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Genetic variation in sexual displays is crucial for an evolutionary response to sexual selection, but can be eroded by strong selection. Identifying the magnitude and sources of additive genetic variance underlying sexually selected traits is thus an important issue in evolutionary biology. We conducted a quantitative genetics experiment with gray treefrogs (Hyla versicolor) to investigate genetic variances and covariances among features of the male advertisement call. Two energetically expensive traits showed significant genetic variation: call duration, expressed as number of pulses per call, and call rate, represented by its inverse, call period. These two properties also showed significant genetic covariance, consistent with an energetic constraint to call production. Combining the genetic variance-covariance matrix with previous estimates of directional sexual selection imposed by female preferences predicts a limited increase in call duration but no change in call rate despite significant selection on both traits. In addition to constraints imposed by the genetic covariance structure, an evolutionary response to sexual selection may also be limited by high energetic costs of long-duration calls and by preferences that act most strongly against very short-duration calls. Meanwhile, the persistence of these preferences could be explained by costs of mating with males with especially unattractive calls.

Even simple biological signals vary in several measurable dimensions. Understanding their evolution requires, therefore, a multivariate understanding of selection, including how different properties interact to determine the effectiveness of the signal. We combined experimental manipulation with multivariate selection analysis to assess female mate choice on the simple trilled calls of male gray treefrogs. We independently and randomly varied five behaviorally relevant acoustic properties in 154 synthetic calls. We compared response times of each of 154 females to one of these calls with its response to a standard call that had mean values of the five properties. We found directional and quadratic selection on two properties indicative of the amount of signaling, pulse number, and call rate. Canonical rotation of the fitness surface showed that these properties, along with pulse rate, contributed heavily to a major axis of stabilizing selection, a result consistent with univariate studies showing diminishing effects of increasing pulse number well beyond the mean. Spectral properties contributed to a second major axis of stabilizing selection. The single major axis of disruptive selection suggested that a combination of two temporal and two spectral properties with values differing from the mean should be especially attractive.

Signals used for mate choice and receiver preferences are often assumed to coevolve in a lock-step fashion. However, sender-receiver coevolution can also be nonparallel: even if species differences in signals are mainly quantitative, females of some closely related species have qualitatively different preferences and underlying mechanisms. Two-alternative playback experiments using synthetic calls that differed in fine-scale temporal properties identified the receiver criteria in females of the treefrog Hyla chrysoscelis for comparison with female criteria in a cryptic tetraploid species (H. versicolor); detailed preference functions were also generated for both species based on natural patterns of variation in temporal properties. The species were similar in three respects: (1) pulses of constant frequency were as attractive as the frequency-modulated pulses typical of conspecific calls; (2) changes in preferences with temperature paralleled temperature-dependent changes in male calls; and (3) preference functions were unimodal, with weakly defined peaks estimated at values slightly higher than the estimated means in conspecific calls. There were also species differences: (1) preference function slopes were steeper in H. chrysoscelis than in H. versicolor; (2) preferences were more intensity independent in H. chrysoscelis than in H. versicolor; (3) a synergistic effect of differences in pulse rate and shape on preference strength occurred in H. versicolor but not in H. chrysoscelis; and (4) a preference for the pulse shape typical of conspecific calls was expressed at the species-typical pulse duration in H. versicolor but not in H. chrysoscelis. However, females of H. chrysoscelis did express a preference based on pulse shape when tested with longer-than-average pulses, suggesting a hypothesis that could account for some examples of nonparallel coevolution. Namely, preferences can be hidden or revealed depending on the direction of quantitative change in a signal property relative to the threshold for resolving differences in that property. The results of the experiments reported here also predict patterns of mate choice within and between contemporary populations. First, intraspecific mate choice in both species is expected to be strongly influenced by variation in temperature among calling males. Second, simultaneous differences in pulse rate and pulse shape are required for effective species discrimination by females of H. versicolor but not by females of H. chrysoscelis. Third, there is greater potential for sexual selection within populations and for discrimination against calls produced by males in other geographically remote populations in H. chrysoscelis than in H. versicolor.

Interactions between species can affect the evolution of their sexual signals, receiver selectivity, or both. One commonly expected outcome is reproductive character displacement, whereby adverse consequences of mismating select for greater differentiation of communication systems in areas of sympatry than in areas of allopatry. We found evidence of reproductive character displacement in the acoustic communication system of green tree frogs (Hyla cinerea). The strength of female preferences for the spectral properties of calls that distinguish conspecific calls from those of a closely related congener, H. gratiosa, was greater in areas of sympatry with H. gratiosa than in areas of allopatry. We also found subtle differences in advertisement calls and in the heights of male calling perches when we restricted our comparisons to localities in which H. gratiosa was also breeding (syntopy) with localities where this species was absent. Hyla cinerea and H. gratiosa show only weak genetic incompatibility, but the calls representative of interspecific hybrids were unattractive to females of both parental species. Hybrids might also be at an ecological disadvantages because of different habitat preferences of the two taxa. Thus, selection against production of less fit or less attractive hybrid or backcross offspring are probably the main causes responsible for the differences documented in this paper.

A mating preference function describes the relationship between variation in a trait in potential mates and the strength of the preference for that trait. Few studies have measured mating preference functions either at a population level or for individuals. We used two-choice playback experiments to determine the mating preference functions of individual female barking treefrogs for two call characteristics: call-repetition rate and fundamental frequency. We tested each female four times with each pair of stimuli and with three to six pairs of stimuli. Individual females exhibited directional preferences for higher call rates and stabilizing preferences for intermediate fundamental fre- quency. These individual preference functions were similar to population-level preferences documented in previous studies. Within a stimulus type (call rate or fundamental frequency), nearly all females exhibited the same general preference function. However, females varied in the minimum difference between stimuli necessary to elicit a unan- imous choice for the higher call rate, and they differed in both the intermediate fundamental frequency they preferred most and the minimum difference in fundamental frequency that elicited a unanimous choice for one of the two alternatives. The variation we observed among females was not repeatable; repeatabilities were in general low and statistically nonsignificant. The observed variation in the preferences of females, even if unrepeatable, should weaken selection on male traits relative to selection in the absence of such variation.