Evolution of the anuran foam nest: Trait conservatism and lineage diversification

Key innovations are organismal traits that trigger adaptive radiation and lineage diversification. The wide range of anuran reproductive strategies from aquatic to terrestrial modes are potential key innovations. One such strategy is the foam nest, a structure with multiple functions that originated independently several times in different continents. Here, we explore several evolutionary aspects associated with the foam nest in multiple lineages to test the hypothesis that this trait is phylogenetically conserved and has influenced diversification. We used the most inclusive anuran phylogeny to date to reconstruct ancestral states, measure phylogenetic signal and evaluate models of trait-dependent diversification. Our results show that the foam nest appeared during the Paleocene-Eocene transition (c. 55 Myr) in three major groups (Leptodactylidae, Limnodynastidae and Rhacophoridae) and has been highly conserved ever since. The foam nest probably originated from an ancestor with aquatic reproduction, except in Rhacophoridae (in which it evolved from an ancestor with jelly nests). Despite possessing several ecological functions, we found evidence of the influence of foam nests on diversification rates only in the South American lineage Leptodactylidae. We suggest that the foam nest was an important adaptation to historical climatic changes, enabling reproduction in dry open areas.