Bears have been known to turn to cannibalism as a food resource. But in a recently released review, an international team of researchers write that many described documented instances are acts of opportunism, not aggression.
Their review, published July 13 in Ursus, covers 39 studies recounting 198 occurrences of cannibalism in bears and explores the phenomenon in four species in particular: polar bears, American black bears, Asiatic black bears, and brown bears.
Across the board, infanticide (which the authors define as consumption of unrelated youth) and conspecific strife (consumption of an adult) are the most common forms of cannibalism and are mostly committed by males, according to the review. Filicide (consumption of one’s own dependent young) was less common, and siblicide (consumption of a sibling) was not documented at all.
The review authors write that it’s important to distinguish between predation-fueled cannibalism and scavenging. Researchers have hypothesized that when it comes to predation-fueled cannibalism, male bears consume other bears to increase their fitness by taking in more calories, and/or to reduce their competition for resources and mating. However, review coauthor Miha Krofel, a wildlife research fellow at the University of Ljubljana in Slovenia, says that while bears have been observed killing other bears to eat them, this situation seems to be more of an exception than the rule. Instead, the review finds that cannibalism most often appears to be a result of bears consuming an already dead bear, and that both sexes engage in this type of scavenging.
“Often the public would see a bear eating another bear as gruesome or immoral in a way, but bears are just out there living their lives, trying to survive,” Krofel says. “So, if there’s a carcass that’s available, they just take the opportunity to eat it.”
He and his coauthors found that in polar bears and brown bears, most instances of cannibalism are infanticide, while most cases by American black bears are conspecific strife or infanticide, and all cannibalism events reported in Asiatic black bears are conspecific strife. Krofel says this could be partly related to ingrained differences among those species, or it may be a product of how data were collected.
“Some species are much more visible than the others, and of course there’ll be higher chances that cannibalism is recorded, or just the fact that some species are really understudied,” he says.
For instance, more than half of the total reported occurrences of cannibalism were in polar bears, which are the most carnivorous and predatory bear species—but also the easiest species to spot while hunting due to their open environment.
Andrew Derocher, a biologist at the University of Alberta in Canada who is not involved in the review, says cannibalism in brown bears likely occurs less frequently than in polar bears due to aggressive mothers deterring predators and cubs having access to trees to climb.
Derocher, who has studied both polar and brown bears for 40 years, says he has seen indications that cannibalism in polar bears may even increase because a decline in sea ice prevents them from hunting seals—an idea the review also supports.
“Nutritionally stressed bears are expected to show increased cannibalistic behavior,” Derocher said. “As sea ice melts earlier and forms later across the Arctic, polar bears are onshore for longer and some bears will deplete their fat stores and become nutritionally stressed. A hungry bear will respond differently to other bears, and if their survival is threatened, they may resort to cannibalism to increase their own likelihood of surviving.”
Moving forward, the review authors call for more research on four less-studied bear species—sloth bears, giant pandas, spectacled bears, and sun bears—and for more documented evidence of cannibalism in all bears.
“There are many bear species out there. And many of them, we don’t know much [about],” Krofel says. “All these tropical species are really understudied and definitely need more attention, both from research as well as conservation.”