Evidence for Selection on Thermoregulation: Effects of Temperature on Embryo Mortality in the Garter Snake Thamnophis elegans
Despite widespread belief that selection molds thermoregulatory behaviors, direct evidence for fitness effects is extremely rare. We studied the effect of developmental temperature on embryo mortality in a viviparous snake. Seventy-four female Thamnophis elegans were maintained at one of nine constant temperatures during pregnancy (21–33 C). The duration of pregnancy was recorded for each female as well as the sex, snout–vent length, mass, and survival of 504 newborns. Embryo survival was highest at an intermediate temperature (26.6 C). The developmental temperature of maximum survivorship in T. elegans corresponds to the temperature that induces minimum developmental abnormality and the optimum temperature for whole organism performance. These three thermal optima together correspond with the average temperatures imposed on embryos by free-ranging pregnant females (26.6 C). This correspondence implies that thermal optima are coadapted to thermoregulation in T. elegans.Abstract
Mother's mass as a function of snout-to-vent length.Mother's condition index is calculated as the residual from this regression. The regression is described by the equation: Log (mother's mass) = (1.49 ± 0.14SE) + (0.0051 ± 0.0002SE)x, where x is the mother's snout-to-vent length in mm (P < 0.001, R2 = 0.87)
Log-odds of embryo survival as a function of developmental temperature in T. elegans.The numerator and denominator of all odds ratios were increased by one to avoid incalculable ratios. The curve is a quadratic regression fitted to the data; Log-odds of survival = (−31.6 ± 9.8SE) + (2.51 ± 0.71SE)x + (−0.0471 ± 0.0129SE)x2, where x is developmental temperature in degrees C. The dashed lines represent the 95% confidence interval for the best-fit line
Contributor Notes
Department of Zoology, 3029 Cordley Hall, Oregon State University, Corvallis, Oregon 97331. Present address: (RPO) Washington Department of Fish and Wildlife, 600 Capitol Way N, Olympia, Washington 98501. (RPO) tsirtalis@hotmail.com and (SJA) arnolds@science.oregonstate.edu. Send reprint requests to RPO.