Mama bears foil hunters by keeping babies close
Swedish brown bears have learned to exploit regulations that forbid shooting nursing mothers. Andrew P Street reports.
The brown bear (Ursus arctos) is a very successful alpha predator which roams Alaska, Canada, Russia, Central Asia, China, Romania and other mainly chilly and forested regions in the northern hemisphere, at one point spreading as far south as Mexico, and still with small (albeit threatened) populations in France, Italy and Spain.
Now, one group has seemingly discovered that parenthood confers certain personal survival advantages.
Specifically, those in Sweden have adjusted to hunting restrictions which make it a crime to shoot a bear mothering a cub – and a new study published in the journal Nature suggests that this shows hunting regulation can affect what biologists call “life history traits”.
In the paper, Joanie van de Walle and colleagues, from the Université de Sherbrooke, in Canada and the University of Southeast Norway, report that because the regulation is based on female reproductive status, it works to favour the survival prospects of nursing mothers.
The bears have not been slow to see the implications of this. Using observations made between 1987 to 2015, the researchers found that mother bears extended the period of care for cubs from 18 months to two and a half years.
Being killed during the annual three-month hunting season is the main cause of early death for Swedish brown bears. Multiple previous studies cited in the paper have found that hunting prompts species to speed up their life histories, resulting in earlier and more frequent reproduction.
This research shows the reverse outcome: imposing regulations that forbid killing bears while they are nursing can result in slower life histories for individual populations.
As brown bears don't breed until the previous batch of cubs has been weaned, the strategy requires a trade-off in terms of fewer breeding opportunities for the mother. However, the researchers calculated the risk of mortality for a solitary adult female bear compared to one with a cub.
They found that there was an almost four times greater risk for bears that roamed solo. The numbers for the slower breeding strategy clearly add up.
In fact, the strategy works even more efficiently as external pressure from hunting increases: it's better for populations to nurse the cubs they have for longer than to try to produce more of them.
What's especially remarkable is that the hunting laws have not been in place for all that long – only since the mid-90s – which means the bears are impressively astute when it comes to statistics. Perhaps that's the greatest discovery here: not only are these bears better parents, they appear to have a head for numbers.