Polar Bears In Depth
Physiology
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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, Øritsland (1970) concluded that polar bears depend on a combination of fur, fat, and subdermal vascularization to maintain their body temperature. Øritsland (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.
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, Øritsland (1970) concluded that polar bears depend on a combination of fur, fat, and subdermal vascularization to maintain their body temperature. Øritsland (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.