Surface Body Temperature Varies Greatly During Hibernation

Unlike core temperature, black bear surface temperature has been shown to fluctuate throughout the day with peaks that jump as much as 20°C at a time. This differs greatly from the consistency seen in body temperature, which stayed within a 1.5°C range for the entire winter sleep period. The peaks in surface temperature have no correlation to air temperature, so what could be the cause or purpose of such variation (see Figure 1; Harlow et al., 2004)?

Figure 1. Reproduced with permission from Dr. Harlow. Surface and Core Temperature in Bear 1 v. Time. This graph shows body and surface temperatures on differing scales for Bear 1 this study. Temperatures were recorded every 10 minutes on the neck and from the abdomin using and internal data recorder. This shows the large fluctuations in black bear surface temperature compared to the small variation in body temperature (Harlow et al., 2004).

This shows a tranquilized female bear in a hollow tree that she was using to hibernate with her young. Around her neck is device similar to that used by Harlow et al. to measure surface temperature in bears. (Image Courtesy of Dr. Dorcas)

Surface Temperature Fluctuations Help Black Bears Maintain Strength

Harlow et al. proposed that the spikes observed were the result of muscle contractions that help preserve muscle over the hibernation period. This explains how black bears can lose less than 25% of their strength and quickly mobilize if aroused. If humans underwent a similar period of torpor, they could expect to lose 80% of their muscleand would have trouble moving. In order to avoid stimulating the arousal of the bear, the heat from the muscle contractions is released across the skin as opposed to into the body, resulting in the observed spikes in surface temperature. When the researchers enlisted students to exercise on a stationary bike and measured their surface temperature with the same collars used on the bears, it produced a similar peak in surface temperature (Figure 2). Therefore, the three to four periods of muscular activity per day are thought to help maintain black bear muscle thoughout their long inactivity (Harlow et al., 2004).