ENERGY COSTS OF GROWTH IN NEONATE REPTILES

Abstract

Do young reptiles avoid paying the metabolic costs of rapid growth? Growing mam- mals and birds must spend extra energy, beyond the metabolic costs they have when not growing, to pay the cost of synthesizing new tissues. This synthesis cost is in addition to the other costs that growing animals pay to forage for, consume, digest and allocate the extra food they need to produce new body substance. Thus, it is reasonable to hypothesize that free-living neonate and juvenile terrestrial vertebrates in general will have basal (standard) and field metabolic rates and feeding rates that are higher than expected for (non-growing) adults of the same body mass. Suckling mammals and neonate precocial birds do have relatively high basal metabolic rates. Among reptiles however, currently available data on standard metabolic rates (SMR) of neonates indicates no sig- nificant increment over adult SMR. In the field, metabolic increments due to the additional growth- associated costs should be relatively large and hence readily detectable in neonate reptiles. To date, the field metabolic rates (FMR's) of neonates and juveniles have been measured in ten species of reptiles: three phrynosomatid lizards, three lacertid lizards, two iguanid lizards, one agamid lizard, and one chelonian. The neonate lizards all had metabolic rates that were similar to those measured simultaneously in free-living adults, after correction for body mass differences (using mass0-80). The chelonian (Gopherus agassizii) juveniles also showed no obvious increment in energy expenditure over adults. More careful measurements of SMR in neonate reptiles should be done to determine if reptiles are free of the large energetic "tax" on growth that is paid by neonate mammals and precocial birds. Studies of neonate altricial birds, which are ectothermic for some time after hatching and have low SMR's, also may be instructive in this regard. Field studies of neonate reptile time- activity budgets are needed to determine how they achieve such low field metabolic rates while simultaneously growing rapidly. Do neonates have lower body temperatures on average during their activity period? Do neonates spend less time active each day than adults? Do neonates use micro- habitat resources in ways that reduce intraspecific competition with adults, but allow for rapid growth anyway? Are the thermal dependencies of digestion, metabolism and growth different in neonates, thereby allowing them to function well and grow rapidly at lower daily integrated body temperatures?