Breathing was associated with an elevated heart rate and apnea with lower rates, the pattern and level of change varying with activity and breathing frequency. Inactive snakes breathing infrequently achieved a low, stable heart rate, called the "apneic heart rate", which is distinct from diving bradycardia. Breathing tachycardia occurred in anticipation of the actual initiation of ventilation. Activity caused an elevation in heart rate during apnea. Resting heart rates were similar to those of other diving reptiles but somewhat lower than those of non-diving species, although comparisons were tentative because of the different methodologies used by different investigators. In the evening, breathing tachycardia was reduced; consequently heart rate differences between breathing and apnea were smaller than during the day. The physiological adaptations of marine snakes to their environment seem to consist of quantitative rather than qualitative departures from the basic reptilian mode.
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