The origins and drivers of sexual size dimorphism in sharks

While sexual size dimorphism (SSD) is abundant in nature, there is huge variation in both the intensity and direction of SSD. SSD results from a combination of sexual selection for large male size, fecundity selection for large female size and ecological selection for either. In most vertebrates, it is variation in the intensity of male-male competition that primarily underlies variation in SSD. In this study, we test four hypotheses regarding the adaptive value of SSD in sharks-considering the potential for each of fecundity, sexual, ecological selection and reproductive mode as the primary driver of variation in SSD between species. We also estimate past macroevolutionary shifts in SSD direction/intensity through shark phylogeny. We were unable to find evidence of significant SSD in early sharks and hypothesise that SSD is a derived state in this clade, that has evolved independently of SSD observed in other vertebrates. Moreover, there is no significant relationship between SSD and fecundity, testes mass or oceanic depth in sharks. However, there is evidence to support previous speculation that reproductive mode is an important determinant of interspecific variation in SSD in sharks. This is significant as in most vertebrates sexual selection is thought to be the primary driver of SSD trends, with evidence for the role of fecundity selection in other clades being inconsistent at best. While the phylogenetic distribution of SSD among sharks is superficially similar to that observed in other vertebrate clades, the relative importance of selective pressures underlying its evolution appears to differ.

Heavy metal pollution alters reproductive performance and mate choice in an anuran, Strauchbufo raddei

Understanding the effects of pollution on reproductive performance and sexual selection is crucial for the conservation of biodiversity in an increasingly polluted world. The present study focused on the effect of environmental heavy metal pollution on sexually selected traits, including morphological characteristics and acoustic parameters, as well as mate choice in Strauchbufo raddei, an anuran species widely distributed in Northern China. The results showed that male courtship signals, including forelimb length, forelimb force, and advertisement calls, have evolved under the pressure of heavy metal pollution in young S. raddei. In addition, the breeding age was lower in the polluted areas, and younger individuals had more mating opportunities. However, males with heightened reproductive performance did not show the expected higher individual quality. The current study suggests that exposure to heavy metal pollution can induce stress in males, altering reproductive performance and further disrupting mate choice.

Predation drives complex eco-evolutionary dynamics in sexually selected traits

Predation plays a role in preventing the evolution of ever more complicated sexual displays, because such displays often increase an individual's predation risk. Sexual selection theory, however, omits a key feature of predation in modeling costs to sexually selected traits: Predation is density dependent. As a result of this density dependence, predator-prey dynamics should feed back into the evolution of sexual displays, which, in turn, feeds back into predator-prey dynamics. Here, we develop both population and quantitative genetic models of sexual selection that explicitly link the evolution of sexual displays with predator-prey dynamics. Our primary result is that predation can drive eco-evolutionary cycles in sexually selected traits. We also show that mechanistically modeling the cost to sexual displays as predation leads to novel outcomes such as the maintenance of polymorphism in sexual displays and alters ecological dynamics by muting prey cycles. These results suggest predation as a potential mechanism to maintain variation in sexual displays and underscore that short-term studies of sexual display evolution may not accurately predict long-run dynamics. Further, they demonstrate that a common verbal model (that predation limits sexual displays) with widespread empirical support can result in unappreciated, complex dynamics due to the density-dependent nature of predation.

Tactics of evasion: strategies used by signallers to deter eavesdropping enemies from exploiting communication systems

