The Arctic can be a harsh and unforgiving environment. It can be particularly tough on polar bear mothers, which fast for at least four months while they give birth to and suckle their cubs in dens they have hollowed out of snowdrifts. Accordingly, in the time before they enter those dens, they gorge, packing on the pounds to see them through the lean times ahead. And when they emerge, they are understandably keen to replenish themselves, and encourage their occasionally recalcitrant youngsters to pick up the pace on their way to the sea ice.
Sometimes, a polar bear will give birth to one cub, sometimes three, most often two. Sometimes, however, the demands are just too great: Lacking the energy to see the pregnancy to its conclusion, a mother's body may reabsorb the fetus or fetuses, allowing her to emerge, resume feeding and, come spring, mate once more; on occasion - how frequently is unknown - she may give birth but be so malnourished that she is forced to eat one or more of her offspring.
A new study in the journal Nature Communications has shown that the size of a litter - and the likelihood of reproductive success - is strongly correlated to the amount of weight that a pregnant female is able to accumulate prior to entering the den. The greater the amount of fat, the larger the energy store to sustain her and her cubs. Of course, the less time the female has to eat, the less opportunity she has to build up that energy store - and, in parts of their range, polar bears have less time to eat than in the past.
Peter Molnar of the University of Alberta and colleagues studied the bears of western Hudson Bay in Canada, which is at the southern edge of the polar bear's range. Every summer, the sea ice in Hudson Bay melts completely, obliging the bears to come ashore, where they await the ice's return in a state of "walking hibernation." That means the fast lasts even longer for pregnant bears there than elsewhere, giving them an even smaller window in which to stock up their energy supplies.
Unfortunately, Molnar and colleagues write, sea ice in western Hudson Bay is breaking up earlier in the summer, and polar bears are forced ashore one to two weeks earlier than was the case in the early 1990s. Given predicted temperature increases as a result of greenhouse gas emissions, by mid-century the average date on which bears will come ashore is estimated to be the 1st of July - one month earlier than twenty years ago.
The authors note that, in the early 1990s, 28 percent of pregnant bears in western Hudson Bay failed to produce any cubs; they calculate that, forced to come ashore a month earlier, between 40 and 73 percent will be unable to raise a litter. Furthermore, the average size of the litters born will also decrease. Should sea ice break up fully two months earlier, they estimate that at least 55 percent, and perhaps 100 percent, of pregnant females will be unable to give birth.
Not surprisingly, they note that "the litter size predictions provided in this study serve as another indicator that the western Hudson Bay population will probably not remain viable under predicted climatic conditions."
Although the Hudson Bay polar bears are especially vulnerable to the impacts of climate change on sea ice, Geoff York, of the World Wildlife Fund's Arctic Program, argues that their fate is relevant for polar bears elsewhere.
"Climate change is fundamentally altering long held assumptions for wildlife management," York - who has studied polar bears extensively but was not involved in the Nature Communications study. "The circumstance for polar bears is both dynamic and complex and will require solutions that understand regional differences and embrace adaptation. There are 19 sub-populations of polar bears across the Arctic and this paper presents one of what will be many different stories unfolding as conditions change."
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