Size, temperature, and fitness: three rules

Question: Associations of body size and of body temperature with fitness have complex relationships for ectotherms, but three general patterns are known. Bigger is better: Larger body size is frequently associated with greater fitness within populations. Hotter is smaller: Smaller adult body sizes typically result from development at higher temperatures. Hotter is better: Greater maximal performance at the optimal temperature is frequently associated with higher optimal temperatures. How do we – or even can we – reconcile these three apparently conflicting empirical patterns about temperature, size, and fitness of ectotherms? Methods: We summarize available evidence supporting or contradicting these three rules. We present a conceptual framework that describes how developmental and adult body temperatures affect causal connections among size, performance, and key components of fitness. Findings: There is strong empirical support for Bigger is better and Hotter is smaller (≥79% of studies/estimates), primarily for terrestrial insects, reptiles, and annual plants. Evidence regarding Hotter is better is still limited (and primarily from terrestrial insects), but most