Phenotypic plasticity in tropical butterflies is linked to climatic seasonality on a macroevolutionary scale

<p>Phenotypic plasticity can be adaptive in fluctuating environments by providing rapid environment-phenotype matching and this applies particularly in seasonal environments. African <em>Bicyclus</em> butterflies have repeatedly colonized seasonal savannahs from ancestral forests around the Late Miocene and many species now exhibit seasonal polyphenism. On a macroevolutionary scale, it can be expected that savannah species will exhibit higher plasticity due to them experiencing stronger environmental seasonality than forest species. We quantified seasonality using environmental niche modelling, and surveyed the degree of plasticity in a key wing pattern element (eyespot size) using museum specimens. We show that species occurring in highly seasonal environments display strong plasticity, while species in less seasonal or aseasonal environments exhibit surprisingly variable degrees of plasticity, including strong to no plasticity. Furthermore, eyespot size plasticity has a moderate phylogenetic signal and the ancestral <em>Bicyclus</em> likely exhibited some degree of plasticity. We propose hypotheses to explain the range of plasticity patterns seen in less seasonal environments, and generate testable predictions for the evolution of plasticity in <em>Bicyclus</em>. Our study provides one of the most compelling cases showing links between seasonality and phenotypic plasticity on a macroevolutionary scale and the potential role of plasticity in facilitating the colonization of novel environments.</p>