Widespread butterflies follow inconsistent trends in the Bergmann's rule and flight morphometry: Implications for conservation in the Western Himalaya

Abstract

The Bergmann's rule relating eco-geographic pattern in body size of species with latitude/elevation, remain unexplored among different animal taxa including insects from the Indian Himalayan Region that ranges in elevation and environmental conditions. The applicability of the rule was tested using body size data set of natural adult male populations of six butterfly species that were common in abundance and occur over a range of elevations in the Western Himalaya. The influence of changes in body size on flight related morphometry was also explored to understand the adaptative and dispersal strategies of the species across elevations. Among the various morphometric traits, the principal component analysis revealed that the forewing length was the most reliable measure of determining the body size clinal pattern in butterflies. Species Danaus chrysippus, Danaus genutia and Vanessa indica strongly followed the Bergmann's rule, while Papilio polytes followed the converse Bergmann's rule. By contrast, Pieris brassicae and Catopsilia pomona exhibited a moderate clinal pattern and partially followed the converse of the rule. The flight related morphometric indices namely, wing load and aspect ratio varied among species and showed no regular pattern along elevational gradients. The results suggested inconsistent trends in intra-specific morphometric variations of widespread butterfly species, crucial to comprehend their conservation strategies in the Western Himalaya.