ANGILLETTA, M.J.*; LEE, V.; SILVA, A.C.: How Do Embryos Incubated at Higher Temperatures Spend Less Energy During Incubation?

In some reptiles, the energetic cost of development remains the same or even decreases with increased incubation temperature. The surprisingly low energy expenditure of embryos at high incubation temperatures may be caused by either of two mechanisms: 1) a shorter incubation periods coupled with a relatively low thermal sensitivity of metabolic rate, or 2) acclimation of metabolic rate to incubation temperature. In the eastern fence lizard, Sceloporus undulatus, incubation period is shortened by 50% when incubation temperature is increased by 10 C. Consequently, embryos must have a Q₁₀ of 2.0 or lower to stabilize the energetic cost of development . Otherwise, thermal acclimation of metabolic rate must occur during development. We quantified the thermal sensitivity of embryonic metabolism in S. undulatus with repeated measures of metabolic rate at 27, 31, and 34 C. Similar to adult reptiles, metabolic rate of embryos increased significantly with increased body temperature, from 1.5 J h^-1 at 27 C to 2.2 J h^-1 at 34 C. The Q₁₀ of metabolic rate from 27 to 31 C was only 1.6, which is much lower than Q₁₀s of 2.0 to 3.0 that have been observed in adults. However, the Q₁₀ of metabolic rate from 31 to 34 C was 2.7. Therefore, thermal acclimation must play a role in reducing the energetic cost of development at incubation temperatures that are higher than 31 C. We found that embryos did acclimate to incubation temperature; embryos incubated at 32 C had a lower metabolic rate than embryos incubated at 28 C, when both were measured at 30 C. Both the low thermal sensitivity and thermal acclimation of metabolic rate play a role in stabilizing the energetic cost of development in S. undulatus.