In the majority of frogs, males but not females produce vocalizations to attract mates. Sexual selection can influence the evolution of these vocalizations by modifying the frog's morphology. The larynx is the main organ responsible for sound production; thus, it constitutes a target of selection. The goal of this study was to determine qualitatively and quantitatively the sexual differences in the developmental sequence of larynges of Physalaemus pustulosus ( = Engystomops pustulosus). We examined larynges of individuals ranging from recent metamorphs to adults. We provide quantitative measurements of laryngeal area and volume, vocal cords, and fibrous mass 1 (FM1), as well as qualitative descriptions of the developmental origin of vocal cords and FM1. Larynges of small male and female juveniles (less than 16 mm SVL) are similar, both externally and internally. The area of these larynges is no more than 1 mm2; no vocal cords or FM1 are present in either sex at this developmental stage. In individuals larger than 16 mm SVL, there is a marked divergence in relative size of the larynx between the sexes. The male larynx begins to exhibit a strong positive allometry until it reaches a plateau at 24 mm SVL. Also, at 16 mm SVL, the vocal cords and FM1 appear as distinctive outgrowths on the wall of the arytenoid cartilage in male larynges. Female growth rate of the larynx, however, is constant and exhibits negative allometry throughout ontogeny. Female vocal cords and FM1 are barely distinguishable as adults, and never attain the size and most likely neither the mobility as in the adult male larynx.
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Vol. 2014 • No. 1