How evolutionary novelties evolve is a major question in evolutionary biology. It is widely accepted that changes in environmental conditions shift the position of selective optima, and advancements in phylogenetic comparative approaches allow the rigorous testing of such correlated transitions. A longstanding question in vertebrate biology has been the evolution of terrestrial life histories in amphibians and here, by investigating African bufonids, we test whether terrestrial modes of reproduction have evolved as adaptations to particular abiotic habitat parameters. We reconstruct and date the most complete species-level molecular phylogeny and estimate ancestral states for reproductive modes. By correlating continuous habitat measurements from remote sensing data and locality records with life-history transitions, we discover that terrestrial modes of reproduction, including viviparity evolved multiple times in this group, most often directly from fully aquatic modes. Terrestrial modes of reproduction are strongly correlated with steep terrain and low availability of accumulated water sources. Evolutionary transitions to terrestrial modes of reproduction occurred synchronously with or after transitions in habitat, and we, therefore, interpret terrestrial breeding as an adaptation to these abiotic conditions, rather than an exaptation that facilitated the colonization of montane habitats.

Chair of Wildlife Ecology and Management University of Freiburg Tennenbacher Straße 4 79106 Freiburg Germany

Department of Environmental Science University of Basel Klingelbergstrasse 27 4056 Basel Switzerland

Department of Life Sciences Natural History Museum London SW7 5BD UK

Department of Life Sciences University of Roehampton London SW15 4JD UK

Dříteč 65 53305 Czech Republic

Ecology Evolution and Developmental Group Department of Wetland Ecology Estación Biológica de Doñana 41092 Sevilla Spain

Institut für Spezielle Zoologie und Evolutionsbiologie mit Phyletischem Museum Friedrich Schiller Universität Jena Erbertstraße 1 07743 Jena Germany

Museum für Naturkunde Berlin Leibniz Institute for Evolution and Biodiversity Science Invalidenstraße 43 10115 Berlin Germany

WSL Swiss Federal Institute for Forest Snow and Landscape Research Birmensdorf Switzerland

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Terrestrial reproduction as an adaptation to steep terrain in African toads