JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. abstract: Antipredator defenses and warning signals typically evolve in concert. However, the extensive variation across taxa in both these components of predator deterrence and the relationship between them are poorly understood. Here we test whether there is a predictive relationship between visual conspicuousness and toxicity levels across 10 populations of the color-polymorphic strawberry poison frog, Dendrobates pumilio. Using a mouse-based toxicity assay, we find extreme variation in toxicity between frog populations. This variation is significantly positively correlated with frog coloration brightness, a viewer-independent measure of visual conspicuousness (i.e., total reflectance flux). We also examine conspicuousness from the view of three potential predator taxa, as well as conspecific frogs, using taxon-specific visual detection models and three natural background substrates. We find very strong positive relationships between frog toxicity and conspicuousness for bird-specific perceptual models. Weaker but still positive correlations are found for crab and D. pumilio conspecific visual perception, while frog coloration as viewed by snakes is not related to toxicity. These results suggest that poison frog colors can be honest signals of prey unpalatability to predators and that birds in particular may exert selection on aposematic signal design.

Our view of the evolution of sexually selected traits and preferences was influenced radically in the 1990s by studies that emphasized how signals interact with sensory properties of receivers. Here, twenty-five years later, we review evidence that has accumulated in support of this idea. We replace the term sensory biases with perceptual biases to emphasize the growing knowledge of how cognitive processes generate selection on sexual traits. We show that mating preferences among conspecifics (e.g., sexual selection by mate choice) often are influenced by perceptual adaptations and constraints that have evolved in other contexts besides mate choice among conspecifics. We suggest that these perceptual biases need not be costly to females when they influence mate choice because in many cases they generate direct benefits. Although we do not reject a role for indirect benefits in mate choice, such as good genes, exclusive focus on eugenic mate choice limits our understanding of the evolution of the remarkable diversity of sexually selected traits.

Aposematic signals may be subject to conflicting selective pressures from predators and conspecifics. We studied female preferences

It is commonly assumed that natural selection imposed by predators is the prevailing force driving the evolution of aposematic traits. Here, we demonstrate that aposematic signals are shaped by sexual selection as well. We evaluated sexual selection for coloration brightness in populations of the poison frog Oophaga [Dendrobates] pumilio in Panama's Bocas del Toro archipelago. We assessed female preferences for brighter males by manipulating the perceived brightness of spectrally matched males in two-way choice experiments. We found strong female preferences for bright males in two island populations and weaker or ambiguous preferences in females from mainland populations. Spectral reflectance measurements, coupled with an O. pumilio-specific visual processing model, showed that O. pumilio coloration was significantly brighter in island than in mainland morphs. In one of the island populations (Isla Solarte), males were significantly more brightly colored than females. Taken together, these results provide evidence for directional sexual selection on aposematic coloration and document sexual dimorphism in vertebrate warning coloration. Although aposematic signals have long been upheld as exemplars of natural selection, our results show that sexual selection should not be ignored in studies of aposematic evolution.aposematism ͉ color ͉ dendrobates ͉ divergence ͉ polymorphism

Many animals advertise their chemical defense to predators with conspicuous coloration and unpalatability, but little is known about the information in these signal elements. To effectively avoid predation, is it more advantageous to invest in increased conspicuousness or greater noxiousness, or to allocate equally to both signal modalities? Using natural variation among poison frog species measured with spectral reflectance and toxicity assays, we tested the relative importance of warning signal components with predator-learning and avoidance experiments. We demonstrate that closely related species use alternative strategies: increasing either conspicuousness or toxicity affords equivalent avoidance by predators and provides protection to nontoxic mimic species. These equally effective predator avoidance tactics demonstrate different aposematic solutions for two potentially costly signal components, providing a mechanism for natural diversity in warning signals.aposematism ͉ chemical defense ͉ predation

Sensory physiology has been shown to influence female mate choice, yet little is known about the mechanisms within the brain that regulate this critical behaviour. Here we examine preference behaviour of 58 female swordtails, Xiphophorus nigrensis, in four different social environments (attractive and unattractive males, females only, non-attractive males only and asocial conditions) followed by neural gene expression profiling. We used a brain-specific cDNA microarray to identify patterns of genomic response and candidate genes, followed by quantitative PCR (qPCR) examination of gene expression with variation in behaviour. Our microarray results revealed patterns of genomic response differing more between classes of social stimuli than between presence versus absence of stimuli. We identified suites of genes showing diametrically opposed patterns of expression: genes that are turned 'on' while females interact with attractive males are turned 'off' when interacting with other females, and vice versa. Our qPCR results identified significant predictive relationships between five candidate genes and specific mate choice behaviours (preference and receptivity) across females exposed to males, with no significant patterns identified in female or asocial conditions or with overall locomotor activity. The identification of stimulus-and behaviour-specific responses opens an exciting window into the molecular pathways associated with social behaviour and mechanisms that underlie sexual selection.

