The temperature-size rule (TSR) describes a decrease in body size with increasing rearing temperature. The TSR has been observed for a wide variety of ectothermic organisms, but we lack a mechanistic understanding of how it arises. Current hypotheses suggest that the TSR arises to maintain access to oxygen or from asymmetries in the temperature-dependence of development time and growth rate. I propose a general model of body size evolution and plasticity that explains the TSR as an adaptive response to changing metabolic demand under a resource supply constraint. Unlike other theories, the model – known as the supply-demand model – makes quantitative predictions about correlated changes in metabolic rate and body size with temperature. The supply-demand model predicts that under a resource supply constraint S, organisms must trade mass-specific resource demand D against size M to maintain complete use of the supply. This generates a ‘rate-size’ trade-off whereby an increase in demand caused by higher temperatures forces a drop in size. Because demand increases with size according to the metabolic scaling exponent b, the model predicts that mass-specific resource demand and body size will be correlated with a slope of -1/b.
Results/Conclusions
In this study, I confronted supply-demand model predictions with experimental data on the protist Actinosphaerium reared under a range of temperatures (20-28C). Importantly, the supply of food was controlled across replicates and treatments to create a match to the theory. In protists, b~1.06, so I predicted that with warming, cell size and mass-specific resource use would covary with a slope of -0.94. As expected, cell volume of Actinosphaerium decreased with temperature (~15% per degree C) and mass-specific biomass production rates increased with temperature. The slope of the relationship (-0.91) was almost identical to the predicted slope, providing strong support for the model. The supply-demand model provides a powerful and simple explanation for the TSR. This is important because understanding the TSR and being able to predict it with simple theory is urgent given the forecasts of global climate change.