The Body Size and Fitness Match and Its Variability in Plastic Response to Temperature

The evolution of the plastic response of organisms to environmental change remains one of the most challenging areas of biological research. Reasons for this include the complex nature of environmental cues and organismal responses, the energetic costs behind phenotypic plasticity performed under different conditions, and the individual capacity to respond, which depends on many developmental factors. A special case is the plastic body size response to temperature, the temperature‐size rule (TSR). We used eight experimental populations of the rotifer Lecane inermis and measured body size and population growth rate r over a wide thermal range to investigate (i) the thermal conditions under which rotifers perform the TSR or canalize their body size (= no plasticity) and how this relates to fitness, and (ii) whether this response varies with organismal thermal preferences. We found a relationship between body size and fitness, confirming that the TSR is only performed within a certain thermal range, beyond which body size is canalized. We did not find the expected relationship between the strength of the TSR and the range of thermal tolerance, but our results do not allow us to reject the existence of such a relationship. Furthermore, we found a high repeatability of the parameters informing thermal tolerance compared to previous studies, reflecting a substantial degree of developmental constancy in the context of the organism's preference for temperature. We describe the special case of plasticity versus canalization for body size response to optimal and suboptimal temperatures in organisms that differ in their thermal tolerance.