Laboratory and field studies of the energy expenditure of endotherms: A comparison.

Field energy expenditures of endotherms have been estimated by combining laboratory measurements of energy expenditure with field time budgets and by direct measurements in the field through the use of doubly-labelled water. Too much variation occurs in field expenditures to rely on scaling alone to predict energy expenditure without losing the unique characteristics of distinctive species. Field expenditures are most effectively analysed when combined with measurements of the same species under 'standard' conditions in the laboratory. The effect of body size on the expenditures associated with maintenance, locomotion and reproduction are also most clearly established in the laboratory.