1. The ‘temperature-size rule’ (TSR) is a widely observed phenomenon within ectothermic species: individuals reared at lower temperatures grow more slowly, but are larger as adults than individuals reared at warmer temperatures. Although the TSR is common and of widespread ecological importance, it is not known whether there is a general physiological mechanism causing the TSR or even if species share a similar pattern of thermal response across ontogeny.
2. We constructed a conceptual model to show that binary division forces growth (g) and development (D) rates to return to a fixed ratio in unicellular organisms exposed to a change in temperature. After a period of decoupling during thermal acclimation, these rates must be restored to maintain a fixed ratio of adult:progeny size. However, the relationship between adult and progeny size need not be fixed in multicellular organisms at different temperatures, and hence growth and development rates need not have a fixed ratio either.
3. We conducted a meta-analysis on data of metazoan ontogenetic responses to temperature which demonstrates that adult size shows a much stronger temperature–size response than progeny size, and reveals variation in size response among other life cycle phases.
4. This study shows fundamental differences in the operation of the TSR in unicellular and multicellular organisms, suggesting that a general physiological mechanism causing the TSR is unlikely. Our findings also reveal the value of analysing shifts in size through the life cycle and across generations: these will yield a more complete quantitative description of how, and potentially provide clues to why, body size responds to temperature.