Butterflies with low thermoregulatory capacity show greatest upwards range shifts along an elevational gradient.

Climate change is a growing threat to global biodiversity, and so there is a pressing need to understand which traits impact species vulnerability, and the capacity of these traits to respond to environmental change. I quantify two thermal traits (thermoregulation, thermal tolerance) and functional traits (wing length, colouration, wing condition, sex) of butterflies across an elevational gradient in central Europe to investigate adaptation to local climatic conditions. There is no evidence of intraspecific variation in thermal traits, suggesting that these may be fixed within species. At low elevations, large species are better than small species at avoiding high body temperatures. Dark species have improved thermoregulatory capacities with decreasing elevation but ultimately have consistently higher body temperatures than pale species. This implies that small and dark species may be particularly vulnerable to extreme heat. I subsequently detect shifts from small dark species at high elevations to large pale species at low elevations. Finally, species with poor thermoregulatory capacity or larger wings have shown greater upwards elevational range shifts. This argues that butterflies are responding to climate change through redistribution of species rather than adaptation in place, and that thermoregulatory performance will be a key driver of ecological responses to climate change.