Black Bears and Elevated Body Temperature

One of the most basic differences between hibernation in black bears and ground squirrels and other “true” hibernators is that bears maintain a relatively high and constant body (or core) temperature. With normal body temperatures of 38°C, black bears normally allow their core temperature to drop only as low as 31-33°C, similar to when they are sleeping. Some have been recorded with temperatures as high as 36.9°C over a 19 day period. These core temperatures stay very stable throughout the hibernation period, varying slightly within a 1.5°C range (Harlow et al., 2004). This lack of temperature variation shows that black bears do not use shivering or periodic awakenings from their hibernation as a means to maintain body temperature, as do many smaller hibernators (Harlow et al., 2002). However, as will be seen in the discussion of surface temperature, this does not necessarily mean a lack of muscle activity (Harlow et al., 2004). Because bears are so much larger than other hibernators, they need a much smaller percentage of their body mass to be stored as food stores for hibernation (see Table 1 below). They also have thick fur and extra body mass which help to insulate their core against the cold environmental temperatures (Nedergaard et al., 1990).

Table 1 (adapted from Nedergaard et al., 1990). Body Size v. Hibernating Feasibility. As body mass increases (and thus metabolic rate decreases), the relative energy needed to be saved by larger animals is less than that for smaller animals. The calculations here were made from the basal metabolic rate calculated by 3.34*(body mass) 0.75 over a 100 day winter period. This shows that smaller animals need to store substantially greater percentages of their body mass than larger animals.

This raises the question of why bears, with all their insulation, maintain such high core temperatures when they could save more energy by allowing their body temperature to hover closer to the environmental temperature. There are a number of explanations for this. This first deals with the costs of rewarming an animal of this size. As can be seen in Table 2, the cost of increasing a bear’s core temperature by 30°C compared to smaller hibernators (such as the marmot) is very high. Therefore, it is more efficient for bears to maintain a higher body temperature rather than deal with the costs of reheating their bodies after hibernation. Other advantages of a high body temperature include enabling them to awake from their winter sleep more quickly in response to predators and other disturbances. This is important because bears have a much harder time concealing their large bodies than many smaller hibernators, and can awaken to defend themselves or their young if disturbed. Lastly, it allows females to give birth during winter and nurse their young, which deeper hibernators cannot accomplish (Nedergaard et al., 1990).

Table 2 (adapted from Nedergaard et al., 1990). The Cost of Rewarming is Relative to Body Size. This shows the cost it takes to reheat an animal from 7°C to 30°C with a specific heat of 4.2 joules per gram degree. The rewarming equivalent is a measure of the hours it would take an animal to rewarm itself to normal temperatures.