HEART-RATES OF NORTHERN ELEPHANT SEALS DIVING AT SEA AND RESTING ON THE BEACH

Heart rates of northern elephant seals diving at sea and during apnoea on land were monitored to test whether a cardiac response to submerge nce is an important factor in their ability to make repetitive, long-d uration dives, Seven juvenile northern elephant seals were captured at Ano Nuevo, CA, instrumented and translocated to release sites around Monterey Bay, Heart rate and dive depth were recorded using custom-des igned data loggers and analogue tape monitors during the seals' return to Ano Nuevo, Heart rates during apnoea and eupnoea were recorded fro m four of the seals after they hauled out on the beach, Diving pattern s were very similar to those of naturally migrating juveniles, The hea rt rate response to apnoea at sea and on land was a prompt bradycardia , but only at sea was there an anticipatory tachycardia before breathi ng commenced, Heart rate at sea declined by 64% from the surface rate of 107+/-3 beats min(-1) (mean +/- S.D.), while heart rate on land dec lined by 31% from the eupnoeic rate of 65+/-8 beats min(-1), Diving he art rate was inversely related to dive duration in a non-linear fashio n best described by a continuous, curvilinear model, while heart rate during apnoea on land was independent of the duration of apnoea. Occas ionally, instantaneous heart rate fell as low as 3 beats min(-1) durin g diving. Although bradycardia occurs in response to apnoea both at se a and on land, only at sea is heart rate apparently regulated to minim ise eupnoeic time and to ration oxygen stores to ensure adequate suppl ies for the heart and brain not only as the dive progresses normally b ut also when a dive is abnormally extended.