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.
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.