It’s been a big year for large adult sons in science news, whether we’re talking about Jupiter, the large adult son of our solar system, or a killer whale trying to get her large adult son laid (seriously). Now, new research shows that human activity is causing Scandinavian brown bears (Ursus arctos) to produce more large adult sons. In bear culture, however, that’s not such a bad thing.
The paper, published in the journal Nature Communications on Tuesday, focuses on the effects of Swedish laws protecting mother bears who have cubs in the nation’s forests. Since fewer mother bears are being killed by hunters, the researchers write, cubs are spending more of their lives under their mothers’ care.
In the paper, the team of researchers from Canada, Norway, Austria, and Sweden describe a trend among bear families, revealed in data collected over 20 years of close monitoring. In the past, bear mothers raised their cubs for 1.5 years. But starting in 1993, after Swedish hunting regulations began protecting mother bears with cubs, about 25 percent of mother bears started caring for their cubs for 2.5 years.
Being a large adult son may have a negative connotation for humans, but for bears, it turns out to be a good thing.
While the additional year mother bears spent caring for their cubs usually translated to lower fertility rates overall, the lifelong benefit of spending more time with their mothers was ultimately better for the bears than higher fertility rates.
“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,” write the authors. “This survival advantage compensates for a reduction in reproductive output for females providing longer maternal care.”
While the two different maternal tactics — shorter-term care for more cubs versus longer-term care for fewer cubs — yielded similar lifetime fitness, the researchers found that as hunting pressure increases, the longer-term maternal care emerged as the better strategy.
These latest findings are complicated by previous research on these brown bear populations, though. A paper published in Proceedings of the Royal Society B in 2014 showed evidence that laws protecting mother bears could have unintended consequences for cubs. When mothers are protected, large males are more likely to be killed by hunters. And as dominant males are killed, new males come in to take their places. This may sound like a pretty normal process, but it involves one nasty twist: New dominant males often kill cubs — who were fathered by another male — so that the female bears will become fertile again, allowing the new male to impregnate them.
As the problematic naturalist John Muir once said, “When we try to pick out anything by itself, we find it hitched to everything else in the universe.” That’s the big lesson that scientists are learning now, in the Anthropocene, the geological era in which everything on Earth is somehow affected by human activity. While hunting laws may lead to bear cubs spending more time with their mothers, these very same laws also seem to lead to higher rates of infanticide as male bears compete for dominance in a power vacuum. In the Anthropocene, every solution brings new consequences. And in the case of these large adult sons, it’s not clear whether the cure or the sickness is more dangerous.
Abstract: As an important extrinsic source of mortality, harvest should select for fast reproduction and accelerated life histories. However, if vulnerability to harvest depends upon female reproductive status, patterns of selectivity could diverge and favor alternative reproductive behaviors. Here, using more than 20 years of detailed data on survival and reproduction in a hunted large carnivore population, we show that protecting females with dependent young, a widespread hunting regulation, provides a survival benefit to females providing longer maternal care. This survival gain compensates for the females’ reduced reproductive output, especially at high hunting pressure, where the fitness benefit of prolonged periods of maternal care outweighs that of shorter maternal care. Our study shows that hunting regulation can indirectly promote slower life histories by modulating the fitness benefit of maternal care tactics. We provide empirical evidence that harvest regulation can induce artificial selection on female life history traits and affect demographic processes.