Local adaptation in thermal tolerance for a tropical butterfly across ecotone and rainforest habitats.

Thermal adaptation to habitat variability can determine species vulnerability to environmental change. For example, physiological tolerance to naturally low thermal variation in tropical forests species may alter their vulnerability to climate change impacts, compared with open habitat species. However, the extent to which habitat-specific differences in tolerance derive from within-generation versus across-generation ecological or evolutionary processes are not well characterized. Here we studied thermal tolerance limits of a Central African butterfly (Bicyclus dorothea) across two habitats in Cameroon: a thermally stable tropical forest and the more variable ecotone between rainforest and savanna. Second generation individuals originating from the ecotone, reared under conditions common to both populations, exhibited higher upper thermal limits (CTmax) than individuals originating from forest (∼3°C greater). Lower thermal limits (CTmin) were also slightly lower for the ecotone populations (∼1°C). Our results are suggestive of local adaptation driving habitat-specific differences in thermal tolerance (especially CTmax) that hold across generations. Such habitat-specific thermal limits may be widespread for tropical ectotherms and could affect species vulnerability to environmental change. However, microclimate and within-generation developmental processes (e.g. plasticity) will mediate these differences, and determining the fitness consequences of thermal variation for ecotone and rainforest species will require continued study of both within-generation and across-generation eco-evolutionary processes. This article has an associated First Person interview with the first author of the paper.