Eavesdropping predators, parasites and parasitoids exploit signals emitted by their prey and hosts for detection, assessment, localization and attack, and in the process impose strong selective pressures on the communication systems of the organisms they exploit. Signallers have evolved numerous anti-eavesdropper strategies to mitigate the trade-off between the costs imposed from signal exploitation and the need for conspecific communication. Eavesdropper strategies fall along a continuum from opportunistic to highly specialized, and the tightness of the eavesdropper-signaller relationship results in differential pressures on communication systems. A wide variety of anti-eavesdropper strategies mitigate the trade-off between eavesdropper exploitation and conspecific communication. Antagonistic selection from eavesdroppers can result in diverse outcomes including modulation of signalling displays, signal structure, and evolutionary loss or gain of a signal from a population. These strategies often result in reduced signal conspicuousness and in decreased signal ornamentation. Eavesdropping enemies, however, can also promote signal ornamentation. While less common, this alternative outcome offers a unique opportunity to dissect the factors that may lead to different evolutionary pathways. In addition, contrary to traditional assumptions, no sensory modality is completely 'safe' as eavesdroppers are ubiquitous and have a broad array of sensory filters that allow opportunity for signal exploitation. We discuss how anthropogenic change affects interactions between eavesdropping enemies and their victims as it rapidly modifies signalling environments and community composition. Drawing on diverse research from a range of taxa and sensory modalities, we synthesize current knowledge on anti-eavesdropper strategies, discuss challenges in this field and highlight fruitful new directions for future research. Ultimately, this review offers a conceptual framework to understand the diverse strategies used by signallers to communicate under the pressure imposed by their eavesdropping enemies.

Evolutionary interactions between thermal ecology and sexual selection

Thermal ecology and mate competition are both pervasive features of ecological adaptation. A surge of recent work has uncovered the diversity of ways in which temperature affects mating interactions and sexual selection. However, the potential for thermal biology and reproductive ecology to evolve together as organisms adapt to their thermal environment has been underappreciated. Here, we develop a series of hypotheses regarding (1) not only how thermal ecology affects mating system dynamics, but also how mating dynamics can generate selection on thermal traits; and (2) how the thermal consequences of mate competition favour the reciprocal co-adaptation of thermal biology and sexual traits. We discuss our hypotheses in the context of both pre-copulatory and post-copulatory processes. We also call for future work integrating experimental and phylogenetic comparative approaches to understand evolutionary feedbacks between thermal ecology and sexual selection. Overall, studying reciprocal feedbacks between thermal ecology and sexual selection may be necessary to understand how organisms have adapted to the environments of the past and could persist in the environments of the future.

Dual function and associated costs of a highly exaggerated trait in a cichlid fish

Exaggerated secondary sexual characteristics are apparently costly and seem to defy natural selection. This conundrum promoted the theory of sexual selection. Accordingly, exaggerated secondary sexual characteristics might be ornaments on which female choice is based and/or armaments used during male-male competition. Males of many cichlid fish species, including the adaptive radiation of Nicaraguan Midas cichlids, develop a highly exaggerated nuchal hump, which is thought to be a sexually selected trait. To test this hypothesis, we conducted a series of behavioral assays in F2 hybrids obtained from crossing a species with a relatively small hump and one with an exaggerated hump. Mate-choice experiments showed a clear female preference for males with large humps. In an open-choice experiment with limited territories, couples including large humped males were more successful in acquiring these territories. Therefore, nuchal humps appear to serve dual functions as an ornament for attracting mates and as an armament for direct contest with rivals. Although being beneficial in terms of sexual selection, this trait also imposes fitness costs on males possessing disproportionally large nuchal humps since they exhibit decreased endurance and increased energetic costs when swimming. We conclude that these costs illustrate trade-offs associated with large hump size between sexual and natural selection, which causes the latter to limit further exaggeration of this spectacular male trait.

Information out of the blue: phenotypic correlates of abdominal color patches in Sceloporus lizards

Colorful ornaments are important visual signals for animal communication that can provide critical information about the quality of the signaler. In this study, we focused on different color characteristics of the abdominal patches of males of six lizard species from the genus Sceloporus. We addressed three main objectives. First, we examined if size, brightness, saturation, and conspicuousness of these ornaments are indicative of body size, condition, immune function, or levels of testosterone and corticosterone. Second, we evaluated if the distinct components of these abdominal patches (blue or green patches and black stripes) transmit similar information about the signaler, which would support the redundant signal hypothesis, or if these components are related to different phenotypic traits, which would support the multiple message hypothesis. Third, we compared the phenotypic correlates of these ornaments among our six species to understand the degree of conservatism in the signaling patterns or to find species-specific signals. Using data collected from males in natural conditions and a multi-model inference framework, we found that in most species the area of the patches and the brightness of the blue component are positively related to body size. Thus, these color characteristics are presumably indicative of the physical strength and competitive ability of males and these shared signals were likely inherited from a common ancestor. In half of the species, males in good body condition also exhibit relatively larger blue and black areas, suggesting that the expression of these ornaments is condition-dependent. Abdominal patches also provide information about immunocompetence of the males as indicated by different correlations between certain color characteristics and ectoparasite load, counts of heterophils, and the heterophil:lymphocyte ratio. Our findings reveal that area and brightness of the abdominal patches signal the size and body condition of males, whereas blue saturation and conspicuousness with respect to the surrounding substrate are indicative of immune condition, thus supporting the multiple message hypothesis. However, some of these correlations were not shared by all species and, hence, point to intriguing species-specific signals.

