When they compared polar bear skulls that were harvested in the early 20th century with modern-day ones, researchers discovered that the older remains were significantly larger in size than the newer specimens. Experts believe that, at this point, the large mammals are being forced to reduce their size on account of the fact that several factors, including human presence and hunting, loss of habitat and global warming, are pressing down on them, the BBC News reports.

According to a new scientific paper published in the latest issue of the Journal of Zoology, it may be that the bears are also losing their size on account of the larger amounts of toxic pollutants in the waters where they live, as well as because more and more seal populations are decimated by warming temperatures and excessive hunting. Additionally, the animals constantly swim longer and longer distances before they can find food or ice to sit on, and this increased effort is not sustainable with a large body mass, evolutionary biologists say.

“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 former] will be much smaller, because it will not have enough energy to allocate to growth,” the lead scientist of the new research, Cino Pertoldi, explains the worrying trend.

He holds a joint appointment, as a professor of biology at the Aarhus University, and also in the Polish Academy of Science. According to the team, the Arctic waters now have much higher concentrations of carbon and halogens (especially fluorine, chlorine, bromine and iodine) than they did a century ago.

“We also know from previous studies that some chlorinated chemical pollutants have affected the fertility of the females,” the expert adds, quoted by the same news outlet. He proposes that, over the past 100 years, the gene pools may have also dwindled in their diversity, which may have led to a higher number of inbreeding instances.

This type of behavior is known to adversely affect future generations in all animals, including humans. Additionally, the team has also revealed that bone density in the polar bears is decreasing, a fact that has been directly correlated with the increased pollution in their natural habitats.