Deep in the forests of south-central Sweden, brown bears (Ursus arctos) emerge from hibernation ravenous. Mama bears and their cubs will consume massive amounts of berries, bees, voles, and maybe other meat to plump up before heading into hibernation again in the fall. This has been the bears’ annual routine for millennia.
But a group of researchers in Scandinavia that has been tracking brown bears there for three decades has started to notice something just a bit different in female bears’ behavior in recent years. “More and more females are keeping their cubs longer,” says Joanie Van de Walle, a graduate student at the University of Sherbrooke in Quebec, Canada, who is studying the ecology of Scandinavian brown bears. While for the most part, mother bears in the region have tended to care for cubs for only about a year and a half, many of the bears in Sweden are now rearing cubs for more than two years.
That’s surprising, because for as long as a mother bear has cubs, she isn’t fertile and therefore won’t mate. The longer she keeps her baby bears around, the fewer offspring she can have in her lifetime, notes Frank van Manen, who studies grizzly bears, a subspecies of brown bear (Ursus arctos horribilis), for the US Geological Survey in Yellowstone National Park.
So why are Sweden’s brown bear mothers adopting such a seemingly disadvantageous reproductive strategy? That’s a question Jon Swenson, an ecologist at the Norwegian University of Life Sciences who has helped to track the Scandinavian bear population, wanted to answer. He and his colleagues have been following female bears from birth to death for 30 years. The team now has data spanning five generations, which it’s using to study the social organization of the roughly 3,000 bears in the region, along with the animals’ mate choices and life histories.
Based on their three decades’ worth of data—the largest and longest-running dataset collected to date for a large carnivore—Swenson, Van de Walle, and colleagues saw that between 1987, when the monitoring project started, and 1995, there was little to no evidence of mother bears staying with their cubs longer than 1.5 years. From 1995 to 2005, the 2.5-year timeframe to raise cubs remained rare, describing only about 7 percent of weaned litters. But between 2005 and 2015, it jumped to around 36 percent.
Now, Swenson and his colleagues think they have found the cause: hunters. In Sweden, legislation that prohibits hunters from killing bears with cubs dates back to at least 1973. Van de Walle, Swenson, and their colleagues found that, for the past 30 years, hunters have been removing mother bears that wean after shorter periods of time from the population, meaning mother bears that keep their cubs longer have a survival advantage over those that don’t (Nat Commun, 9:1100, 2018). As a result, the authors suggest, the hunting regulation promotes slower life histories among Scandinavian brown bears.
It’s hardly the first example of harvesting practices acting as an artificial selective pressure on animal populations. Fishing, for instance, has targeted the largest individuals of certain species, with the result that fish lose reproductive opportunities as they grow larger, selecting for individuals that reach sexual maturity sooner and leading to an overall reduction in the size of individuals within a population. And in animal species with economically valuable or prized trophy features such as horns, antlers, and tusks, those with larger outgrowths are often hunted out of the population, leaving individuals with smaller adornments to mate.
For Scandinavian bears, extra time with mom might carry advantages for the cubs, too. It “might actually make these cubs better prepared for life,” Miha Krofel, a carnivore ecologist at the University of Ljubljana in Slovenia who was not involved in the new study, tells The Scientist by email. “There is a lot for bears to learn before gaining independence.” They need to know where to find food, which habitats to use in which season, how to interact with other bears, and how to avoid people. Cubs that live longer with their mothers could even be less prone to conflicts with humans.
There is a lot for bears to learn before gaining independence.—Miha Krofel, University of Ljubljana
Van Manen says the reported change in cub rearing and its association with hunting is interesting, but he cautions against “interpreting this observation as a cause and effect, because there may be alternative hypotheses that would be worth exploring.” It’s puzzling, for instance, that in the Scandinavian brown bear population the 1.5-year weaning tactic was dominant throughout the study period. “The norm for brown bear populations worldwide is that cubs stay with their mothers for 2.5 years or more,” he says. “So another way of thinking about the results of this study is why the 1.5-year tactic was relatively common in the Scandinavian brown bear population to begin with.” Van Manen suggests the shift in parenting strategy back toward the species norm might have been the result of decreasing densities of aggressive adult male bears, which can attack and kill cubs, presumably to gain access to adult female mates.
Still, the team’s observation that hunting changes female bears’ strategies for raising cubs has support from researchers studying bear populations elsewhere. Since the 1930s, Slovenia has protected its bear populations and, in 1986, made it illegal to kill a mother with her cubs. There, Krofel says, he and his colleagues are seeing slightly different effects: females first give birth up to a year earlier than females in other brown bear populations around the world. “Understanding these effects is crucial for managers to be able to formulate hunting regulations, which will decrease unwanted side effects of hunting, as well as to help the public decide whether bear hunting is acceptable in [a] given situation or not,” Krofel says.
Such decision-making was recently in play for the Yellowstone grizzly bear population. The bears had been on the endangered species list and protected from hunting until last year. Now, the population has reached at least 700 individuals, and the Wyoming Fish and Game Commission unanimously voted to allow the hunting of about two dozen bears east of Yellowstone National Park this year. Some researchers argue that killing females could threaten the stability of the grizzly population, but van Manen argues otherwise. The hunt will be extremely well regulated, he says, and “hunting pressure will be very low compared to the Scandinavian study.”
For now, Van de Walle notes that the Scandinavian study has raised many more questions than it is has answered. One particular mystery she would like to solve is how the change in parenting strategy is being transferred between generations—whether female bears know instinctively to wean their cubs later, or if they learn to do it from their mothers, she says. “We need a longer study period to verify that.”