Researchers have concluded that polar bears are shrinking and that stress is to blame. The study concludes that bear skulls have changed in both size and shape compared with the earlier half of the 20th century. Bear skulls in the latter decades of the century were between two to nine per cent smaller. The stress is linked to an increase of pollutants in the bears’ bodies and the reduction of sea ice, which now demands extra energy to hunt. “Polar bears are one of the most polluted mammals on the globe,” says Christian Sonne, a veterinary scientist involved with the study.