The relative effects of pace of life-history and habitat characteristics on the evolution of sexual ornaments: A comparative assessment

Selection may favor greater investment into sexual ornaments when opportunities for future reproduction are limited, for example, under high adult mortality. However, predation, a key driver of mortality, typically selects against elaborate sexual ornaments. Here, we examine the evolution of sexual ornaments in killifishes, which have marked contrasts in life-history strategy among species and inhabit environments that differ in accessibility to aquatic predators. We first assessed if the size of sexual ornaments (unpaired fins) influenced swimming performance. Second, we investigated whether the evolution of larger ornamental fins is driven primarily by the pace of life-history (investment into current vs. future reproduction) or habitat type (a proxy for predation risk). We found that larger fins negatively affected swimming performance. Further, males from species inhabiting ephemeral habitats, with lower predation risk, had larger fins and greater sexual dimorphism in fin size, compared to males from more accessible permanent habitats. We show that enlarged ornamental fins, which impair locomotion, evolve more frequently in environments that are less accessible to predators, without clear associations to life-history strategy. Our results provide a rare link between the evolution of sexual ornaments, effects on locomotion performance, and natural selection on ornament size potentially through habitat differences in predation risk.

Dancing in the rain: how do abiotic conditions influence sexually selected behaviors in the White-ruffed Manakin?

Tropical animals respond to rainfall in population-specific ways. In extremely wet regions, endotherms experience heavy rains as stressors with consequences for behavior and demography. Ultimately, such stressors can affect the relative strength of abiotic selection, reducing the scope for sexual selection and other biotic sources of selection. We studied population-level differences in the response to rainfall in White-ruffed Manakins (Corapipo altera) on the Caribbean slope of Costa Rica, a species having a lek mating system subject to strong sexual selection. Between 2008 and 2013, we studied reproductive behavior in a population inhabiting an extremely wet site; estimates of apparent survival were low, and the turnover of display courts and dominant males was high. Males also engaged in coordinated display, and sub-adult males practiced in the presence of adults. Over three breeding seasons (2017-2019), we studied a population of the same species at a site only 110 km away, but in a location receiving roughly half as much rain. We tested behavioral predictions of three alternative mechanisms-indirect abiotic effects, direct mortality effects, and direct behavioral effects-linking rainfall to sexual selection in these two populations. Data derived from over 4300 hr of observations at 105 display sites revealed high interannual variation in nearly all response variables, including turnover of display sites, retention of alpha status, male display behavior, and time females spent assessing male display. Additionally, we detected spatial differences in drivers of display site turnover. Notably, age distribution of males was skewed toward older individuals at the drier location. Based on these findings we infer that indirect abiotic effects on forest structure leading to display site transience and direct effects of mortality increasing turnover in the male population likely underlie links between rain and the spatial and temporal differences we documented. Our results are consistent with rain constituting an important source of abiotic selection for tropical endotherms and modulating the scope for sexual selection near the extremes of a species' hygric niche.

