Circulatory responses of seals to periodic breathing: heart rate and breathing during exercise and diving in the laboratory and open sea

TLDR

Seals must maintain a steady state while spending a large fraction of time submerged, a physiological aspect that has received less attention than dive duration but is crucial for understanding their morphological and physiological specializations. We confirmed these patterns using very high‑frequency and acoustic telemetry in freely diving harbour seals in the open sea. Laboratory exercise showed heart rate during breathing bouts remained high and unchanged with increasing metabolic rate, while during dives it was 25–33 % of the breathing rate and insensitive to metabolic rate; overall heart rate rose with exercise due to increased breathing time, and at sea surface heart rate stayed ~120 bpm regardless of dive duration, dropping to ~40 bpm underwater, with average heart rate largely determined by time spent below surface and only minimally by dive length, and the exceedingly low heart rates seen during forced dives in the laboratory were never observed at sea.

Abstract

In terms of a seal's success as a predator, the ability to spend a high fraction of time submerged and yet remain in a steady state is at least as important as that to make long dives. This aspect of seal physiology has received much less attention than has the ability to dive for extended periods and yet is important to the context in which dive-related morphological and physiological specializations are considered. We found, in seals exercising in the laboratory, that as [Formula: see text] increased, heart rate during breathing bouts was high and unchanging. Heart rate during dives was 25 to 33% of the rate during breathing and was insensitive to metabolic rate. However, the fraction of time seals spent breathing and overall average heart rate increased with exercise. We have confirmed these patterns using very high frequency and acoustic telemetry in freely diving harbour seals in the open sea. Heart rate at the surface was virtually constant at 120 beats/min and independent of dive duration. While the seals were diving, heart rate fell to 40 beats/min. Average heart rate depended almost entirely on the proportion of time spent below the surface and only to a minor extent on dive length. The exceedingly low heart rates seen during forced dives in the laboratory were never observed at sea.