Dynamic thermal reaction norms and body size oscillations challenge explanations of the temperature-size rule

Background: The temperature-size rule (TSR) describes a decrease in body size with environmental warming. There is little agreement about why the TSR occurs, but potential explanations include that smaller size (1) maintains aerobic scope, (2) is generated by differential responses of development and growth to temperature, and (3) balances the demand for resources with the expected supply. Organism: The ciliate Tetrahymena thermophila. Methods: We grew microcosm populations at three temperatures and measured population density and cell volume for 11 days (c. 20-53 generations depending on temperature). Results: Populations at all temperatures showed typical sigmoidal population growth, but cell volumes oscillated widely. The oscillations reveal a dynamically shifting relationship between cell volume and temperature, such that the TSR was observed only when population sizes stopped growing, while a reverse TSR was observed during the exponential growth phase. Conclusion: The dependence of the TSR on roughly equilibrium conditions challenges the hypotheses that maintaining aerobic scope and differential responses of growth and development to temperature drive the TSR. Reversals of the TSR and oscillations in cell volume are consistent with the idea that balancing resource demand with environmental supply drives body size changes (the supply-demand model), but the oscillations suggest a role for generational lags in achieving an optimal size.