Heart rates of muskrats diving under simulated field conditions: persistence of the bradycardia response and factors modifying its expression

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Abstract
A marked, rapidly deployed bradycardia accompanied every voluntary dive by muskrats diving under a wide range of simulated field conditions in the laboratory. Telemetered heart rates were typically stable during submergence, with little evidence of postdive tachycardia or anticipatory changes in cardiac frequency prior to onset or termination of spontaneous dives. The extent of bradycardia varied with the type of dive; heart rate was highest during foraging trips and lowest during escape dives provoked by the investigators. Diving heart rate was positively correlated with the predive rate in escape dives but not foraging or exploratory dives. In animals trained to swim an underwater maze separating a simulated lodge from a feeding station, the extent of bradycardia increased with the period of submergence. This relationship was strongest for escape dives and was attributed mainly to a reduction in motor activity with time submerged. Several escape dives more than 3 min in duration were documented in both field and laboratory and all were characterized by prolonged periods of inactivity underwater. Heart rate during exploratory and foraging dives varied positively with both water (4–30 °C) and telemetered abdominal (30–38.4 °C) temperature. Type of dive, body temperature, and water temperature accounted for 31% of the variance in heart rate of diving muskrats. These results suggest that while the classic diving response is largely preserved in freely diving muskrats, the extent of bradycardia varies with the nature of the dive and is subject to cortical and thermal influences.

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