Scientists studying polar bear growth have revealed that the bears have shrunk in size over the last century. The scientists compared polar bear skulls from the early 20th century to those from the late 20th century.
BBC News reports that the study in the Journal of Zoology says changes in the size and shape of polar bears may be linked to a reduction in sea ice and an increase in pollution.
The team said that the bears growth could be limited by the pollutants now present in the bears bodies which could lead to physical stress for them. Another factor could be the increase in effort required in the need to find food.
The lead scientist, Cino Pertoldi, professor of biology from Aarhus University and the Polish Academy of Science, explained: "Because the ice is melting, the bears have to use much more energy to hunt their prey. 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, including some of the professors colleagues from the Aarhus University Department of Arctic Environment, also found there to be differences in the shape of the bears skulls from the different time periods.
Dr Pertoldi said that development was slightly more mysterious. He said that while it was not possible to exactly determine the cause, the changes were likely to be linked to environmental issues and highlighted the pollutants now present in the Arctic and also within the polar bears bodies.
The aim of the study was to compare between two groups of animals living through periods of time when sea ice levels and pollution levels were very different.
The scientists focused on certain pollutants and these were compounds containing carbons and halogens, fluorine, chlorine, bromine or iodine.
Some of the compounds are no longer used but some remain important to industry and they include use in solvents, pesticides, refrigerants, adhesives and coatings.
The team also says a reduction in the genetic diversity of the species could also be a factor in the changes. Dr Pertoldi said that over the last century hunting could have depleted the gene pool causing polar bears to suffer the effects of inbreeding. He added: "We also know from previous studies that some chlorinated chemical pollutants have affected the fertility of the females."
Another member of the research team from Aarhus University, Rune Dietz explained that he and colleagues had confirmed a link between reduced bone mineral density in polar beers and the man made "persistent organic pollutants". This leaves the bears vulnerable to injuries and osteoporosis.
The Zoological Museum of Copenhagen provided almost 300 polar bear skulls for the research.
A veterinary scientist at the Aarhus University and also a member of the research team, Christian Sonne said that this was a unique and "fantastic sample" of the development of the bears over a century.
He said that concentrations of man-made pollutants in the Arctic have significantly increased in that time.
He also added: "Polar bears are one of the most polluted mammals on the globe."