CORRELATES OF AVERAGE DAILY METABOLISM OF FIELD-ACTIVE ZEBRA-TAILED LIZARDS (CALLISAURUS DRACONOIDES)

The extent of variation in reptile field metabolism, and its causal ba ses, are poorly understood. We studied the energetics of the insectivo rous lizard Callisaurus draconoides at a site in the California Desert (Desert Center) and at a site at the southern tip of the Baja Peninsu la (Cabo San Lucas; hereafter, Cabo). Reproducing Callisaurus were sma ller at Cabo than at Desert Center. The allometry of metabolism with b ody mass can account for most differences in whole-animal metabolism. There was no significant effect of sex or source population on mass-ad justed metabolic rate in the laboratory (resting metabolism, measured by closed-system respirometry) or in the field (field metabolism, meas ured with doubly labeled water). The mass-adjusted resting metabolism and field metabolism of gravid females and the field metabolism of juv enile lizards were not significantly different from those of nonreprod uctive adults. Temperature had a significant effect on resting metabol ism (Q(10) = 2.7); fed lizards had resting metabolism that was 22% hig her than that of fasted lizards; field metabolism was positively corre lated with growth rate in juveniles; and field metabolism of adults in creased from spring to late summer at Desert Center by 25%, probably b ecause of longer activity period length and slightly higher activity p eriod body temperature. We calculated from water influx and field meta bolism that juveniles allocated 18% of their metabolizable energy inta ke to growth and that most energy deposited into eggs was transferred from energy stores rather than ingested in the weeks prior to laying.