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The Journal of Experimental Biology 204, 2127-2132 (2001)
© 2001 The Company of Biologists Limited

Postprandial exercise: prioritization or additivity of the metabolic responses?

Albert F. Bennett^* and James W. Hicks

Department of Ecology and Evolutionary Biology, University of California at Irvine, Irvine, CA 92697, USA

*e-mail: abennett{at}ci.edu

Accepted March 28, 2001

Monitor lizards (Varanus exanthematicus) were used to examine the prioritization or additivity of the metabolic responses associated with exercise and digestion, either of which can elevate metabolic rate independently. Rates of oxygen consumption (._O2) and ventilation (._E) were measured in lizards during fasting exercise, postprandial rest and postprandial exercise. In fasting animals, ._O2 increased with walking speed to a maximal value of 15.9mlO₂kg^-1min^-1 at 1.25kmh^-1. Postprandial resting metabolic rate was elevated significantly above fasting levels (4.1 versus 2.0mlO₂kg^-1min^-1). During postprandial exercise, ._O2 increased to a maximal value of 18.8mlO₂kg^-1min^-1 at 1.25kmh^-1. At every level of exercise, ._O2 was significantly higher in postprandial animals by a similar increment; the maximal rate of oxygen consumption was significantly increased by 18% in postprandial individuals. Maximal ._E did not differ in fasting and postprandial animals and, therefore, the greater ._O2max of postprandial animals cannot be attributed to a higher ventilation rate. Air convection requirement (._E/._O2) is significantly lower in postprandial animals at rest and at all levels of exercise, indicating a relative hypoventilation and increased pulmonary oxygen extraction efficiency. We suggest that this increased oxygen extraction may be due to decreased cardiopulmonary shunts and/or to lower mixed venous oxygen content. The data unequivocally support an additivity model rather than prioritization models for the allocation of elevated metabolic rate: the postprandial metabolic increment is not suspended during exercise, but rather is added onto the cost of exercise. It is clear that fasting exercise did not elicit truly maximal levels of cardiopulmonary oxygen transport in these animals, indicating problems for design models that make this assumption.

Key words: activity, digestion, exercise, lizard, maximal aerobic speed, maximal rate of oxygen consumption, oxygen consumption, postprandial, prioritization, reptile, specific dynamic action, symmorphosis, Varanus exanthematicus, ventilation.

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