Liver Toxicity. Polar bear liver can contain very high levels of vitamin A (Rodahl and Moore 1943; Lewis and Lentfer 1967; Russell 1967). The concentration varies greatly among individuals, but does not seem to be age dependent. The liver is toxic to humans if eaten. Rodahl and Moore (1943) summarized the variety of human health effects reported by Arctic explorers who had eaten polar bear liver. Effects ranged from drowsiness, headache, and general irritability to large-scale peeling of the skin. Peeling was often localized, but sometimes covered victims from head to foot. Variation in the quantity of liver consumed and the vitamin A concentrations within each liver probably accounted for the diversity of reported symptoms.
Thermoregulation. Polar bears appear to be highly specialized for life in the arctic marine environment. However, Scholander et al. (1950) reported relatively high thermal conductivity for polar bear fur in both air and water. Likewise, �ritsland (1970) concluded that polar bears depend on a combination of fur, fat, and subdermal vascularization to maintain their body temperature. �ritsland (1970) and Best (1982) showed that polar bears can increase effective peripheral insulation with vasomotor controls. Such controls could be most effective during water immersion. All adaptations, however, were inadequate to contend with either exceptionally cold or hot air temperatures; polar bears may depend on postural and behavioral mechanisms during extremes of air temperature.
Newborn cubs have short, thin hair and no subcutaneous fat (Blix and Lentfer 1979). Therefore, they are poorly equipped for survival outside of the maternal dens in which they are born. On emergence from the den, however, cubs are much better equipped for outside exposure. Blix and Lentfer (1979) reported a lower critical temperature for a 12.5-kg cub of �30� C. At �45� C, the cub's oxygen consumption increased only 33%, and there was no decrease in core temperature. Immersion of this cub into ice water resulted in a precipitous and immediate drop of body temperature. Despite the small size and minimal subcutaneous fat, it appears that cubs are ready to face the outside world at the time of den departure. They are, however, not ready for immersion.
Locomotion. �ritsland 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 (�ritsland 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 4�8 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).
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