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Evolution of Vocal Sacs in Anura
Of the many features that make frogs and toads unique, vocal sacs are among the most remarkable. Vocal sacs are inflatable, elastic chambers present in adult males of most anurans and are key elements in their social interactions. Traditionally vocal sacs have been associated primarily with acoustic communication, but their functions are currently being reinterpreted, and there is increasing evidence that they play a wider role in anuran biology. We surveyed the anatomical and histological structure of vocal sacs in all major clades of frogs by examining 777 specimens representing 605 species. Herein we characterize the morphological diversity of the three elements that compose the vocal sac: the gular skin, the superficial submandibular musculature, and the internal mucosa. We describe major anatomical patterns and define characters that we optimize on a comprehensive phylogenetic hypothesis of Anura. Integrating this anatomical information with images and videos of vocalizing frogs, we produce an updated morphological classification that includes 20 patterns of vocal sac morphology, each of which can be diagnosed by internal and external structures. Applying this classification to 4358 species, we discuss major evolutionary trends, taking ontogeny, homology, and multimodal communication into consideration. A single, spherical vocal sac is the most widely distributed vocal sac shape (present in 63% of known species), but some degree of lateralization (bilobate or paired sacs) has evolved in almost all anuran families. Some groups, such as Hylidae and Ranidae, are particularly diverse and contain more than 10 different vocal sac morphologies. Vocal sacs are absent in 18% of anurans and have been lost between 146 and 196 times, an astounding number considering their biological importance. Lastly, we review the morphological diversity and taxonomic relevance of vocal sac structures for each of the 58 families of recent anurans.
