Thermoregulation in Heliconius butterflies is determined by habitat association and morphology but not phylogeny

Temperature determines butterflies' physiological processes and performance. It varies strongly across time and space, with tropical butterflies experiencing temperatures close to their thermal limits. Climate change with more frequent heat waves may push species closer to these limits. Butterflies are known to thermoregulate, which is influenced by their morphology and behaviour. However, in tropical butterflies, little is known about how thermoregulatory abilities vary across habitats and if morphology or species relatedness are their drivers. We conducted a field and laboratory study on buffering ability-the ability to regulate body temperature with respect to air temperature-, take-off temperature and heating rate of mimetic Heliconius butterflies from different habitats. We tested whether butterfly thermoregulation varies across habitats and morphologies. We also tested whether closely related species exhibit more similar thermoregulatory abilities and explored the importance of behaviour. Mimetic Heliconius pairs are two species that have nearly identical wing coloration and are often in different phylogenetic clades. Studying these pairs allows us to quantify the contribution of morphology and phylogenetic relatedness to thermoregulation. Because mimetic species pairs are often restricted to specific habitats, thermoregulatory abilities can be compared across habitats. We found that butterflies from different habitats differed in their buffering ability, likely due to differences in morphology, such as wing colour. Take-off temperature was lowest in the only high-altitude species we studied, whereas species from lowland habitats did not differ. Live heating rate did not differ across habitats, yet it decreased with iridescence and forewing length. There was no phylogenetic signal in thermoregulatory abilities, indicating that more closely related species did not thermoregulate more similarly than expected by chance. Our results showed that the convergent wing patterns in Heliconius butterflies may contribute to thermoregulatory ability and that thermoregulation should be considered when investigating the habitat specificity of mimetic pairs. Furthermore, Heliconius thermoregulation is not constrained by shared ancestry, suggesting that the morphological traits observed in the different mimetic pairs may be advantageous in certain thermal environmental conditions. Overall, species differences in thermoregulatory abilities may shape how Heliconius distributions respond to climate change-driven temperature shifts.