| Summary: | While diving, harbour seals (Phoca vitulina) manage their oxygen stores through cardiovascular adjustments including bradycardia, a concurrent reduction in cardiac output, and peripheral vasoconstriction. At the surface, post-dive tachycardia facilitates rapid reloading of these oxygen stores. Although harbour seals can tolerate over 20 min of submergence, the majority of their natural dives are only 2 to 6 min and are usually followed by surface intervals that are less than 1 min, so they spend about 80 % of their time at sea submerged. Given that harbour seals meet their ecological needs through repetitive short aerobic dives, I was interested in the functional role, if any, of the cardiovascular dive response during these short dives. During voluntary diving in an 11 m deep tank, the cardiovascular responses to submergence of five captive harbour seals were manipulated using specific pharmacological antagonists, and the effects on diving behaviour were observed. Effects of pharmacological blockade on mean heart rate and on heart rate variability were also examined in order to assess the autonomic control of heart rate during voluntary diving. Heart rate was recorded using subcutaneous electrodes and a custom data logger while diving behaviour was monitored using a video camera. The muscarinic blocker methoctramine blocked diving bradycardia; the α-adrenergic blocker prazosin blocked diving vasoconstriction; and the β-adrenergic blocker metoprolol blocked post-dive tachycardia. Mean heart rate analysis revealed that diving bradycardia is primarily modulated by the vagus, and post-dive tachycardia is due to increased sympathetic stimulation of the heart as well as parasympathetic withdrawal. Results of power spectral analysis of heart rate variability supported the conclusion that the parasympathetic nervous system predominates during diving but not during surface intervals. None of the pharmacological blockers had any effect on average dive or surface interval duration. Mean dive duration ranged from 2.3±0.47 to 2.9±0.10 min (mean±S.E.M.) for all treatments including controls, and mean surface interval duration ranged from 23 to 29 sec. Seals maintained a high percent dive time (83 to 87 %) in all treatments including controls. Thus, harbour seals do not need the dive response during short dives in order to maintain an efficient dive strategy. Science, Faculty of Zoology, Department of Graduate
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