantage.
Distinguishing Cannibalism from Predation
Researchers must distinguish between infanticide (the killing of a cub) and cannibalism (the consumption of the carcass). Infanticide can be driven purely by the reproductive motive, where the male kills the cub and leaves the body behind. Cannibalism suggests a nutritional motive, regardless of whether the bear killed the cub or was scavenging.
Direct observations of these events are rare in the Arctic, making it difficult to determine the exact motive in every case. A bear feeding on a cub may have preyed upon it or found a cub that died from starvation or natural causes. Distinguishing between predation-fueled infanticide and simple scavenging remains a significant challenge for researchers.
While the biological imperative for infanticide has always existed, the increase in observed cannibalism strongly suggests a link to the intense energetic crisis caused by habitat loss. This behavior is becoming a more common survival strategy.