By Victoria Gill
Science reporter, BBC News
Environmental stresses could be causing physical changes in the bears
Polar bears have shrunk over the last century, according to research.
Scientists compared bear skulls from the early 20th Century with those from the latter half of the century.
Their study, in the Journal of Zoology, 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, the team said.
The researchers used the skulls as indicators of body size. The skulls from the later period were between two and 9% 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."
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, the team says, could also be related to a reduction in the genetic diversity of the species.
The team used skulls from the Zoological Museum of Copenhagen
Hunting over the last century, said Dr Pertoldi, could have depleted the gene pool, leaving polar bears to suffer the effects of inbreeding.
"We also know from previous studies that some chlorinated chemical pollutants have affected the fertility of the females," he continued.
Rune Dietz from Aarhus University was another member of the research team.
He explained that he and his colleagues had already determined a link between man-made "persistent organic pollutants" and reduced bone mineral density in polar bears - which could leave the animals vulnerable to injury and to the bone disease osteoporosis.
The maximum sea ice extent is declining by about 2.7% per decade
The collection of almost 300 polar bear skulls was provided by the Zoological Museum of Copenhagen in Denmark.
Christian Sonne, a veterinary scientist from Aarhus University who worked with the team, said that this provided a unique and "fantastic sample", providing a window into the bears' development over a whole century.
During that time, he said, concentrations of many man-made pollutants in the Arctic have significantly increased.
He said: "Polar bears are one of the most polluted mammals on the globe."