Evolutionary drivers of sexual signal variation in Amazon Slender Anoles

Phenotypic variation among populations, as seen in the signaling traits of many species, provides an opportunity to test whether similar factors generate repeated phenotypic patterns in different parts of a species' range. We investigated whether genetic divergence, abiotic gradients, and sympatry with closely related species explain variation in the dewlap colors of Amazon Slender Anoles, Anolis fuscoauratus. To this aim, we characterized dewlap diversity in the field with respect to population genetic structure and evolutionary relationships, assessed whether dewlap phenotypes are associated with climate or landscape variables, and tested for nonrandom associations in the distributions of A. fuscoauratus phenotypes and sympatric Anolis species. We found that dewlap colors vary among but not within sites in A. fuscoauratus. Regional genetic clusters included multiple phenotypes, while populations with similar dewlaps were often distantly related. Phenotypes did not segregate in environmental space, providing no support for optimized signal transmission at a local scale. Instead, we found a negative association between certain phenotypes and sympatric Anolis species with similar dewlap color attributes, suggesting that interactions with closely related species promoted dewlap divergence among A. fuscoauratus populations. Amazon Slender Anoles emerge as a promising system to address questions about parallel trait evolution and the contribution of signaling traits to speciation.

Predation and Resource Availability Interact to Drive Life-History Evolution in an Adaptive Radiation of Livebearing Fish

Predation risk and resource availability are two primary factors predicted by theory to drive the evolution of life histories. Yet, disentangling their roles in life-history evolution in the wild is challenging because (1) the two factors often co-vary across environments, and (2) environmental effects on phenotypes can mask patterns of genotypic evolution. Here, we use the model system of the post-Pleistocene radiation of Bahamas mosquitofish (Gambusia hubbsi) inhabiting blue holes to provide a strong test of the roles of predation and resources in life-history evolution, as the two factors do not co-vary in this system and we attempted to minimize environmental effects by raising eight populations under common laboratory conditions. We tested a priori predictions of predation- and resource-driven evolution in five life-history traits. We found that life-history evolution in Bahamas mosquitofish largely reflected complex interactions in the effects of predation and resource availability. High predation risk has driven the evolution of higher fecundity, smaller offspring size, more frequent reproduction, and slower growth rate—but this predation-driven divergence primarily occurred in environments with relatively high resource availability, and the effects of resources on life-history evolution was generally greater within environments having high predation risk. This implies that resource-driven selection on life histories overrides selection from predators when resources are particularly scarce. While several results matched a priori predictions, with the added nuance of interdependence among selective agents, some did not. For instance, only resource levels, not predation risk, explained evolutionary change in male age at maturity, with more rapid sexual maturation in higher-resource environments. We also found faster (not slower) juvenile growth rates within low-resource and low-predation environments, probably caused by selection in these high-competition scenarios favoring greater growth efficiency. Our approach, using common-garden experiments with a natural system of low- and high-predation populations that span a continuum of resource availability, provides a powerful way to deepen our understanding of life-history evolution. Overall, it appears that life-history evolution in this adaptive radiation has resulted from a complex interplay between predation and resources, underscoring the need for increased attention on more sophisticated interactions among selective agents in driving phenotypic diversification.

Signaler-receiver-eavesdropper: Risks and rewards of variation in the dominant frequency of male cricket calls

Signals are important for communication and mating, and while they can benefit an individual, they can also be costly and dangerous. Male field crickets call in order to attract female crickets, but gravid females of a parasitoid fly species, Ormia ochracea, are also attracted to the call and use it to pinpoint male cricket hosts. Conspicuousness of the call can vary with frequency, amplitude, and temporal features. Previous work with this system has only considered temporal variation in cricket calls, both large scale, that is, amount of calling and at what time of evening, and small scale, that is, aspects of chirp rate, pulse rate, and numbers of pulses per chirp. Because auditory perception in both crickets and flies relies on the matching of the peak frequency of the call with the peripheral sensory system, peak frequency may be subject to selection both from female crickets and from female flies. Here, we used field playbacks of four different versions of the same male Gryllus lineaticeps calling song that only differed in peak frequency (3.3, 4.3, 5.3, and 6.3 kHz) to test the relative attractiveness of the calls to female crickets and female flies. Our results clearly show that lower frequency calls enhance male safety from fly parasitism, but that the enhanced safety would come at a cost of reduced attraction of female crickets as potential mates. The results imply that eavesdropper pressure can disrupt the matched coevolution of signalers and receivers such that the common concept of matched male-female signaler-receiver coevolution may actually be better described as male-female-predator signaler-receiver-eavesdropper coevolution.

