Thermal effects on the energetics of lizard embryos: Implications for hatchling phenotypes

In many ectotherms, incubation temperature has profound effects on the timing of hatching and size of hatchlings, but the mechanisms underlying these effects are poorly understood. We studied the energetics of embryonic development and growth in the lizard Sceloporus undulatus. Eggs were incubated at six constant temperatures, ranging from 28°to 38°C, and embryonic metabolism, incubation period, and body size at hatching were determined. The duration of embryonic development decreased significantly from 55 d at 28°to 40 d at 32°C but did not differ significantly between 32° and 34°C. Embryos incubated at temperatures above 34°C did not survive to hatching. Metabolic rate at specific stages of development (percentage of total incubation period) did not differ among embryos incubated at 28°, 30°, and 34°C. As a result, the total amount of energy expended during the incubation period at 28°C (2.0 kJ) was greater than that at 30°-34°C (1.7-1.8 kJ). However, the difference in energy expenditure did not affect body size at hatching; neither snout-vent length nor body mass varied significantly with incubation temperature, and both were the same as those of hatchlings collected in the field. Thus, there was no apparent trade-off between hatching date and body size of lizards at hatching. In a natural population in New Jersey, USA, we quantified soil temperatures at potential nesting sites and studied the thermoregulatory behavior of gravid females to examine the possible consequences of female behavior for hatchling phenotypes. In females and at potential nest sites, embryos would experience temperatures that resulted in high mortality in the laboratory experiment (>32°C). Gravid females had a field body temperature of 33.9°C (95% CI = 0.8°C) and selected a body temperature of 33.3°C (95% CI = 1.0°C) when placed in thermal gradients in the laboratory. Soil temperatures rose above 32°C for several hours each day. Embryos must be able to survive intermittent exposure to temperatures that were lethal under conditions of chronic exposure in the laboratory. Selection of relatively high body temperatures by gravid females, coupled with tolerance of acute exposure to relatively high soil temperatures, would reduce the incubation period without a concomitant reduction in body size at hatching.