A lack of Arctic sea ice is causing polar bear litters to decrease in size, according to a new study published today in the journal Nature Communications. This trend is predicted to continue over the next several decades, potentially threatening the health of the species.  "Polar bear litter size is decreasing," says study lead author Peter Molnar of the University of Alberta, now a post-doctoral student at Princeton University. The study compared observations from the early 1990s to today, and found that mother bears are producing fewer triplets and twins now than they did then.  "There were many twins and even some triplets in the early 1990s, whereas now there are practically no triplets, fewer twins and more females with just a single cub," says Molnar. The study 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.  Polar bears feed on seals while on the ice during the eight coldest months of the year, and fast on land during the four warmer months. With less time on the ice, there's less time to eat and store food.  "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.  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.  This puts the viability of the population in jeopardy, says Molnar. This is the first study of its kind to quantify this, he adds.  "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.  To report corrections and clarifications, contact Standards Editor  . For publication consideration in the newspaper, send comments to  . Include name, phone number, city and state for verification. To view our corrections, go to
Study: Melting Arctic sea ice puts polar bear populations at risk - Science Fair: Science and Space News - USATODAY.com