Polar bears have shrunk over the last century, according to research.

研究显示，上个世纪北极熊的体态有所下降。

Environmental stresses could be causing physical changes in the bears

Scientists compared bear skulls¹ from the early 20th Century with those from the latter half of the century.

Their study, in the Journal of Zoology³, describes changes in size and shape that could be linked an increase in pollution and the reduction in sea ice.

Physical "stress" caused by pollutants⁴（污染物质） 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 indicators⁵ 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 allocate⁶（分派，分配） 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（卤族） - fluorine（氟）, chlorine（氯）, bromine（溴） or iodine⁷（碘）.

Some of these compounds have already been phased（定相的） out, but many still have important uses in industry. These include solvents⁸（溶剂）, pesticides⁹, refrigerants, adhesives¹⁰（粘接材料） and coatings.

Genetic¹¹ brink¹³

The changes, the team says, could also be related to a reduction in the genetic diversity of the species.

Hunting over the last century, said Dr Pertoldi, could have depleted¹⁴（贫化） the gene¹² pool, leaving polar bears to suffer the effects of inbreeding.

"We also know from previous studies that some chlorinated（绿色的） chemical pollutants have affected¹⁵ 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 determined¹⁶ a link between man-made "persistent¹⁷ organic pollutants" and reduced bone mineral density¹⁸ in polar bears - which could leave the animals vulnerable to（易受……攻击） injury and to the bone disease osteoporosis（骨质疏松症）.

Skull² collection

The collection of almost 300 polar bear skulls was provided by the Zoological Museum of Copenhagen in Denmark.

Christian¹⁹ 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."