Physiology of Hibernating Bears

Abstract

devoted to the more interesting cardiovascular adjustment. Gradually, over a period of two to four weeks, the sleeping heart rate of the bear becomes lower and lower, changing from a summer heart rate of 54 bpm in young bears to a winter sleeping level of 24 bpm (a reduction to 43 per cent of summer rate), and in older bears from a summer sleeping heart rate of 40 bpm down to 10 or 8 bpm (a reduction to 25 per cent of summer rates). In our initial experiments, the long life, short-range transmitters usually had a life of only six months. This meant that transmitters placed in the summer would have a battery which would become depleted during the bear's dormancy period in the winter. Be? cause of this condition in the implanted transmitter, some investigators have asked us whether the low heart rates attained were due to the change in the mercury battery of the transmitter during the last month of its activity. We have never carefully explained in the literature that this is by no means the case. For the following reasons, it can be categorically stated that low heart rates in bears in winter are always accurately recorded with the Iowa trans? mitter up to within one day of the cessation of the life of the transmitter. In the