Energy Expenditure of a Large Herbivorous Lizard

Abstract

The total daily (24 hr) energy expenditure of the large (mean weight 240 g) herbivorous lizard Egernia cunninghami has been calculated, and partitioned into the daily costs of maintenance, food-gathering, and thermoregulatory activity. Energy expenditure was calculated from equations relating oxygen consumption rate to activity, body weight and body temperatures. Duration of activity was recorded for lizards of known weight observed continuously during daylight hours in the field. The daily cycle of body temperature of each observed lizard was assembled from the normal activity temperatures, heating and cooling rates and retreat temperatures. Although the weight-relative energy costs of maintenance and activity are much smaller for E. cunninghami than the 4 g insectivore Uta stansburiana, the cost per individual is considerably greater for the larger lizard. However, E. cunninghami is active for a much shorter portion of its time abroad (< 8%, < 25 min day-1) than U. stansburiana (30%, 235 min day-1) and uses less of the total daily energy expenditure in activity (10% cf. 40%). The average duration of thermoregulatory movement was 2.6 min for E. cunninghami cf. at least 78 min for U. stansburiana. Little time is spent by E. cunninghami in foraging for the leaves and flowers of legumes; food-gathering effectiveness, the percentage of metabolized energy used for activities other than food-gathering, is estimated between 91 and 95%. It is suggested that the paucity of large carnivorous lizards results from the prohibitive costs of gathering vertebrate prey.