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A new study, led by researchers from Canada and Norway, has revealed an unexpected trend—female brown bears are spending an extra year caring for their cubs.

Mother bears typically stay with their young for about 1.5 years. The longer a bear spends with her cubs, the fewer offspring she will have over her lifetime, as she cannot mate and reproduce when she is already caring for a litter.

But in a population of brown bears in Sweden, a hunting policy may be influencing female bears to stick with their cubs for an additional year.

The researchers analyzed more than 20 years’ worth of data collected from approximately 500 tagged brown bears (Ursus arctos). They found that prior to 2005, just seven percent of female adult bears stayed with their cubs for 2.5 years. But by 2015, 36 percent of mother bears were extending their care to 2.5 years – demonstrating that a behavior once considered a rarity has become quite common.

The team believes the extended caregiving may be linked to a hunting regulation in Sweden that prohibits hunters from shooting a mother or her cubs when they are together. Therefore, the hunting policy may be filtering out the females that chose not to keep their young for the extra time, the study suggests.

The analysis of data shows that both the mother and her cubs can increase their survival rates by staying together for the extra year. Adult females that are alone are about four times more likely to be killed than females accompanied by their cubs during the hunting season, according to the researchers.

According to the study, there were only about 130 individual brown bears in the region in the 1930s. But the population boomed, growing to 2,800 by 2013 as a result of the protective hunting measures that were implemented. The benefits and increased survival rates among adult bears who care for the cubs longer outweigh the consequences of having fewer cubs overall throughout the mother’s lifetime, the researchers conclude.

“Prolonged maternal care provides a buffer against high hunting pressure, as it protects adult females, which are the most productive segment of the population, as well as yearlings, which are the most vulnerable individuals,” write the authors. “This implies that the relative frequency of female reproductive tactics may alternate over time in the population depending on the level of human exploitation.”

The study, published this week in the journal Nature Communications, shows that the first documented litter in the study population that was raised with the 2.5-year tactic was born in 1993 and weaned in 1995.

The study authors do address other potential factors that could be causing the increase in extended care, such as a reduction in available resources. But the abundance of billberry, which is one of the most important food resources for Scandinavian brown bear females, has not declined in the last decade. And even if some females struggled to find a sustainable amount of billberry, the bears can adjust and search for alternatives such as crowberry when needed.

The team believes that if levels of hunting pressure on brown bear populations continues to remain high or even increase, then the relative occurrence of the 2.5-year tactic will also increase.

“We provide empirical evidence that harvest regulation can induce artificial selection on female life history traits and affect demographic processes.”

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