Adult thermoregulatory behaviour does not provide, by itself, an adaptive explanation for the reflectance–climate relationship (Bogert's pattern) in Iberian butterflies

Abstract

Abstract The association of darker, less reflective insect wings with cooler environments (Bogert's rule) is thought to be related to adult thermoregulation, but the adaptive explanation and the implications for sensitivity to climate warming are yet to be tested. We re‐evaluate the pattern for butterflies using finer resolution data than in previous approaches, both geographically and morphologically, and test its correlation with recent evidence of impacts of warming on butterflies. We compared reflectance–climate relationships at different grid sizes, selected the best subset of reflectance measurements and tested the contribution of the species basking mode, the phylogenetic structure of the data and the correlation between reflectance and published abundance or altitudinal shifts. We used standardised RGB (Red, Green, Blue) values from 222 species from the Iberian Peninsula, and regional mean temperature and precipitation data from the study area (10 and 50‐km resolutions) and Europe (50‐km resolution). Correlations between reflectance and temperature increased at finer geographical and morphological resolutions. However, the butterfly basking mode did not improve the statistical explanation of the pattern. Reflectance shows a strong phylogenetic structure, while variance partitioning indicated a poor pure contribution of the climate variables in the reflectance–climate correlation. Overall, mean temperature and precipitation were only modest predictors of butterfly reflectance. No correlation between reflectance and recent abundance or altitudinal shifts was found using the hypothesised best estimates of reflectance. Although significant correlations between butterfly shading and altitudinal shifts were found for two of the reflectance measurements, this is interpreted as weak, probably artifactual evidence on the predictive power of this relationship. The strong phylogenetic pattern of the reflectance and the low fraction of the reflectance measures analysed suggest that tests for alternative explanations are still needed to shed light on the meaning of the colour–environment relationships in butterflies, which probably are of a complex nature. From an adaptive point of view, unravelling the basis of Bogert's pattern in butterflies requires a closer, habitat‐level approach and alternative variables to adult thermoregulatory behaviour to be tested.