Steven R. Waldschmidt’s research while affiliated with University of Wisconsin–Madison and other places

Standard metabolic rates (SMR) are frequently used to estimate the maintenance costs of free-ranging ectotherms. Ecologically relevant sources of variation in the rate of metabolism not accounted for by SMR (e.g. nutritional status, time of day) can significantly affect estimates of the maintenance costs of free-ranging ectotherms. This study reports the effects of temperature, sex, population and time of day on the rate of oxygen consumption of resting Sceloporus merriami. The rate of oxygen consumption of fasted lizards (standard conditions) was strongly dependent on temperature, sex and population, and it was not constant over the inactive period (scotophase). Average rates of oxygen consumption were as much as four times higher than the minimal rate (SMR) for all individuals. Our results indicate that the cost of maintenance of S. merriami calculated using SMR is from two to four times lower than the cost estimated from the average rates of oxygen consumption of fasted lizards. These results have important implications for ecologists interested in comparing lizard productivity with similarly sized endotherms. Rates of oxygen consumption at 32 and 37-degrees-C increased by as much as 37% after lizards were fed 10-15% of their live mass in crickets. This effect was not observed at 25-degrees-C. Because oxygen consumption is not constant over the scotophase and because free-ranging lizards usually have food in their guts, we conclude that when estimating the energy budgets of free-ranging lizards ecologists should use the rate of oxygen consumption averaged over the scotophase of fed animals, not SMR.

In the laboratory, we measured the pulmocutaneous and ocular evaporative water loss (EWL) in dry air from side-blotched lizards, Uta stansburiana, at wind speeds of 0.44 and 0.96 m s⁻¹ and body temperatures of 30 and 38 C. At 38 C, Uta's rate of pulmocutaneous EWL was 34% higher than it was at 30 C. Wind speed, however, had no effect on pulmocutaneous EWL. Wind speed affected the rate of EWL only when lizards had their eyes open. When the lizards opened their eyes at 30 C, the average rate of total EWL increased by 11% at 0.44 m s⁻¹ and by 37% at 0.96 m s⁻¹. Similarly, when lizards opened their eyes at 38 C, the average rate of total EWL increased by 19% at 0.44 m s⁻¹ and by 41% at 0.96 m s⁻¹. We developed a model of ocular EWL based on heat- and mass-transfer equations. In these equations, we assumed that water freely evaporated from the eyes. Because the rate of ocular EWL we calculated was statistically indistinguishable from the values we measured, we conclude that the increased rate of EWL from lizards that had their eyes open was primarily attributable to ocular water loss. We used our model of ocular EWL to predict the rates of ocular EWL for Uta exposed to ecologically realistic values of wind speed and relative humidity. On the basis of these simulations, we predict that in Uta, at a mean selected body temperature of 36 C, ocular EWL will rarely exceed 18% of their pulmocutaneous EWL.

At a site in western Nebraska, we studied the effects of supplemental food and water on growth, survival, and habitat utilization of hatchling iguanid lizards, Sceloporus undulatus. Hatchlings were randomly assigned to one of four experimental groups and received either (1) supplemental food only, (2) supplemental water only, (3) supplemental food and water, or (4) neither supplemental food nor water (control). Neither supplemental food nor supplemental water affected growth rate in length, growth rate in mass, or size-specific body mass (i.e. robustness) at the end of the treatment period. Over the 45 day treatment pierod, there were no significant differences among treatment groups in survival, home-range size, or habitat utilization. There were, however, significant differences in the index of activity. Hatchlings in the treatment groups receiving supplemental water were sighted more often than hatchlings in the control group. We conclude that, during our study, neither food nor water limited growth but that water limited activity.

In laboratory experiments, we determined the effects of body temperature and the quantity of food consumed on the probability of feeding, the passage time of food, and the digestive coefficient in the iguanid lizard Uta stansburiana. Between body temperatures of 20 and 36 C, the probability of eating increased curvilinearly with body temperature but was inversely related to the quantity of food offered. For lizards fed an unrestricted ration, the average consumption rate increased from 0 to 50 mg g⁻¹ day⁻¹ between body temperatures of 20 and 28 C and remained constant between 28 and 36 C. The passage time of food decreased curvilinearly as body temperature increased. For lizards fed an unrestricted daily ration, the mean passage time ranged from 4.6 days at 22 C to 1.2 days at 32 C. For lizards fed a restricted ration, the mean passage time ranged from 9.2 days at 22 C to 1.8 days at 32 C. Body temperature and feeding regime each had a small but significant effect on the digestive coefficient. In the rest...