How Rearing Temperature Affects Optimal Adult Size in Ectotherms

1. Rearing temperature may affect juvenile mortality, growth and development rates, adult mortality rate, and/or population growth rate. Increased juvenile growth rate may affect the trade-off curve relating fecundity to development period, lowering it and making it less steep. Here we identify the overall effects of temperature on optimal adult size. Larger bodies are favoured by decreased juvenile mortality rate and decreased population growth rate (the sum of these is termed discounted juvenile mortality rate). 2. The effects of the other variables depend on whether environmental heterogeneity is spatial or temporal. Under spatial heterogeneity larger adult bodies are favoured by increased juvenile growth rate but varying adult mortality rate has no effect. Under temporal heterogeneity larger adult bodies are favoured by increased adult mortality rate but the effects of juvenile growth rate depend on the details of the changes in the fecundity-development period trade-off curve. 3. If discounted juvenile mortality rate does not increase at higher temperatures, some of these predictions are hard to reconcile with the finding that higher rearing temperatures generally result in smaller adult bodies. Using available data, we conclude that temperature effects on juvenile growth in spatially heterogeneous environments, and on adult mortality in temporally heterogeneous environments, would generally predict larger rather than smaller size. 4. Predictions involving juvenile mortality and population growth rate are also evaluated using the available data, and mechanisms linking temperature to increased juvenile mortality are discussed. They include increased molecular damage, ectotherm predation and parasitism, drought and oxygen shortage.