Evol Ecol Res 10: 251-268 (2008)     Full PDF if your library subscribes.

Size, temperature, and fitness: three rules

Joel G. Kingsolver1* and Raymond B. Huey2

1Department of Biology, University of North Carolina, Chapel Hill, NC 27599 and 2Department of Biology, University of Washington, Seattle, WA 98195, USA

Author to whom all correspondence should be addressed.
e-mail: jgking@bio.unc.edu


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 available information supports the rule. Analyses of counterexamples are particularly instructive. The rules operate at different levels. Bigger is better describes phenotypic variation within populations. Hotter is smaller describes phenotypic plasticity of a genotype. Hotter is better describes evolved variation in reaction norms among genotypes or between species.

Conclusions: We unify these three rules into a path diagram that describes how temperature impacts critical rate processes throughout the life cycle. Adult body size and development time are key traits that are not only consequences of temperature-dependent processes, but also are causes of variation in fitness. An unresolved issue involves how to determine the appropriate fitness metric for a particular ecological context (population and environment). For example, the intrinsic rate of population increase (r) is strongly influenced by generation time (and development time), whereas net reproductive rate (R0) is strongly influenced by fecundity (and size). Because the relative strengths of different paths contributing to fitness change differ for these fitness metrics, the choice of metric can affect whether Hotter is better is ‘better’ than Bigger is better.

Keywords: body size, development time, fitness metrics, haiku, phenotypic plasticity, reaction norm, temperature.

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        © 2008 Joel G. Kingsolver. All EER articles are copyrighted by their authors. All authors endorse, permit and license Evolutionary Ecology Ltd. to grant its subscribing institutions/libraries the copying privileges specified below without additional consideration or payment to them or to Evolutionary Ecology, Ltd. These endorsements, in writing, are on file in the office of Evolutionary Ecology, Ltd. Consult authors for permission to use any portion of their work in derivative works, compilations or to distribute their work in any commercial manner.

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