Closed Systems: Resolving Potentially Conflicting Demands of Diving and Exercise in Marine Mammals

Abstract

Since terrestrial mammals usually have continuous access to air while exercising, most investigations on the oxygen demands of exercise have looked at the physiological or biochemical limits to oxygen transport and utilization. Recently there has also been a substantial interest in exercise at altitudes where oxygen supplies start to become limiting (Sutton et al. 1982). However, there have been only a few studies of mammalian exercise under the extreme conditions of complete inaccessibility to oxygen: marine mammals swim, hunt, and work underwater without access to air — they must carry their oxygen supplies with them while diving. It was quickly realized that diving mammals possess two major adaptations that allow such underwater work: increased oxygen storage capacity relative to terrestrial mammals and the ability to utilize a “diving reflex” — a physiological response that shunts the oxygen-enriched blood away from the peripheral organs towards the primary aerobic tissues such as the heart, lung, and brain (Irving 1939; Scholander 1940). In recent years it has been found that this reflex is actually a response of variable nature and tends to be maximized under laboratory diving conditions and minimal during natural diving within the animals aerobic dive limits (Hill et al. 1984; Kooyman et al. 1980, 1983). However, as more information on natural diving becomes available another interesting controversy has arisen: the requirements for diving, which include bradycardia, reduced metabolism and some peripheral vasoconstriction, conflict with the standard physiological adjustments seen in aerobically exercising terrestrial mammals.