Species radiations where a single ancestor diversifies into an array of species can result in explosive diversity in a short time. Species radiations often arise when ancestral populations colonise a new habitat and/or begin to utilise a novel resource. Savannah grasslands represented a novel niche during the Miocene and their colonisation from ancestral forests has resulted in rapid diversification of numerous plants, invertebrates and vertebrates. However, colonising seasonal savannahs from relatively stable ancestral forests is expected to be challenging. Then, how were species able to colonise this novel but challenging environment? Tropical Mycalesina butterflies are an excellent system to investigate this question due to their interesting evolutionary history and ecology.

Butterflies of sub-tribe Mycalesina (Family: Nymphalidae) have undergone spectacular geographic radiations in Africa, Asia and Madagascar comprising some 320 species. Reconstructing ancestral states for habitat preference revealed a striking pattern that Mycalesina species have independently colonised savannahs multiple times across geographic radiations during the late Miocene and Pliocene. This naturally leads to the question posed above: how were these butterflies able to colonise seasonal savannahs from ancestral forests?

Firstly, by studying reproductive strategies using field samples and museum material, I showed that forest species reproduce throughout the year, while savannah species undergo reproductive diapause during the dry season, and that both habitat preference and reproductive strategies evolve in a correlated fashion. I further show that the timing of the evolution of reproductive diapause coincides with the colonisation of savannahs. Secondly, by rearing a replicate pair of forest and savannah species from the African and Malagasy radiation and using published data on species from Asian radiation, I demonstrate that savannah species consistently exhibit a faster pace-of-life than forest species across all radiations. Thus, savannah species in comparison to forest species, lay smaller but more eggs, have faster egg-, larval- and pupal development times, and show reduced adult longevity. Thirdly, by sampling three communities of Mycalesina butterflies spanning both highly seasonal and less seasonal locations, I show that predictability in the precipitation and temperature is important for the maintenance of seasonal polyphenism. Finally, I show that seasonal polyphenism in Mycalesina butterflies is a key innovation.

To finally answer the main question about how Mycalesina butterflies were able to colonise savannahs from forested habitats, I show that adaptations in a suite of phenotypic and life-history traits, which form components of seasonal polyphenism, were crucial