Summary 1.Females from many species assess fluctuating asymmetry (FA; nondirectional deviations from symmetry in bilateral traits) when choosing mates. However, the hypothesis that FA indicates the quality of a potential mate is controversial because of the lack of consistent evidence that FA is conditional or heritable. 2. We present evidence that FA provides information about growth rate in the swordtail fish Xiphophorus multilineatus, and a model for how variation in growth strategies could obscure the relationship between FA and stress. 3. Males from one of the genetically influenced size classes of males in this species (Y-II) are more often asymmetrical for the sexually selected trait vertical bars. We also detected a significant relationship between the absolute difference in otolith ring counts and absolute difference in vertical bar number, supporting the hypothesis that FA in vertical bars indicates developmentally instability. 4. In a diet manipulation study, we found support for an alternative growth strategy and a relationship between growth and FA. The Y-II males that grew faster on a high-quality diet had a relationship between higher growth rates and asymmetry that was not detected in the other genotypes. 5. Our results suggest a hypothesis to explain why FA does not always increase with stress: differences across genotypes in the threshold for environmental stress needed to shift the allocation of resources from growth to developmental quality will obscure the relationship between FA and environmental stress. Our results also provide an explanation for why the larger females of some species of swordtails have been shown to prefer asymmetry: in some conditions, choosing a mate that optimizes growth rate over developmental stability (asymmetry) may be adaptive.

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Animals do not make decisions in a bubble but often refer to previous experience when discriminating between options. Contrast effects occur when the value of a stimulus affects the response to another value of the stimulus, and the changes in value and response are in the same direction. Although contrast effects appear irrational, they could benefit decision makers when there is spatial or temporal variation and autocorrelation in the value of stimuli that elicit decisions. Here, we examined whether contrasts influence female evaluation of male performance-based sexual signals. We exposed female Lincoln's sparrows (Melospiza lincolnii) to one week of songs that we had experimentally reduced or elevated in performance, followed by a novel song of intermediate performance. We found that high-performance songs were more attractive to females than low-performance songs. Moreover, the intermediate songs were more attractive following exposure to low-than to high-performance songs. These results indicate that contrast can influence evaluation of performancebased sexual stimuli. By examining contrast effects in the ecologically relevant context of mate choice for performance, we can better understand both the adaptive value of comparative evaluation as well as the mechanisms that underlie variation in mate choice and sexual selection.

Sensory responses to courtship signals can be altered by reproductive hormones. In seasonally-breeding female songbirds, for example, sound-induced immediate early gene expression in the auditory pathway is selective for male song over behaviourally irrelevant sounds only when plasma estradiol reaches breeding-like levels. This selectivity has been hypothesized to be mediated by release of monoaminergic neuromodulators in the auditory pathway. We previously showed that in oestrogen-primed female white-throated sparrows, exposure to male song induced dopamine and serotonin release in auditory regions. In order to mediate hormone-dependent selectivity, this release must be (1) selective for song and (2) modulated by endocrine state. Therefore, in the current study we addressed both questions by conducting playbacks of song or a control sound to females in a breeding-like or non-breeding endocrine state. We then used high performance liquid chromatography to measure turnover of dopamine, norepinephrine, and serotonin in the auditory midbrain and forebrain. We found that sound-induced turnover of dopamine and serotonin did in fact depend on endocrine state; hearing sound increased turnover in the auditory forebrain only in the birds in a breeding-like endocrine state. Contrary to our expectations, these increases occurred in response to either song or artificial tones; in other words, they were not selective for song. The selectivity of sound-induced monoamine release was thus strikingly different from that of immediate early gene responses described in previous studies. We did, however, find that constitutive monoamine release was altered by endocrine state; whether the birds heard sound or not, turnover of serotonin in the auditory forebrain was higher in a breeding-like state than in a non-breeding endocrine state. Our results suggest that dopaminergic and serotonergic responses to song and other sounds, as well as serotonergic tone in auditory areas, could be seasonally modulated. This article is protected by copyright. All rights reserved.