Abstract
Heart rate and peripheral blood flow distribution are the primary determinants of the rate and pattern of oxygen store utilisation and ultimately breath-hold duration in marine endotherms. Despite this, little is known about how otariids (sea lions and fur seals) regulate heart rate (fH) while diving. We investigated dive fH in five adult female California sea lions (Zalophus californianus) during foraging trips by instrumenting them with digital electrocardiogram (ECG) loggers and time depth recorders. In all dives, dive fH (number of beats/duration; 50±9 beats min(-1)) decreased compared with surface rates (113±5 beats min(-1)), with all dives exhibiting an instantaneous fH below resting (<54 beats min(-1)) at some point during the dive. Both dive fH and minimum instantaneous fH significantly decreased with increasing dive duration. Typical instantaneous fH profiles of deep dives (>100 m) consisted of: (1) an initial rapid decline in fH resulting in the lowest instantaneous fH of the dive at the end of descent, often below 10 beats min(-1) in dives longer than 6 min in duration; (2) a slight increase in fH to ~10-40 beats min(-1) during the bottom portion of the dive; and (3) a gradual increase in fH during ascent with a rapid increase prior to surfacing. Thus, fH regulation in deep-diving sea lions is not simply a progressive bradycardia. Extreme bradycardia and the presumed associated reductions in pulmonary and peripheral blood flow during late descent of deep dives should (a) contribute to preservation of the lung oxygen store, (b) increase dependence of muscle on the myoglobin-bound oxygen store, (c) conserve the blood oxygen store and (d) help limit the absorption of nitrogen at depth. This fH profile during deep dives of sea lions may be characteristic of deep-diving marine endotherms that dive on inspiration as similar fH profiles have been recently documented in the emperor penguin, another deep diver that dives on inspiration.
Highlights
The diving physiology and capabilities of breath-hold divers are crucial to their ability to exploit prey resources, thereby influencing their role in the ecosystem
Cardiovascular regulation is critical during diving because changes in heart rate and cardiac output affect blood O2 uptake from the lung and the rate and magnitude of blood O2 delivery to tissues
We investigated the dive ƒH response in naturally diving adult California sea lions on maternal foraging trips using a self-contained electrocardiogram (ECG) recorder and a time depth recorder (TDR)
Summary
The diving physiology and capabilities of breath-hold divers are crucial to their ability to exploit prey resources, thereby influencing their role in the ecosystem. The dive capacity of breath-hold divers is dependent on both the available O2 stores and the rate at which the O2 stores are depleted. Cardiovascular regulation is critical during diving because changes in heart rate (ƒH) and cardiac output affect blood O2 uptake from the lung and the rate and magnitude of blood O2 delivery to tissues. Received 16 October 2013; Accepted 15 January 2014 has long been considered central to a decreased rate of O2 consumption in breath-hold divers, and, to their dive capacity. During forced submersion, a severe bradycardia (decrease in ƒH to below resting values) results in the isolation of muscle and peripheral organs from blood flow, thereby conserving blood oxygen for the heart and brain (Scholander, 1940; Scholander et al, 1942; Zapol et al, 1979; Blix et al, 1983). More recent studies on trained and freely diving animals indicate that this ‘dive response’ is variable and often more moderate, with declines in ƒH dependent on dive duration and activity (Andrews et al, 1997; Hindle et al, 2010; Davis and Williams, 2012; Noren et al, 2012)
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