The paper explores three empirical rules governing the relationships between body size, temperature, and fitness in ectotherms: "Bigger is better," "Hotter is smaller," and "Hotter is better." These rules describe how body size and temperature influence fitness in different ways. "Bigger is better" suggests that larger body size is generally associated with higher fitness. "Hotter is smaller" indicates that higher developmental temperatures result in smaller adult sizes. "Hotter is better" implies that higher optimal temperatures are linked to greater maximal performance. The authors summarize existing evidence supporting these rules, noting strong support for "Bigger is better" and "Hotter is smaller," primarily in terrestrial insects, reptiles, and annual plants. Evidence for "Hotter is better" is more limited but still supports the rule. The rules operate at different levels: "Bigger is better" describes phenotypic variation within populations, "Hotter is smaller" describes phenotypic plasticity of a genotype, and "Hotter is better" describes evolved variation in reaction norms among genotypes or species. The authors propose a path diagram that integrates these rules, showing how temperature affects critical rate processes throughout the life cycle. Adult body size and development time are key traits that influence fitness. The choice of fitness metric (e.g., intrinsic rate of population increase, r, or net reproductive rate, R₀) can affect whether "Hotter is better" is more important than "Bigger is better." The paper also discusses the interactions between the rules, noting that they can sometimes be synergistic or antagonistic. For example, in colder environments, the temperature-size rule and Bergmann's rule may work together, while in warmer environments, they may conflict. The authors conclude that these rules are not mutually exclusive and that understanding their interactions is crucial for predicting evolutionary changes in response to environmental factors. The study highlights the need for further research to fully integrate these rules and understand their implications for ecological and evolutionary processes.The paper explores three empirical rules governing the relationships between body size, temperature, and fitness in ectotherms: "Bigger is better," "Hotter is smaller," and "Hotter is better." These rules describe how body size and temperature influence fitness in different ways. "Bigger is better" suggests that larger body size is generally associated with higher fitness. "Hotter is smaller" indicates that higher developmental temperatures result in smaller adult sizes. "Hotter is better" implies that higher optimal temperatures are linked to greater maximal performance. The authors summarize existing evidence supporting these rules, noting strong support for "Bigger is better" and "Hotter is smaller," primarily in terrestrial insects, reptiles, and annual plants. Evidence for "Hotter is better" is more limited but still supports the rule. The rules operate at different levels: "Bigger is better" describes phenotypic variation within populations, "Hotter is smaller" describes phenotypic plasticity of a genotype, and "Hotter is better" describes evolved variation in reaction norms among genotypes or species. The authors propose a path diagram that integrates these rules, showing how temperature affects critical rate processes throughout the life cycle. Adult body size and development time are key traits that influence fitness. The choice of fitness metric (e.g., intrinsic rate of population increase, r, or net reproductive rate, R₀) can affect whether "Hotter is better" is more important than "Bigger is better." The paper also discusses the interactions between the rules, noting that they can sometimes be synergistic or antagonistic. For example, in colder environments, the temperature-size rule and Bergmann's rule may work together, while in warmer environments, they may conflict. The authors conclude that these rules are not mutually exclusive and that understanding their interactions is crucial for predicting evolutionary changes in response to environmental factors. The study highlights the need for further research to fully integrate these rules and understand their implications for ecological and evolutionary processes.