Although private communication is considered an important diversifying force in evolution, there is little direct behavioural evidence to support this notion. Here, we show that ultraviolet (UV) signalling in northern swordtails (Xiphophorus) affords a channel for communication that is not accessible to their major predator, Astyanax mexicanus, the Mexican tetra. Laboratory and field behavioural experiments with swordtails (X. nigrensis) and predators (A. mexicanus) demonstrate that male UV ornamentation significantly increases their attractiveness to females but not to this predator, which is less sensitive to UV. UV reflectance among swordtail species correlates positively with tetra densities across habitats, and visual contrast estimates suggest that UV signals are highly conspicuous to swordtails in their natural environment. Cross-species comparisons also support the hypothesis that natural selection drives the use of UV communication. We compared two species, one with high (X. nigrensis) and one with low (X. malinche) Mexican tetra densities. Xiphophorus nigrensis males reflect significantly more UV than X. malinche, exhibit significant UV sexual dimorphism, and UV is a salient component of the sexual communication system. In X. malinche, however, males reflect minimally in the UV, there is no UV sexual dimorphism, and UV does not play a part in its communication system.

1 . Mimetic advantage is considered to be dependent on frequency because an increase in mimic abundance leads to breakdown of the warning signal 2,3 . Where multiple toxic species are available, batesian polymorphism 4 is predicted-that is, mimics diversify to match sympatric models. Despite the prevalence of batesian mimicry in nature 5 , batesian polymorphism is relatively rare 6 . Here we explore a poison-frog mimicry complex comprising two parapatric models and a geographically dimorphic mimic that shows monomorphism where models co-occur. Contrary to classical predictions, our toxicity assays, field observations and spectral reflectances show that mimics resemble the less-toxic and less-abundant model. We examine "stimulus generalization" 7 as a mechanism for this nonintuitive result with learning experiments using naive avian predators and live poison frogs. We find that predators differ in avoidance generalization depending on toxicity of the model, conferring greater protection to mimics resembling the lesstoxic model owing to overlap of generalized avoidance curves. Our work supports a mechanism of toxicity-dependent stimulus generalization 8 , revealing an additional solution for batesian mimicry where multiple models coexist.In batesian mimicry, an edible species co-opts a warning signal from an unpalatable species to gain advantage through predator deception 1 . If batesian mimics are too common, however, this advantage breaks down as predators learn to ignore the warning signal. Where more than one model species is available 4 , diversifying frequency-dependent selection predicts the evolution of polymorphism in which mimics diverge in appearance to resemble sympatric models 6,9,10 . Batesian polymorphism is suggested to distribute warning signal degradation over several defended model species, enabling the mimic to increase in abundance. Reported accounts of such mimetic polymorphism, however, are relatively rare 6 and unknown in vertebrate mimicry systems 11,12 .Here we investigate a mimicry system that is inconsistent with the predictions of frequency dependence. We examine a poison-frog mimicry complex composed of two parapatric models and a geographically varying mimic (Fig. 1). The model Ecuadorian poison frogs Epipedobates bilinguis and Epipedobates parvulus share a similar warning signal of a bright red-spotted dorsum but differ in axilla and groin colouration (Fig. 1b). Their phylogenetically distant relative 13 , Allobates zaparo, is geographically dimorphic, matching each warning signal where models are parapatric (Fig. 1b). Where the two models co-occur, however, the mimic resembles only a single model (E. bilinguis; Figs 1 and 2). Here we use spectral reflectances, toxicity assays, field abundance measurements and predator learning experiments to investigate mechanisms that may be contributing to this pattern in nature.Theoretical and empirical studies predict that coexistence of aposematic models may lead to (1) batesian polymorphism 6,9,10 , (2) evolution of a mimic phenotype intermed...