Seasonal plasticity and life-cycle adaptations in butterflies

Abstract

Plasticity is a major component of phenotypic variation, interrelated with genetic differentiation through the existence of genetic variation in reaction norms. Phenotypic plasticity also makes experimental tests of life-history theory possible, because responses to the environment can be tested against predictions. I report results from studies on life-cycle regulation and seasonal plasticity in life-history adaptations in temperate butterflies, especially Polygonia c-album, Pararge aegeria and Pieris napi. The optimal development time of individuals should vary within and among populations, and life-history theory would expect size differences to follow as a consequence. Predictions regarding variation in development time and size were tested using “split-brood” techniques to obtain population-level estimates of individual reaction norms. In all three species, individuals destined for the diapause pathway took longer to develop than those in the same brood destined for reproduction the same season, and females took longer than males (as predicted by protandry theory). P. aegeria and P. c-album also varied development time within pathways, in a quantitative response to photoperiods indicating the progression of the season. In all three species, variation in development time was mostly a result of plasticity in rates of growth and development, and size at emergence therefore typically was not strongly affected. I conclude that it is often profitable to view life histories and developmental pathways as “chosen” by the insect rather than passively given by the environment, and hence to apply the methods of behavioural ecology: experimental techniques and optimality theory.