Deep computer vision reveals the mediation of thermal-melanism and body size by precipitation and sex in a threatened alpine butterfly, Parnassius smintheus

Abstract

Insect morphologies are strongly tied to selective forces, yet due to variation in these forces and finite resources, insects must strategically invest in select morphologies while deprioritizing others. Melanism and body size may be one such potential trade-off that insects navigate and these two factors are important for fecundity, dispersal, thermoregulation, anti-desiccation, and immunity. In this work, we examined how sex and environmental factors mediate a potential body-size/melanism trade-off in the cold-adapted butterfly Parnassius smintheus (Lepidoptera: Papilionidae). We used deep computer vision approaches and museum specimen photography to process over 1,000 images of the species. We found that body-size and melanism are strongly mediated by temperature and elevation (thermal-melanism hypothesis) and that precipitation mediates these slopes for males and females differently. Notably, under the wettest conditions, females exhibit stronger concordance with the thermal-melanism hypothesis while the relationship for males is inverted, suggesting increased competition among males in cold-wet environments. Our results highlight the importance of considering sex when examining how the environment influences intraspecific morphological variation, especially under projected scenarios of global climate change.