Locomotion. Øritsland et al. (1976), Hurst et al. (1982a, 1982b), and Best (1982) concluded that polar bears are relatively inefficient walkers. Measurements were made from two polar bears walking on treadmills. Oxygen consumption and heat storage were higher than might have been predicted for other mammals of comparable size. Inefficient walking was attributed to aspects of polar bear morphology, specifically the massive forelimbs evolved for capture of prey (Øritsland et al. 1976; Hurst et al. 1982a, 1982b). Economy of transport, they suggested, was compromised by considerations of thermoregulation and hunting strategy. Nonetheless, the typical daily, seasonal, and annual movements of polar bears place them among the most mobile of all quadrupeds (Amstrup et al. 2000). Locomotion in polar bears is clearly an area where additional research is in order.

Hibernation. Like other ursids, polar bears have evolved a very specialized winter dormancy. Females occupy maternal dens of ice and snow for periods of 4­8 months. During that time, they neither eat nor drink and they do not urinate or defecate (Nelson et al. 1973; Folk and Nelson 1981; Nelson 1987; Watts and Hansen 1987; Ramsay and Stirling 1988). In hibernating bears, normal mineral levels are maintained, lean body mass is constant, blood electrolyte balance is preserved, and levels of blood metabolites are largely unchanged despite loss of nearly half of their total body mass after den entry (Nelson et al. 1973; Folk and Nelson 1981; Guppy 1986; Nelson 1987; Atkinson and Ramsay 1995). They appear able to maintain constant fluid levels by using metabolic water produced from fat catabolism (Guppy 1986; Nelson 1987).

Polar bears may be even more highly evolved with regard to their ability to survive food deprivation than the other ursids. Behavior and physiology of polar bears are well adapted to a feast-and-famine feeding regimen (Lunn and Stirling 1985; Watts and Hansen 1987; Ramsay and Stirling 1988; Derocher and Stirling 1990; Derocher et al. 1990). It now appears that they can alter their metabolism during periods of food deprivation at any time of the year (Nelson et al. 1983). Atkinson and Ramsay (1995) and Derocher et al. (1990) demonstrated that polar bears, unlike other bears, can shift as needed into a hibernation-like metabolic pattern when confronted by a period of food shortage. Facultative changes into and out of a hibernation-like state would magnify the value of summer and winter shelter denning described by Messier et al. (1994) and Ferguson et al. (2000a). This ability could make polar bears the most advanced of all mammals when it comes to dealing with food and water deprivation (Nelson 1987).