Why Are Some Mama Bears Caring for Cubs Longer?

brown bear

Story at-a-glance -

  • Researchers, who have been following more than 500 Scandinavian brown bears closely, some since 1984, have detected a not-so-subtle shift in the length of time some mama bears are caring for their cubs
  • It used to be a rare occurrence for a mama bear to stay with her cubs for 2.5 years, but it appears to be getting increasingly common
  • In Sweden, it’s illegal to hunt family groups, and a single female bear is four times more likely to be shot than a bear with a cub; in short, hunters are shooting more of the females that only keep their cubs for a year
  • Both mother bears and their cubs gain an increased survival advantage with longer care periods, which compensates for the reduced reproductive rates that occur as a result

By Dr. Karen Shaw Becker

In Sweden, brown bears typically stay with their cubs for 1.5 years, after which they’re left to forge through life on their own. But researchers, who have been following more than 500 Scandinavian brown bears closely, some since 1984, have detected a not-so-subtle shift in the length of time some mama bears are caring for their cubs.1 It used to be a rare occurrence for a mama bear to stay with her cubs for 2.5 years, but it appears to be getting increasingly common.

At first glance, the trend seems to go against the laws of nature, which would suggest that female bears should have as many offspring as possible in order to further the species. By keeping their offspring with them longer, they miss out on mating opportunities and give birth to fewer young over the course of their lifetime. Yet, the population is not suffering as a result. Quite the opposite.

In fact, female bears who stay with their young longer tend to live longer than other mama bears — a direct result of hunting trends in the area, which are actually influencing the way bears parent their cubs.

Human Hunting Alters How Long Bears Stay With Their Cubs

In Sweden, bear hunting is a popular pastime, with about 300 bears shot each year from 2010 to 2014. With bear hunting on the rise, there’s one bear population that has remained safe: females with their cubs. This is because, in Sweden (as in many countries) it’s illegal to hunt family groups. As such, professor Jon Swenson, Ph.D. from the Norwegian University of Life Sciences (NMBU) explained, a single female bear is four times more likely to be shot in Sweden than a bear with a cub.2

Because of this hunting trend, the number of female bears that keep their cubs for 2.5 years is on the rise, increasing from 7 percent in 2005 to 36 percent in 2015. It’s not that the bears are catching on to the strategy and choosing to keep their young with them, the researchers noted, as the length of time bears keep their cubs seem to be a fixed trait. Instead, "This basically means that we are shooting more of those females that only keep their cubs for a year,” Swenson said in a news release.3

The survival advantage that the bears’ gain from caring for their cubs an extra year compensates for their reduced reproductive rates, the study noted, especially in areas with heavy hunting. In areas with high hunting pressure, “the fitness benefit of prolonged periods of maternal care outweighs that of shorter maternal care.”4 What’s more, it’s not only the mama bears that gain a survival advantage.

In the study, all the female cubs that stayed with their mothers longer survived to at least their third birthday, but among the cubs who got only 1.5 years of mother’s love, 22 percent did not.5 The young bears may have fallen victim to hunters and they’re also more likely to die in battles with other bears. “We show that being in a family group and providing longer maternal care results in a survival advantage for both adult females and dependent offspring,” the researchers wrote.6

What this means for bears over time remains to be seen, but the researchers are curious whether female cubs that stayed with their moms for 2.5 years will go on to do the same for their young. Interestingly, in North America brown bears routinely stay with their cubs for 2.5 years.7

Venturing Closer to Humans May Help Mother Bears Protect Their Cubs

Human activities influence bear habits in many ways, including perhaps even acting as a “shield” for mother bears seeking to protect their cubs from males. One-third of brown bear cubs are killed during mating season, most often by unrelated males seeking to mate with their mothers. The male bears may try to kill the cubs in order to force the female to stop lactating, which allows her to become pregnant again.

In nature, animals will often locate themselves close to the enemies of their predators, which gives them a natural shield against predation. Moose in Yellowstone, for instance, are known to have their calves near roads, which helps protect their offspring from brown bears, which tend to avoid traffic.8 Female brown bears, it turns out, may also use “human shields” by venturing closer to human populations, which male brown bears tend to avoid.

In a study of Scandinavian brown bears, mothers who ventured closer to humans were more likely to have surviving litters during mating season, compared to those who did not (and were more likely to suffer from a complete litter loss).9 It’s not known if this strategy is widespread, but it’s one more intriguing insight into how human activities influence the lives of animals around them, for better or worse.

It’s even been shown that bears reduce their foraging activity during morning hours of hunting season, when they’re most likely to be shot. Over time, it’s thought this could lead to reduced food intake or intake of poorer-quality food, leading to reduced health that could influence reproduction.10 From influencing their diet to the places they raise their cubs to how long they stay together as a family, researchers continue to unravel bits and pieces about how human interventions alter individual species and entire ecosystems at once.

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