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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. [ABSTRACT FROM AUTHOR]

The acquisition of signals used in intrasexual communication is not well studied. To better understand the possible contribution of early experience to the acquisition of intrasexual responses, we reared Neotropical Tungara frogs, Physalaemus pustulosus, in four acoustic environments: 1) conspecific chorus, 2) heterospecific chorus, 3) acoustic isolation, and 4) noise. For the heterospecific chorus, we chose the calls of a congener, P. enesefae, with which P. pustulosus is sympatric in parts of its distribution. We measured the evoked vocal responses (EVR) of these frogs in response to calls of the conspecifics, to calls of the congener, and to calls representing an intermediate between the two species. Male P. pustulosus reared hearing the calls of P. enesefae produced more calls overall and more complex calls in response to the call of P. enesefae. This is consistent with the hypothesis that the EVR of male anurans can be influenced by early experience and is the product of an interaction between genes and the environment. [ABSTRACT FROM AUTHOR]

Anurans have long been a model system for studies of animal communication, but little is known about how individuals acquire acoustically linked mating behaviors. The manner in which behaviors are acquired may be a source of variation in these behaviors. In this study, we reared Physalaemus pustulosus in four acoustic treatment groups: hearing a conspecific chorus of P. pustulosus frogs, acoustically isolated from all frog calls, hearing a chorus of the congener P. enesefae, and hearing noise. We then measured the spectral and temporal characteristics of the calls produced by males and tested them for differences between treatment groups. Males reared in isolation produced calls that were shorter in duration. Females showed discrimination against these shorter calls. Early experience can alter the advertisement call of male P. pustulosus, and acoustic isolation leads to the acquisition of calls that are less attractive to females. Males reared in other groups produced species-typical calls. The calls of male P. pustulosus may be the product of a gene by environment interaction. [ABSTRACT FROM AUTHOR]

The article pays tribute to Austin Stanley Rand, a scientist at the Smithsonian Tropical Research Institute.

Populations of Physalaemus petersi show extraordinary variation in the expression of facultative complex calls. Previous studies have shown that a structure in the larynx, the fibrous mass (FMI), is correlated with variation in production of the complex call among taxa in the Physalaemus pustulosus species group. We tested the hypothesis that the variation in the presence of call complexity that occurs within P. petersi mirrors the same laryngeal variation associated with complex calls among other species of the genus. We examined calls and larynges of P. petersi from populations on a north-south transect on the western side of its Amazonian distribution. All four populations of P. petersi (Peru and Brazil) in the south produce complex calls, whereas only one population of five in the north (Ecuador) produce complex calls. We compared larynges of four populations of P. petersi, two that produce complex calls and two that do not. We then compared them to larynges of P. pustulosus, which produces complex calls, and to Physalaemus coloradorum, which does not produce complex calls. The FM1 is significantly larger in populations of P. petersi that produce complex calls than those without complex calls. FMI sizes of populations of P. petersi were more similar to other species with the same call type than they were to populations of their own species with a different call type. These data suggest several interpretations of the evolution of complex calls and the associated underlying morphology. [ABSTRACT FROM AUTHOR]

The evolution of sexual signaling systems is influenced by natural and sexual selection acting on complex interactions among traits. Natural hybrid zones are excellent systems for assessing fitness effects on sexual phenotypes. Most documented hybrid zones, however, show little variation in sexual signals. A hybrid zone between the swordtails Xiphophorus birchmanni and Xiphophorus malinche is characterized by numerous recombinants for male sexual traits. Analyses of geographic variation in morphological and isozyme traits in the Río Calnali, Hidalgo, Mexico, reveal an upstream-to-downstream gradient from X. malinche- to X. birchmanni-type traits. A second hybrid zone, likely isolated from the R. Calnali, occurs in the nearby Arroyo Pochutla. Although the presumed female preference for swords predicts the introgression of swords from X. malinche-like populations into hybrid populations, the opposite pattern was observed. Swords are reduced in populations otherwise characterized by X. malinche traits. Sexually dimorphic traits were poorly correlated within individuals, indicating that sexual selection does not act against recombinant phenotypes. Hybrid males also exhibit trait values outside the range of parental variation. These patterns are consistent with predictions that females are permissive, preferring generally conspicuous males without attending to specific features. [ABSTRACT FROM AUTHOR]