Tuesday, February 15, 2011

Fewer cubs for polar bear mothers


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.

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