Estimating Water Loss During Hibernation in the American Black Bear (Ursus americanus)

Abstract

The American black bear can hibernate for up to 6 months. During this period they do not eat, drink, urinate, or defecate: they elicit minimal movements and typically do not leave their dens. Still, they lose minimal skeletal muscle mass and strength, and do not suffer any adverse consequences of either starvation or dehydration. In previous reports bears have been shown to maintain normal blood compositions, suggesting that they sustain homeostasis of their blood and other bodily fluids. To maintain this water balance without any fluid intake, bears must minimize water loss throughout their bodies, including their respiratory systems.Radio‐collared wild black bears in northern Minnesota were tracked to their dens during early or late hibernation. Bears were anesthetized, after which the relative amount of water vapor expired was analyzed using a hygrometer/mask system. We designed a respiratory mask to fit on the muzzle of the bear to collect expired air while still allowing them to breathe normally. From these data we estimated the amount of water vapor lost by bears during hibernation and were able to estimate that a 200–250 lbs bear would lose only 2.3L of water in 5 months of hibernation. This ability to conserve water during respiration is required to sustain long periods without intake through diet or drinking. Funding by NIH training grant 5T32AR007612 and Medtronic professorship funding.