Adult Size in Ectotherms: Temperature Effects on Growth and Differentiation

Abstract

A proximate, biophysical model is proposed describing temperature-modulated variation in growth rate and differentiation rate in ectotherms, based upon the Sharpe-Schoolfield equation connecting enzyme kinetics and biological rates. Like the Sharpe-Schoolfield equation, the model assumes (1) that growth rate and differentiation rate can be described as controlled by one rate-limiting enzyme; in addition the model assumes (2) that the temperature coefficients of growth and differentiation are different. The model is used to predict temperature-dependent size variation of ectotherms at maturation as a result of the interaction of growth and differentiation. It is shown that the difference between the activation energy constants of growth and differentiation determines the slope of the size-temperature reaction norm. The structural and heritable variation in enzymes determines reaction norm shape without inferring regulatory genes. All thermodynamic parameters of the Sharpe-Schoolfield equation can be estimated empirically with non-linear regression techniques. The biophysical model provides a proximate framework for genotypic models of reaction norm evolution; genetic variation in either growth or differentiation would lead to genotype by environment interaction. This proximate model of temperature sensitivity and temperature tolerance clarifies how temperature dependence of body size would evolve.