Local adaptations in wing-pattern and life history trait plasticity in a butterfly: humidity as a cue where temperature is unreliable

Many butterflies have wet and dry season morphs with large and small wing eyespots respectively. Eyespot size plasticity is adaptive because the morphs function to avoid predation in their respective season. Eyespot size has been shown to be regulated by rearing temperature in many species. However, temperature is unreliable in some regions because it poorly predicts seasons, and other cues such as humidity may be more reliable. We investigated inter-population differences in cue use for eyespot plasticity in the butterfly Melanitis leda. We reared butterflies from three Indian populations under combinations of temperature and humidity. Butterflies from a population (Vithura) where humidity differentiates seasons but temperature does not, responded only to humidity. Butterflies from a population (Tirunelveli) where seasonality is not distinct, and where temperature and humidity are both unreliable, also responded only to humidity. Butterflies from another population (Coimbatore) where seasonality is not distinct, but where temperature has the highest intra-annual variation, responded only to temperature. This suggests local adaptation in cue use. Life-history traits also differed among populations, with the two populations from more arid regions developing faster and attaining larger body sizes than the one from the humid region. Fast development may be adaptive in dry regions where suitable host plants are available only briefly, while large body size may confer desiccation resistance. We show for the first time that humidity can regulate eyespot size, and that reaction norms vary across populations, fitting to regional climates.