No geographic variation in thermoregulatory colour plasticity and limited variation in heat‐avoidance behaviour in Battus philenor caterpillars

Phenotypic plasticity can help organisms cope with variation in their current environment, including temperature variation, but not all environments are equally variable. In the least variable or extreme environments, plasticity may no longer be used. In this case, the plasticity could be lost altogether, or it could persist with either the same or an altered reaction norm, depending on factors such as the plasticity's costs. In the pipevine swallowtail caterpillar (Battus philenor), I tested for changes in two forms of heat‐avoidance plasticity, colour change and refuge‐seeking behaviour, across the species’ range in the United states, including the cooler eastern parts of its range where colour change has not been observed and is unlikely to be needed. I found that both heat‐avoidance behaviour and colour change persisted in all surveyed populations. Indeed, the reaction norm for colour change remained nearly unaltered, whereas the threshold for refuge‐seeking only changed slightly across populations. These results suggest that the costs of these plastic traits are low enough for them to be maintained by whatever minimal gene flow the population receives. I show that plasticity can be maintained unaltered in populations where it is not used and discuss the potential consequences of this persistence for both the ecology and evolution of plasticity.