Sexual and natural selection on pollen morphology in Taraxacum

PREMISE

Spiny pollen has evolved independently in multiple entomophilous lineages. Sexual selection may act on exine traits that facilitate male mating success by influencing the transfer of pollen from the anther to the body of the pollinator, while natural selection acts to increase pollen survival. We postulated that relative to sexual congeners, apomictic dandelions undergo relaxed selection on traits associated with male mating success.

METHODS

We explored sexual selection on exine traits by measuring the propensity for Taraxacum spp. pollen to attach to hairs of flower-visiting bumblebees (Bombus spp.) or flies (Diptera: Syrphidae and Muscoidea) and assessed natural selection by testing whether pollen traits defend against consumption.

RESULTS

Pollen picked up by bumblebees exhibited a narrower subset of spine-spacing phenotypes, consistent with stabilizing selection. Flies picked up larger pollen from flowers than expected at random. Surveys of corbiculae (pollen basket) contents from foraging bumblebees and feces of flies showed that pollen grains consumed by both kinds of visitors are similar in spine characteristics and size to those produced by the donor. When bees visit inflorescences of apomictic T. officinale, they pick up pollen with spine-spacing phenotypes above the mean and shifted toward those of sexual T. ceratophorum.

CONCLUSIONS

We demonstrate that traits under sexual selection during pollen pickup vary among pollinators, while natural selection for pollen defense is nil in T. ceratophorum. In hybrid zones between apomictic and sexual dandelions, pollen traits place apomictic donors at a dispersal disadvantage, potentially reinforcing reproductive isolation.

Risk of social colours in an agamid lizard: implications for the evolution of dynamic signals

The forces of sexual and natural selection are typically invoked to explain variation in colour patterns of animals. Although the benefits of conspicuous colours for social signalling are well documented, evidence for their ecological cost, especially for dynamic colours, remains limited. We examined the riskiness of colour patterns of Psammophilus dorsalis, a species in which males express distinct colour combinations during social interactions. We first measured the conspicuousness of these colour patterns on different substrates based on the visual systems of conspecifics and predators (bird, snake, canid) and then quantified actual predation risk on these patterns using wax/polymer lizard models in the wild. The black and red male state exhibited during courtship was the most conspicuous to all visual systems, while the yellow and orange male aggression state and the brown female colour were least conspicuous. Models bearing the courtship colour pattern experienced the highest predator attacks, irrespective of the substrate they were placed on. Thus, social colours of males are not only conspicuous but also risky. Using physiological colours to shift in and out of conspicuous states may be an effective evolutionary solution to balance social signalling benefits with predation costs.

3D Printing: Applications in evolution and ecology

In the commercial and medical sectors, 3D printing is delivering on its promise to enable a revolution. However, in the fields of Ecology and Evolution we are only on the brink of embracing the advantages that 3D printing can offer. Here we discuss examples where the process has enabled researchers to develop new techniques, work with novel species, and to enhance the impact of outreach activities. Our aim is to showcase the potential that 3D printing offers in terms of improved experimental techniques, greater flexibility, reduced costs and promoting open science, while also discussing its limitations. By taking a general overview of studies using the technique from fields across the broad range of Ecology and Evolution, we show the flexibility of 3D printing technology and aim to inspire the next generation of discoveries.

Trade-offs between larval survival and adult ornament development depend on predator regime in a territorial dragonfly

Trade-offs between juvenile survival and the development of sexually selected traits can cause ontogenetic conflict between life stages that constrains adaptive evolution. However, the potential for ecological interactions to alter the presence or strength of these trade-offs remains largely unexplored. Antagonistic selection over the accumulation and storage of resources could be one common cause of environment-specific trade-offs between life stages: higher condition may simultaneously enhance adult ornament development and increase juvenile vulnerability to predators. We tested this hypothesis in an ornamented dragonfly (Pachydiplax longipennis). Higher larval body condition indeed enhanced the initial development of its intrasexually selected wing coloration, but was opposed by viability selection in the presence of large aeshnid predators. In contrast, viability selection did not oppose larval body condition in pools when aeshnids were absent, and was not affected when we manipulated cannibalism risk. Trade-offs between larval survival and ornament development, mediated through the conflicting effects of body condition, therefore occurred only under high predation risk. We additionally characterized how body condition influences several traits associated with predator avoidance. Although body condition did not affect burst distance, it did increase larval abdomen size, potentially making larvae easier targets for aeshnid predators. As high body condition similarly increases vulnerability to predators in many other animals, predator-mediated costs of juvenile resource accumulation could be a common, environment-specific limitation on the elaboration of sexually selected traits.

