Comparative morphological study of the swollen third finger in Ameerega,_ Anomaloglossus, _Hyloxalus, and Allobates (Anura: Dendrobatoidea)

Amphibians possess a great diversity of skin glands, among which male breeding glands are common secondary sexual characters that can form macroglands often referred to as “swollen” regions. Dendrobatoidea is the only clade in which the third finger of some species is swollen in adult males but not in adult females. Although the swollen third finger of males presents diverse morphologies and occurs in at least some species of seven dendrobatoid genera, detailed descriptions of external morphology are available for few species and the histology of the third finger has never been examined. In order to elucidate the basis of the reported morphological variation and test the homology of the swollen third finger among dendrobatoids, we studied the histology of the third finger of adult males and females of Allobates talamancae, Ameerega parvula, and Hyloxalus pulchellus, representing individuals without externally evident swelling, and Al. trilineatus, Am. hahneli, Anomaloglossus stepheni, and H. jacobuspetersi, representing individuals with conspicuously swollen third fingers in adult males. Typical granular glands with round nuclei at the periphery of their syncytium and a multicellular gland with cuboid epithelium filled with granules and a lumen were both present in all males and females of all species. Swollen third fingers also presented at least one additional type of gland that is morphologically similar to the breeding glands reported in other amphibians. Despite the apparent absence of third finger swelling in adult male Am. parvula, the third finger of adult males possess glands that are apparently identical to those observed in the conspicuously swollen third finger of adult male Am. hahneli.