There is substantial behavioural and physiological evidence to suggest that most feeding dives by aquatic birds and mammals are aerobic in nature, with no net production of lactate. Any increase in lactate production is matched by increased removal. This does not mean, however, that there are no cardiovascular adjustments associated with such dives. Nonactive parts of the body (including the large pectoral muscles in diving ducks) may be hypoperfused and consume oxygen at a reduced rate. For example, in marine mammals, such as the Weddell seal, reduced perfusion of the gut during a feeding period (which can last for up to 12 h) would reduce the energy expenditure associated with the digestion and assimilation of food (specific dynamic action). Reperfusion during the nonfeeding period would contribute to an unusually high "resting" oxygen uptake. Although some tissues in seals at least can tolerate periods of ischaemia, there is no evidence to suggest that enhanced anaerobic production of ATP is a key factor in the survival of marine mammals during unusually long periods underwater. There may, in fact, be an overall reduction in the ATP requirements of certain tissues, possibly as a result of a reduction in the permeability of cell membranes to some ions, but most certainly as a result of reduced body temperature. During relatively long dives, lactate production eventually exceeds its rate of removal and it accumulates. Precisely what occurs in the muscles is not known. One suggestion is that periods of vasoconstriction are interrupted by vasodilatation, when the oxygen stores are replaced.
