Why do mother bears kill cubs? The answer is complicated.
Popular Science...
We give them names like Otis, Chunk, and Diver. We gamify their annual salmon run (and indulge in some friendly competition ourselves) as we watch them gorge on sockeye salmon during what’s affectionately known as Fat Bear Week. We fall in love with the spring cubs, otherwise known as cubs of the year (COYs), delighting in their silly antics as we watch them grow and learn.
From our safe remove in front of our laptops or phones, it’s sometimes easy to forget that the roly-poly brown bears we’re watching on the Brooks River livecams, a beloved livestream stationed at the Brooks River inside Alaska’s Katmai National Park, aren’t animated characters on a kids’ TV show. But then something happens to remind us that what we are really watching is nature, at its most untamed, uncensored, and—at times—most cruel.
Over the past few weeks, two different mother bears at Brooks River have killed cubs that weren’t their own. Park naturalists believe these mark the first documented cases of female-perpetrated infanticide at Brooks River.
“These situations are challenging, often difficult, and frequently confusing,” Mike Fitz, resident naturalist for Explore.org, the nonprofit that runs Katmai’s live bear cams, said during a livestream discussion park rangers led after the first incident. “We love to celebrate the success of bears with full stomachs and ample body fat. But their ferocity is real, the risks they face are real.”
Even the experts who watch these bears every day admit they’re confused. So what would drive mother bears to kill another’s cub?
Unwanted adoptions might fuel mother bears to attack cubs
Although experts don’t have a definitive answer for why these back-to-back infanticides happened, plain aggression doesn’t appear to be the cause. According to Dr. Stephen Stringham, a bear researcher and author whose recent work has focused on bear infanticides, it comes down to something stranger: adoption in the animal kingdom.
Stringham says in cases of bear infanticide, the killings often follow a pattern: cubs from two different mothers end up mingling—playing, wrestling, and otherwise producing some of the adorable footage we watch on livestreams. In doing so, they also start to pick up each other’s scent. If this mingling continues, an unrelated cub can end up nursing, seeking protection, or otherwise drawing on resources meant for the mother bear’s own cubs.
“They can’t tell kids, ‘Go home, go to your mother, I’m tired of you,’” Stringham says. “[These unrelated cubs] are competing with her cubs for her milk and for her attention. It’s not to her advantage as a general rule to allow that to happen.”
Something close to this pattern occurred in the first of the two Brooks River incidents. Bear 806, a mother already caring for two cubs of her own, had allowed an unrelated spring cub to play alongside them. Then she chased the cub down and killed it.
Making it stranger still is the fact that just months earlier, Bear 806 had taken in a cub that wasn’t biologically hers: a cub known as Biggie, who’d been abandoned in the spring by his mother. The same maternal instinct that led her to adopt one unrelated cub didn’t extend to another.
“It’s not fully known,” Stringham says, of what actually tips the balance between adoption and infanticide, triggering a bear to attack a cub she had previously tolerated.
The second incident, involving an unidentified mother bear attacker, occurred away from the cameras, downstream from where the park’s cams are stationed. However, the infanticide was documented by a park visitor and confirmed by rangers. Jolly, a six-year-old bear known to rangers, was identified as the victim’s mother. She was later seen in the area where her cub had been killed, appearing to search for the cub.
Dominance may play a role in infanticides
Stringham’s research offers another clue into these infanticides: the possibility that the relationship between the victim cub’s mother and the attacker may play a role.
“In the cases where we’ve been able to distinguish there was a difference in rank, it’s always been [the bear] that was acting in a more dominant fashion,” Stringham says, meaning that the more dominant bear is often the one that ends up killing the other mother’s cub. “I’m not talking about there being a long history of Bear A always dominating Bear B, but that there’s been a general tendency in that direction.” Whether or not this was the case in either of the Brooks River incidents has not been confirmed.
Food scarcity might lead to similar bear/cub attacks
Stringham added that competition over food is unlikely to have been a driver in these specific attacks, but could potentially fuel other mother bears to kill another mother’s cubs. During times of scarcity, mother bears might become increasingly intolerant of adopted cubs intermingling with their own.
“When you’ve got several different litters out there simultaneously, there’s a much higher chance that cubs are going to intermingle with each other and start smelling like each other,” he says. “And if there’s lots of fish, the females may tolerate that. But when they choose to leave the falls, and certainly when fish get scarce, things might get a little more dicey.” When the food supply dwindles, mother bears look out for their own.
Nevertheless, Stringham is not certain whether that was the case in these specific incidents.
“That would be pure speculation,” he says.
When something runs from a bear, predator instinct kicks in
Rangers did observe one other factor in the case of Bear 806: the cub ran when it saw the mother bear, which may have caused the situation to suddenly escalate into a predatory chase behavior. This echoes the advice rangers give to humans: Don’t run from a bear. Though Stringham notes, cubs run constantly during play without triggering an attack from their own mothers.
Bears, however, don’t operate on human logic, which is why Katmai rangers are quick to remind livestream viewers not to judge these animals using our own morality.
“We shouldn’t villainize a bear because they killed another bear. We shouldn’t blame a bear because she wasn’t able to protect her cub,” Christine Loberg, a Katmai ranger, said during the discussion.
For Katmai park ranger Sarah Bruce, the upsetting event is a useful reminder to the rest of us: that even from our comfortable vantage points in front of our screens, we’re not as detached from the harsh realities of nature as we might think.
“I constantly have to remind myself that I too am a part of this wild planet,” she said. “This happening on bear cam is not separate from me also living on this earth. This is just kind of what it means to kind of be alive.”
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