Patterns or mechanisms? Bergmann's and Rapoport's rule in moths along an elevational gradient

Bergmann's rule predicts increasing body sizes at higher elevations. The elevational Rapoport's rule predicts an increase of elevational range size with higher elevations. Both rules have often been related to effects of temperature. Larger bodies allow more efficient heat preservation at lower temperature, explaining Bergmann's rule. Higher temperature variability may select for adaptations that allow increased range sizes, explaining Rapoport's rule. The generality of both rules has been challenged and evidence towards explanatory mechanisms has been equivocal. We investigated temperature and its variability as explanations for Bergmann's and Rapoport's rule in moths along an elevation gradient in Switzerland. In particular, we tested for relationships between elevation, temperature and body size across almost 300 species of Macrolepidoptera along a gradient from 600 to 2400 m a.s.l. The gradient was resampled throughout the vegetation season, which allowed assessing temperature effects independently from elevation. We controlled analyses for covariate traits of moths and their phylogeny. We found a positive relationship between body size and elevation, but no link with temperature. Furthermore, there was no positive link between average elevation and elevational range, but there was between temperature variability and elevational range. We conclude that mechanisms other than temperature can lead to increasing body sizes with elevation (supporting Bergmann's pattern, but not the mechanism). Contrary to that, data support the mechanism for Rapoport's rule: high temperature variability is associated with large ranges. However, because temperature variability is not necessarily increasing with elevation, it may not always lead to the geographic pattern predicted.