From erotic excrescences to pheromone shots: structure and diversity of nuptial pads in anurans

The nuptial pad is a secondary sexual character found in anuran amphibians. It includes modified epidermal and dermal tissues and is usually located on the first digit of the hand of males. In this study, we review the structure and diversity of nuptial pads based on a large-scale sample of morphological and phylogenetic diversity. Our findings show that all nuptial pads are characterized by the presence of specialized mucous glands in the dermal component. We also report the co-occurrence of other types of glands in some species and characterize their histochemistry. We describe three primary nuptial pad morphologies: (1) nuptial pads with papillary epidermal projections (or simply, papillae), in which the epidermal projections are formed by an epidermal and dermal evagination; (2) nuptial pads with non-papillary epidermal projections, in which the epidermal projections lack a dermal core forming a papilla; and (3) smooth nuptial pads, which lack projections but have a slight thickening of the dermal area with respect to the adjacent skin. We compare nuptial pads with other secondary sexual traits and discuss several related topics, including morphological diversity, coloration, hormonal control, taxonomic usefulness, and hypotheses regarding their role in reproduction.