Scientists have found that polar bear litter size is decreasing and it appears to be correlated with melting Arctic sea ice, according to a new study published Feb. 8, 2011 in the journal Nature Communications. If the melting trend continues as is predicted, polar bear reproduction could be imperiled.
"Polar bear litter size is decreasing," study lead author Peter Molnar of the University of Alberta, now a post-doc at Princeton University, told USA Today. Specifically, the scientists analyzed a group of polar bears near western Hudson Bay in Canada, the population of which dropped from 1,200 to 900 in the past 20 years.
Twins and triplets less common
"There were lots of triplets and twins in the 1990s, and now there are more females with just a single cub or twins," says Molnar.
Earlier ice break up means drastic reproduction declines
According to the study, 28 percent of the females were not reproducing in the early 1990s. Using computer models to predict what will happen in 2050, the study found that if the ice breaks up just one month earlier, then 40 to 70 percent will not reproduce.
Surviving on the ice
Arctic ice is critical for polar bears. For eight months out of the year the bears live on the ice and feed on seals. During the warmer months the bears fast on land. As the ice melts, the bears have less time at sea which means less time to eat and store fat. This can affect the bears’ fertility.
Food scarcity results in lower reproductive rates
"When little food is available," the study states, "polar bears are known to rely on stores of energy for survival and reproduction. The reliance on energy stores in pregnant females however, limits the survival rates of their cubs."
Arctic ice has been breaking up earlier and earlier each year, says Molnar. Just last week, the National Snow and Ice Data Center reported that Arctic sea ice was at its lowest January level on record.
Current litter size indicates future populations
"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," the authors write in the study.
"Outside western Hudson Bay, over one-third of the global polar bear population follows similar patterns of seasonal on-ice feeding and on-shore fasting."
Thus, the study concludes, similar litter size declines may occur in over one-third of the global polar bear population. —Rachel Walker