Energetics of the Lizard Cnemidophorus Tigris and Life History Consequences of Food-Acquisition Mode

Energy budgets for the wide-foraging "arthropodivorous" lizard Cnemidophorus tigris were constructed for the reproductive season using doubly labeled water measurements of field metabolic rate (FMR). Rates of body mass change, clutch sizes, and clutch intervals were also investigated. FMRs of both sexes (males, 298 J g-I d-1; females, 247 J g-I d-1) were greater in the reproductive season than during the postreproductive season. This was not due to differences in resting metabolism, but, instead, was due to higher activity costs during the reproductive season. Although males had signficantly higher FMR than females, males and females had similar feeding rates (as reflected by water influx rates). The ratio of energy intake to expenditure was higher in females than in males. Females produced eggs but did not grow. Large males did not grow, but small 1 st-yr males similar in size to females did grow. Females laid at least two consecutive clutches during a single reproductive season; clutch interval was -25 d. Clutch size varied with time of laying (first or second clutch), female body size, and year. We compare the reproductive energetics of C. tigris and other wide-foraging Cnemidophorus with the energetics of ambush iguanids. The wide forager C. tigris and the ambusher Uta stansburiana apparently do not differ in the proportion of the energy budget devoted to reproductive production (reproductive ffort, REP). But total reproductive ffort, REt, which includes metabolism associated with reproduction, is much lower in Cnemidophorus tigris. Review of the literature indicates Cnemidophorus tend to lay smaller clutches and larger eggs than iguanids, but the number of eggs laid per unit time generally equals that of most iguanids. Cnemidophorus also deposit energy into their eggs at rates almost 60% higher than ambush iguanids. We consider higher rates of production in wide foragers to be permitted by their higher rates of net energy intake while foraging, compared with most ambushers. These higher rates of production in Cnemidophorus apparently allow them either to reach a larger size than iguanids of similar age at first reproduction or to be younger than iguanids of the same size at first reproduction. Life history characters may be influenced by differences in rates of production associated with different food-acquisition modes.