The polar bear, Ursus maritimus, is a predator uniquely adapted to the harsh, ice-covered environment of the Arctic. As the apex hunter, its survival depends on hunting seals, primarily ringed and bearded seals, from the sea ice platform. The Arctic demands immense fat reserves and energy expenditure from this massive marine mammal. This struggle for survival can occasionally lead to surprising and unsettling behaviors within the species.
Defining Intraspecific Predation in Polar Bears
Scientists refer to one polar bear consuming another as intraspecific predation or consumption. This term acknowledges that the consumption occurs between members of the same species, regardless of the primary motivation for the killing. The behavior is a documented part of the species’ ecology, where the victim is typically a smaller, more vulnerable individual. In nearly all observed cases of active killing, the perpetrator is a large adult male, and the targets are most often cubs-of-the-year, yearlings, or subadults.
Active predation involves a dominant bear hunting, killing, and consuming a conspecific, which is a rare but recorded event. A more common form of consumption is opportunistic scavenging, where a bear feeds on the carcass of another that died from injury or starvation. This carrion feeding is a practical way to acquire calories, as bears are effective terrestrial scavengers. Instances of intraspecific killing have also included adult females, demonstrating that vulnerability, not just size, can determine the victim.
Ecological and Nutritional Triggers for the Behavior
The primary driver for intraspecific predation is severe nutritional stress, linked to changes in the Arctic environment. Polar bears rely on sea ice to hunt seals, but climate warming has lengthened the ice-free season in many regions. This reduction in hunting time leads to prolonged fasting periods, forcing bears to subsist on stored fat reserves. Extended fasting causes a decline in the bear’s body condition, pushing the animal into a state where the reward of a high-calorie meal outweighs the risk of attacking a conspecific.
Hunting success for seals is declining in some areas, forcing bears, particularly thin adult males, to seek alternative, high-risk food sources. A secondary, non-nutritional motivation for infanticide by males is sexually selected infanticide, observed in other bear species. By killing a female’s cub, the male can induce the female to re-enter estrus, providing a reproductive advantage. This infanticide is often accompanied by consumption of the carcass.
The energy demands of polar bears make the caloric density of prey paramount for survival. A conspecific, though risky to acquire, offers substantial fat and protein when other food is unavailable. Predation instances are frequently observed during late summer and autumn when bears have been fasting the longest and are nutritionally depleted. This behavioral shift is a survival tactic, responding to a lack of traditional resources, rather than a normal feature of their social structure.
Frequency of Occurrence and Documentation
Intraspecific killing and consumption are generally infrequent events in the overall ecology of the polar bear, though more common than in most other bear species. Reports of the behavior have long existed through the traditional knowledge of Inuit hunters. Scientific documentation has traditionally been opportunistic, relying on chance observations from researchers in the field.
A systematic review documented 107 events for polar bears, representing over half of all recorded instances across all bear species. These events were most frequently associated with infanticide and conspecific strife, both often perpetrated by males. Active predation events are rare; researchers who have spent decades studying bears in specific regions have only confirmed a handful of cases.
Scientists use several methods to document these incidents. These include tracking footprints to reconstruct the sequence of events, and using necropsies and teeth collection to determine the age of the victim and perpetrator. The analysis of parasites, such as Trichinella infection, also provides indirect evidence that bears consume conspecifics as carrion. While intraspecific predation is part of the bear’s behavioral repertoire, its increased observation in recent years is interpreted as a symptom of environmental pressure, particularly rapid sea ice loss. This suggests that the frequency of the behavior is highly sensitive to the availability of their primary seal prey.