Environmental stress might be shrinking polar bears.

A new research has indicated that environmental stress could be causing physical changes in the bears, making them shrink over the last century.

According to a report by BBC News, the conclusion was arrived at by scientists after comparing bear skulls from the early 20th Century with those from the latter half of the century.

Their study describes changes in size and shape that could be linked an increase in pollution and the reduction in sea ice.

Physical "stress" caused by pollutants in the bears' bodies, and the increased effort needed to find food, could limit the animals' growth, according to the team.

The researchers used skulls as indicators of body size. The skulls from the later period were between two and 9 percent smaller. "Because the ice is melting, the bears have to use much more energy to hunt their prey," explained Cino Pertoldi, professor of biology from Aarhus University and the Polish Academy of Science, and lead scientist in this study. "Imagine you have two twins, one is well fed during its growth and one is starving. (The starving) one will be much smaller, because it will not have enough energy to allocate to growth," he said.

The team, which included colleagues from Aarhus University's Department of Arctic Environment, also found shape differences between the skulls from the different periods. "This development was slightly more mysterious," said Dr Pertoldi.

He explained that it was not possible to determine the cause, but that the changes could be linked to the environment - more specifically to pollutants that have built up in the Arctic, and in the polar bears' bodies.

The aim of the study was to compare two groups of animals that lived during periods when sea ice extent and pollution levels were very different. The pollutants that the scientists focused on were compounds containing carbon and halogens - fluorine, chlorine, bromine or iodine.

Some of these compounds have already been phased out, but many still have important uses in industry. These include solvents, pesticides, refrigerants, adhesives and coatings. The changes, according to the team, could also be related to a reduction in the genetic diversity of the species.

Hunting over the last century could have depleted the gene pool, leaving polar bears to suffer the effects of inbreeding, according to Dr Pertoldi.

"We also know from previous studies that some chlorinated chemical pollutants have affected the fertility of the females," he said.