Mother Bears Are Staying With Their Cubs Longer, Study Finds

Mar 27, 2018
Originally published on March 27, 2018 9:14 pm

Each spring in the global north, brown bears around the world emerge from their dens with new cubs. The cubs come into the world hapless and fragile. Their fathers are long gone; bear mothers must find a way to raise the cubs while surviving themselves.

Female bears generally spend either 1.5 or 2.5 years with their young. In many ways, the pressures of bear life favor the shorter option — a mother with cubs cannot mate, so the more time she spends with each litter, the fewer offspring she'll have over her lifetime.

But a new study of brown bears in Sweden has found a surprising trend: More and more mothers are spending the extra year with their cubs.

"There's about a 30 percent increase in females staying to care for young for an extra year," explains Fanie Pelletier, an ecologist at Sherbrooke University in Quebec, and an author on the study published Tuesday in the journal Nature Communications. "Early on, in the '80s, almost all females stayed with their young for 1 year and a half," she says. "It's only since 2005 that we have witnessed this increase in the population [that are] staying with their young the extra year."

The trend is tied to a hunting regulation that protects family groups from hunters. It's illegal to shoot mother or cubs when they are together.

"For females, if you leave your cubs at one year and a half, then you become a target during the next hunting season," explains Pelletier. But "if you stay for a bit longer with your cubs, you're protected an extra year. The hunting is filtering out the females that keep their young for a smaller amount of time."

The analysis used decades of data collected from tagged brown bears in Sweden — the European cousins of North American grizzly bears. But the findings may be applicable outside Scandinavia, in any place where hunting rates are high. Hunting policies that protect mothers with cubs are widespread. In the U.S., many states have such laws in place for a variety of game animals.

In many countries, family groups of mother bears with cubs are protected from hunters by law.
Ilpo Kojola / Nature

Last year, Congress rolled back a law that banned killing bears and wolves in their family dens on 16 federally owned Alaskan wildlife refuges.

The new findings are a reminder to policymakers that hunting regulations can have long-term effects on the animals that survive. Past studies have found that hunting and fishing rules can have similarly fundamental changes on other types of wildlife.

Fish mature at smaller sizes when humans harvest large fish. Trophy hunting for caribou with large antlers can lead to smaller antlers and tusks in future generations. Rams hunted for their large horns eventually develop smaller horns.

"We assume when the hunting period is closed, hunting does not affect animals anymore," says Pelletier. But "even the ones that are still alive are affected by our policies, so that's something to keep in mind when we decide something."

And, she points out, when animals adapt to hunting policies, it often leads to fewer available animals for hunters. Mother bears staying with their cubs longer means fewer mature female bears can be shot, even if the overall population is growing.

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A new study of brown bears in Sweden has found a surprising trend - mother bears are spending longer with their cubs than they used to. As NPR's Rebecca Hersher reports, hunting policies have a lot to do with the change.

REBECCA HERSHER, BYLINE: Brown bears are large, dangerous and shy, so the best way to study them is to tag them.

FANIE PELLETIER: We can recognize them because they wear a GPS collar and tags. So it's possible to link what happened to a bear, let's say, early in its life and then what happened to it later in its life.

HERSHER: Fanie Pelletier studies predators at Sherbrooke University in Quebec. We talked over Skype. She's been studying bear motherhood for years. Right around this time of year, mother bears emerge from their dens with their cubs. This is when the real child rearing begins. And how long it takes depends.

PELLETIER: They care for their cubs either 1 1/2 year or 2 1/2 years. So there's variability in how long a female will stay with her cubs.

HERSHER: But Fanie analyzed decades of data and found more and more bear mothers are choosing to spend that extra year with their young, a 30 percent increase since 2005. The reason?

PELLETIER: So females and cubs, they are protected from hunting.

HERSHER: Sweden, where the data she analyzed came from, is one of many places where it's illegal to shoot a bear with cubs. You might think mothers are sticking around to protect their kids, but it's actually the other way around.

PELLETIER: If you leave your cubs at one year and a half, then you become a target. But if you stay for a bit longer with your cubs, then you're protected an extra year. So there's a clear benefit of being longer in family groups.

HERSHER: This isn't the first time a hunting policy has had unintended consequences for animals. Preferentially harvesting larger fish has led some species to mature at smaller sizes. Rams hunted for their large horns eventually develop smaller ones. Pelletier says changes like that are a reminder that hunting rules can have profound effects on the animals left behind. Rebecca Hersher, NPR News. Transcript provided by NPR, Copyright NPR.