Bradycardia of the Polar Bear

Résumé

For several years two male polar bears have been studied by long-life implanted physiological radio capsules at the Naval Arctic Research Laboratory, at Point Barrow. One of the siblings has consistently been larger than the other. A regular measurement has been the resting heart rate; for this we used the lowest heart rate obtained during night-time sleep. Night-time sleep is used because different heart rate levels are obtained during day-time sleep and night-time sleep. The two bears, during the continuous light of the summer, sleep regularly from about 11 PM until 9:00 AM; occasionally short bouts of sleep occur around early afternoon. Sleeping heart rates have been obtained throughout the lifetime of these two individuals. For example, during the summer of 1971, the larger polar bear, Irish (310 kg), had mean resting heart rates of 54 ±2 SD b/m (N=12); in 1972 at 332 kg his rate was 48 ±5 SD b/m (N=12); the smaller bear in 1971, at 286 kg, had a lower sleeping heart rate (50 ±5 SD b/m). ... A technique for demonstrating bradycardia (rapid slowing of heartbeat) in marine mammals was developed by Irving and Scholander, namely instrumenting the animal and training it to place its head under water. We decided to try this technique with the larger of the polar bears (Irish). The bear was separated from its companion, deprived of food and water overnight, and then recorded during three routine situations: 1) during high activity time; 2) during the filling of the water tub (approximately 300 litres) and 3) during feeding time. Heart rates were taken by the stopwatch every 15 seconds. ... Head immersion and diving bradycardia were evident for periods lasting up to 2 minutes; during that time the animal appeared to be searching for food at the bottom of the tub. Although diving bradycardia was evident (rate reduced 10 per cent to 72 b/m), it is apparent that there was much more bradycardia during the period of eating (reduced 20 per cent to 60 b/m). This slow rate during eating was remarkably close to the sleeping heart rate. One might have expected the heart rate to go up during the excitement of feeding. Also, competition for the food was not entirely lacking; the companion polar bear in the adjoining cage was constantly reaching through the bars as it attempted to obtain some of the food. One might have expected this activity to increase the excitement. We have found no other reference to bradycardia during feeding.