Bugs and Bergmann’s rule: a cross-taxon large-scale study reveals idiosyncratic altitudinal and latitudinal body size patterns for different insect taxa

Bergmann’s rule posits that an organism’s body size or mass increases with decreasing temperature. While generally established in homeotherms, Bergmann’s rule has been largely inconsistent in insects due to limited taxonomic and spatial coverage and variable sampling methods applied to the same taxonomic group. To rectify these shortfalls in tests of Bergmann’s rule in insects, we sampled ants, ichneumonid wasps, carabid beetles, and geometrid moths simultaneously from three locations (representing tropical, subtropical, and subalpine altitudinal gradients) across Yunnan, Southwest China, where temperature and productivity generally decline with latitude. We sought generalities in Bergmann’s rule in insects by investigating whether community-level altitudinal body size patterns within each location were dependent on geographic locations, microhabitats (forest canopy vs understory), focal taxa (insect families), and taxonomic scale (subfamilies within individual families). We found that altitudinal clines, when present, varied across geographic locations; carabids and geometrids showed positive clines in the tropical altitudinal gradient and negative clines in the subtropical and subalpine altitudinal gradients. Understory and canopy geometrids showed similar patterns while different subfamilies showed variable patterns. Carabid and geometrid body size patterns complied with the combined resource allocation model, which posits that the body size varies with resource availability along altitudinal and latitudinal ranges. Overall, our study revealed idiosyncratic altitudinal and latitudinal body size patterns for different insect taxa. Our study also illustrates the value of fully standardized, large-scale studies in revealing generalities (or the lack thereof) in Bergmann’s rule in insects and we suggest incorporating the natural history and biology of target groups to better explain patterns of body size variation.