Heart rates and diving behavior of leatherback sea turtles (Dermochelys coriacea) were monitored at sea during the internesting interval. Instruments that recorded the electrocardiogram and the depth and duration of dives were deployed on six female leatherback turtles as they laid eggs at Playa Grande, Costa Rica. Turtles dived continually for the majority of the internesting interval and spent 57–68 % of the time at sea submerged. Mean dive depth was 19+/−1 m (mean +/− s.d.) and the mean dive duration was 7.4+/−0.6 min. Heart rate declined immediately upon submergence and continued to fall during descent. All turtles showed an increase in heart rate before surfacing. The mean heart rate during dives of 17.4+/−0.9 beats min-1 (mean +/− s.d.) was significantly lower than the mean heart rate at the surface of 24.9+/−1.3 beats min-1 (P<0.05). Instantaneous heart rates as low as 1.05 beats min-1 were recorded during a 34 min dive. The mean heart rate over the entire dive cycle (dive + succeeding surface interval; 19.4+/−1.3 beats min-1) was more similar to the heart rate during diving than to the heart rate at the surface. Although dive and surface heart rates were significantly different from each other, heart rates during diving were 70 % of heart rates at the surface, showing that leatherback turtles do not experience a dramatic bradycardia during routine diving.

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