The body size and fitness match and its variability in plastic response to temperature

The ability of organisms to respond plastically to environmental change remains one of the most challenging areas of ecological studies. Reasons for this include the complex nature of environmental cues and responses at the organismal level, the costs that organisms face in performing phenotypic plasticity under different conditions, and the individual capacity to respond, which depends on many internal and external factors. The special case in this regard is an example of the plastic body size response to temperature, known as the Temperature-Size Rule (TSR). We used five clonal and three multiclonal experimental populations of the Lecane inermis rotifer. We measured two traits over a wide thermal range: body size and population growth rate r . We investigated (i) the thermal conditions under which different experimental populations perform the TSR or canalize (= no plasticity) their body size, (ii) how this response relates to a direct measure of fitness, and (iii) whether this response varies with the thermal preferences of the studied organisms. 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. Additionally, we found a relatively high repeatability of thermal tolerance in experimental populations in comparison with the previous studies. Finally, we suggest the role of the level of genetic composition within a population in its ability to respond plastically to temperature. This is the first study to describe in detail the special case of plasticity versus canalization, for body size response to optimal and suboptimal temperatures, in organisms that differ in their tolerance to temperature.