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

In many ectotherms, incubation temperature has profound effects on the timi ng of hatching and size of hatchlings, but the mechanisms underlying these effects are poorly understood. We studied the energetics of embryonic devel opment and growth in the lizard Sceloporus undulatus. Eggs were incubated a t six constant temperatures, ranging from 28 degrees to 38 degreesC, and em bryonic metabolism, incubation period,and body size at hatching were determ ined. The duration of embryonic development decreased significantly from 55 d at 28 degrees to 40 d at 32 degreesC but did not differ significantly be tween 32 degrees and 34 degreesC. Embryos incubated at temperatures above 3 4 degreesC did not survive to hatching. Metabolic rate at specific stages o f development (percentage of total incubation period) did not differ among embryos incubated at 28 degrees, 30 degrees, and 34 degreesC. As a result, the total amount of energy expended during the incubation period at 28 degr eesC (2.0 kJ) was greater than that at 30 degrees -34 degreesC (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 wit h incubation temperature, and both were the same as those of hatchlings col lected in the field. Thus, there was no apparent , trade-off between hatchi ng 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 g ravid females to examine the possible consequences of female behaviour for hatching phenotypes. In females and at potential nest sites, embryos would experience temperatures that resulted in high mortality in the laboratory e xperiment (>32 degreesC). Gravid females had a field body temperature of 33 .9 degreesC (95% CI = 0.8 degreesC) and selected a body temperature of 33.3 degreesC (95% CI = 1.9 degreesC) when placed in thermal gradients in the l aboratory. Soil temperatures rose above 32 degreesC 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. Se lection of relatively high body temperatures by gravid females, coupled wit h tolerance of acute exposure to relatively high soil temperatures, would r educe the incubation period without a concomitant reduction in body size at hatching.