Much more than a clasp: Evolutionary pattern of amplexus diversity in anurans

Abstract

The evolution and diversification of animal reproductive modes have been pivotal questions in behavioral ecology. Amphibians present the highest diversity of reproductive modes among vertebrates, involving various behavioral, physiological and morphological traits. One of such features is the amplexus, the clasp or embrace of males on females during reproduction, which is almost universal to anurans. Hypotheses about the origin amplexus are limited and have not been thoroughly tested, nor had they taken into account evolutionary relationships in most comparative studies. However, these considerations are crucial to understand the evolution of reproductive modes. Here, using an evolutionary framework, we reconstruct the ancestral state of amplexus in 686 anuran species; investigate whether the amplexus type is a conserved trait; and test whether sexual size dimorphism (SSD) could have influenced the amplexus type or male performance while clasping females. Overall, we found evidence of at least 35 evolutionary transitions in amplexus type across anurans. We also found that amplexus exhibits a high phylogenetic signal (it is conserved across Anura evolutionary history) and the amplexus type does not evolve in association with SSD. We discuss the implications of our findings on the diversity of amplexus types across anurans.