Evolutionary history, not ecogeographic rules, explains size variation of tropical insects along elevational gradients

Abstract

One of the best-known biogeographic rules for ectotherms is the temperature-size rule, which asserts that ectotherms produce smaller adults at warmer temperatures. Although this is often true, it has become clear that there is no single process behind the pattern and many exceptions to the rule. To disentangle such complex temperature–size relationships, individual clades must be examined at ecological and evolutionary scales. We examined temperature–size relationships for 2,106 individuals from 64 populations and 40 species of Cephaloleia rolled-leaf beetles (Chrysomelidae; Cassidinae) occurring along two tropical elevational gradients: Barva and the Talamanca Cordillera in Costa Rica, Central America. We tested whether the temperature-size rule applied to interspecific elevational assemblages, intraspecific elevational populations or different rearing temperatures for individual populations. At the interspecific scale, evolutionary history, rather than elevation, explains body size. At the intraspecific scale, only one of seven species followed the temperature–size rule across elevations. When larvae were reared at different temperatures, only one of five populations followed the temperature–size rule. Most populations grew to a fixed size regardless of temperature. Size in Cephaloleia beetles is constrained by their evolutionary history and responds to factors that rarely correlate with temperature. As temperature increases, ectotherms will not universally shrink, but determining if and why their size will change will require further investigation. A free Plain Language Summary can be found within the Supporting Information of this article.