Heart rates of emperor penguins diving at sea: implications for oxygen store management

Abstract

Heart rate (ƒH) contributes to control of blood oxygen (O2) depletion through regula- tion of the magnitude of pulmonary gas exchange and of peripheral blood flow in diving verte- brates such as penguins. Therefore, we measured ƒH during foraging trip dives of emperor pen- guins Aptenodytes forsteri equipped with digital electrocardiogram (ECG) recorders and time depth recorders (TDRs). Median dive ƒH (total heartbeats/duration, 64 beats min �1 ) was higher than resting ƒH (56 beats min �1 ) and was negatively related to dive duration. Median dive ƒH in dives greater than the 5.6 min aerobic dive limit (ADL; dive duration associated with the onset of a net accumulation of lactic acid above resting levels) was significantly less than the median dive ƒH of dives less than the ADL (58 vs. 66 beats min �1 ). ƒH profile patterns differed between shallow ( 250 m), with values usually declining to levels near resting ƒH in shal- low, short-duration dives, and to levels as low as 10 beats min �1 during the deepest segments of deep dives. The total number of heartbeats in a dive was variable in shallow dives and consistently high in deep dives. A true bradycardia (ƒH below resting levels) during segments of 31% of shal- low and deep dives of emperor penguins is consistent with reliance on myoglobin-bound O2 stores for aerobic muscle metabolism that is especially accentuated during the severe bradycardias of deep dives. Although ƒH is low during the deepest segments of deep dives, the total number and distribution of heartbeats in deep, long dives suggest that pulmonary gas exchange and peripheral blood flow primarily occur at shallow depths.