The anatomy, chemical composition, and metabolism of adipose tissue in wild polar bears (Ursus maritimus)

Adipose tissue was dissected completely from 14 polar bears (Ursus maritimus) killed in the southeastern Northwest Territories and northern Manitoba in November 1988. Mean adipocyte volume, collagen content, the activities of hexokinase and phosphofructokinase, and the fatty acid composition of the triacylglycerols were measured in samples of adipose tissue from several superficial, intra-abdominal, and inter-muscular depots homologous to those of other terrestrial mammals. The total adipocyte complement was calculated from the mass of each depot and its site-specific adipocyte volume. All the adipose depots found in other Carnivora and most other mammals are present in polar bears. The superficial layer of adipose tissue in polar bears arises from thickening and lateral expansion of depots that are discrete in most other mammals. All depots except the cardiac adipose tissue expand with increasing fatness, but the superficial depots expand faster than any of the internal depots, almost entirely by adipocyte proliferation. The gross anatomy of the superficial adipose tissue is determined mainly by the effect of body mass on the ratio of the surface area of the body to its volume. The superficial depots account for a greater proportion of the total adipose tissue in larger bears and in fatter specimens. The total adipocyte complement is about two to three times greater than that predicted from allometric equations relating adipocyte complement to body mass in non-arctic carnivores. The fatty acid composition of the triacylglycerols in the adipose tissue of polar bears is similar to that of the milk and the serum, but there were fewer long-chain polyunsaturated fatty acids than in their principal prey, ringed seals. There was no clearcut biochemical evidence for the existence of a thermal gradient between the inner and outer sides of the superficial adipose tissue. We conclude that in spite of their arctic habitat, neither the gross distribution of adipose tissue of polar bears nor its biochemical properties are adapted to thermal insulation. The enlarged superficial layer of adipose tissue is primarily an adaptation to increased energy storage.