Like many millennials, bear cubs in Sweden are living at home with their moms longer than previous generations — and researchers think it's because female bears are trying to stay safe from hunters.

Brown bears in Europe are considered the same species as the grizzly bears that live in North America, although the European bears tend to be smaller.

Scandinavian brown bears in south-central Sweden used to care for their cubs for just one and a half years. But starting in 1993, researchers began to notice some bears keeping their cubs around for an extra year. By 2015, more than a third of those Swedish mother bears were caring for their cubs until they were two and a half years old, reports a new study led by Joanie van de Walle, a PhD student in animal ecology at the University of Sherbrooke.

"We were quite surprised," said van de Walle.

They were surprised because mothers can't produce a new litter until their older cubs have left, and having them leave sooner allows them to produce far more cubs over their lifetime. That should give them an advantage under natural selection.

A Scandinavian brown bear family group composed of a female with her dependent cubs. Family groups have been protected from hunting by law since 1986. (Ilpo Kojola)

So why would some bears start keeping their cubs around longer? Van de Walle and her team believe it's to protect themselves and their cubs from hunters.

A new regulation introduced in 1986, banned shooting bears in family groups.

Because of that regulation, van de Walle said, the researchers found that a female bear is four times more likely to be killed when she's alone compared to when she's with cubs.

"So there's really a survival advantage for females to keep the cubs as long as possible," van de Walle said.

In any given year, 30 per cent of bears eligible to be hunted are killed by hunters. The average age at mortality for a female bear is 4.8 years, even though most don't successfully produce cubs until they are five years old.

"Most of them die before they even get the chance to reproduce," van de Walle added.

Johanna Wagstaffe explains why hunting laws are believed to be the cause 1:46

Interestingly, bears in North America typically keep their cubs with them until they're two and a half years old.

And in fact, the researchers think the Swedish bears started keeping their cubs around for a shorter period of time and reproducing more quickly in response to hunting in decades prior to 1986. At that time, a lack of regulation meant female bears with cubs could be hunted too, so reproducing as quickly as possible would have been the optimal strategy.

Van de Walle is currently studying exactly how mother and cubs end up separating. Right now, she said, it appears that it's the mother who decides it's time for the cubs to head out on their own and chases them away in spite of their protests.

The study was led by Joanie van de Walle, a PhD student at the University of Sherbrooke, seen here holding an antenna used to locate bears wearing tracking collars. (Joanie van de Walle)

The new study, which involved Canadian, Norwegian and Austrian researchers was published in Nature Communications. It was funded by the Fonds de Recherche du Québec-Nature et Technologies, the Natural Sciences and Engineering Research Council of Canada, the Swedish Environmental Protection Agency, the Norwegian Directorate for Nature Management, the Austrian Science Fund, and the Swedish Association for Hunting and Wildlife Management.