Craniometric characteristics of polar bear skulls from two periods with contrasting levels of industrial pollution and sea ice extent

Abstract

A morphometric study was conducted on six skull traits and seven teeth traits of 282 polar bear Ursus maritimus skulls sampled in East Greenland from 1892 to 2002, the polar bear material originated from two distinct periods: one period covering 1892–1939 and the other from 1961–2002. The first period being before the introduction of organochlorines in the Arctic environment and having more extensive sea ice cover when compared with the later period. Admixture analysis, followed by multivariate analyses provided evidence for morphometric differences in both the size and the shape of individual skulls collected in the two periods. These findings are possibly a consequence of environmental factors, such as exposure of organohalogens and changed extension of sea ice, ultimately affecting the amount of prey available, a general weakening of the immune system and reduced reproductive success, factors that can affect the individual growth and the realized size at maturity. The process of reduced reproductive success due to a high concentration of organochlorine and/or changes in the amount of food resources may also have affected the polar bears' genetic composition and effective population size. Changes in the genetic composition of the population are suggested to have contributed to the observed morphometric changes with time. The fact that environmental and genetic changes produce different combinations of patterns of morphometric changes allows us to individuate the causes of the morphometrical modifications.