Why Do Bears Sometimes Kill Each Other’s Cubs?

You’ve received questions; the bear has solutions. Bear Month 2026 is right here, and that signifies that our resident bruin recommendation columnist is as soon as once more fielding your queries about bear biology, security, and extra. Learn on for the most recent, or try the remainder of our Bear Month tales.

Q: Why do bears typically kill one another’s cubs? – N.  Fielding, London, OH

A: Jeepers. I’m normally completely satisfied to behave as a consultant for bearkind, however this can be a subject I actually don’t like to speak about. It’s not one thing we bears are significantly pleased with—and even one thing that occurs that always. I’ve definitely by no means seen it myself. However I’ve heard the tales.

Each as soon as and some time, when cubs are younger—normally only a few months outdated—an grownup bear will spot them and go into assault mode, typically killing them on the spot. The perpetrator is normally a male, although I do know of not less than one documented case of a feminine killing one other she-bear’s cubs in Alaska.

A well mannered, charismatic bear like me can’t even fake to know what comes over these killers. So, I spoke to Frank T. van Manen, a USGS scientist emeritus who led the Interagency Grizzly Bear Research Group from 2012 to 2025. He’s one of many nation’s foremost bear specialists—after yours really, after all—and over his 35 years of analysis, he’s had loads of time to consider these sorts of issues.

“One speculation is that males will often kill cubs they didn’t sire in order that, after the feminine loses her cubs, she will be able to return into estrus that very same 12 months and breed once more,” says van Manen. “So, for a male that didn’t sire these cubs, from an evolutionary standpoint, it might make sense to kill the cubs and have the chance to mate with that feminine.”

That speculation is just a little controversial. It additionally comes from analysis performed in Scandinavia, which could not completely mirror the habits and behaviors of bears within the Decrease 48 the place I dwell. It hasn’t fairly matched van Manen’s observations both.

“We do see greater cub mortality areas with greater [population] density, like Yellowstone,” he says, “however we don’t essentially see proof that it’s males in search of a brand new mating alternative.”

As an alternative, the cub-killing conduct appears to be a built-in inhabitants management mechanism. It tends to kick in when bear density will get too excessive for the surroundings to assist. In that situation, there are fewer assets on the panorama, and, in occasions of desperation, hungry bears may merely see cubs as a straightforward supply of diet.

Yep, you learn that proper: After the assault, the grownup bear will typically eat what’s left. I do know that probably sounds grotesque to your human sensibilities, however I’m simply going to stage with you right here: We bears see energy as energy. As quickly as one thing is useless, it’s only a piece of meat.

That’s a philosophy a couple of of my compatriots bore out in surprisingly public vogue throughout Fats Bear Week in 2024, when two grownup males fought over the physique of a lately slain feminine bear on dwell video. The victor—referred to as 32 Chunk—dug in. Viewers had been horrified.

Bears aren’t the one animals that eat their very own, although. Sharks, jaguars, and salamanders have all been identified to dabble in cannibalism sometimes, probably for a similar cause: It helps maintain the inhabitants in verify. In different phrases, it may possibly profit the species over the long term—regardless that I’d a lot favor we maintain it off digicam.

— BEAR