Personality differentially affects individual mate choice decisions in female and male Western mosquitofish (Gambusia affinis)

Consistent individual differences in behavioral tendencies (animal personality) can affect individual mate choice decisions. We asked whether personality traits affect male and female mate choice decisions similarly and whether potential personality effects are consistent across different mate choice situations. Using western mosquitofish (Gambusia affinis) as our study organism, we characterized focal individuals (males and females) twice for boldness, activity, and sociability/shoaling and found high and significant behavioral repeatability. Additionally, each focal individual was tested in two different dichotomous mate choice tests in which it could choose between computer-animated stimulus fish of the opposite sex that differed in body size and activity levels, respectively. Personality had different effects on female and male mate choice: females that were larger than average showed stronger preferences for large-bodied males with increasing levels of boldness/activity (i.e., towards more proactive personality types). Males that were larger than average and had higher shoaling tendencies showed stronger preferences for actively swimming females. Size-dependent effects of personality on the strength of preferences for distinct phenotypes of potential mating partners may reflect effects of age/experience (especially in females) and social dominance (especially in males). Previous studies found evidence for assortative mate choice based on personality types or hypothesized the existence of behavioral syndromes of individuals’ choosiness across mate choice criteria, possibly including other personality traits. Our present study exemplifies that far more complex patterns of personality-dependent mate choice can emerge in natural systems.

Sexual selection on the multicomponent display of black morph male Girardinus metallicus (Pisces: Poeciliidae)

Sexually selected displays often include suites of integrated traits. Black morph males of the poeciliid fish Girardinus metallicus perform courtship and aggressive displays that exhibit their conspicuous yellow and black coloration. Body size, gonopodium size and ventral black area are correlated with intermale aggression, which is key for access to mates. A previous study showed that females may prefer dominant males prior to watching them fight; however, that result was obtained in trials that allowed for male-male interactions across partitions, and to date no study has uncovered the traits important in female choice. We performed a more comprehensive investigation of the multicomponent sexual display including measures of male yellow hue, saturation and brightness. We examined the behavior of size-matched males paired to maximize the difference in yellow saturation, and measured female choice exclusive of male-male interactions and chemical cues. We found no female preference for any traits in the multicomponent sexual display. Males with brighter and more saturated yellow coloration were more likely to be dominant, and dominant males courted and attempted copulations more. Our results suggest that yellow coloration is sexually selected; however, the courtship display requires further investigation because we did not identify targets of female preference, and we discuss possible explanations for this finding.

Dissolved organic carbon and unimodal variation in sexual signal coloration in mosquitofish: a role for light limitation?

Natural selection plays an important role in the evolution of sexual communication systems. Here, we assess the effect of two well-known selection agents, transmission environment and predation, on interpopulation variation in sexual signals. Our model system is a series of 21 populations of Bahamian mosquitofish subjected to independent variation in optical conditions and predation risk. We show that optically diverse environments, caused by locally variable dissolved organic carbon concentrations, rather than spatial variation in predation, drove divergence in fin coloration (fin redness). We found a unimodal pattern of phenotypic variation along the optical gradient indicating a threshold-type response of visual signals to broad variation in optical conditions. We discuss evolutionary and ecological mechanisms that may drive such a pattern as well as the implications of non-monotonic clines for evolutionary differentiation.

Anthropogenic ecosystem fragmentation drives shared and unique patterns of sexual signal divergence among three species of Bahamian mosquitofish

When confronted with similar environmental challenges, different organisms can exhibit dissimilar phenotypic responses. Therefore, understanding patterns of phenotypic divergence for closely related species requires considering distinct evolutionary histories. Here, we investigated how a common form of human-induced environmental alteration, habitat fragmentation, may drive phenotypic divergence among three closely related species of Bahamian mosquitofish (Gambusia spp.). Focusing on one phenotypic trait (male coloration), having a priori predictions of divergence, we tested whether populations persisting in fragmented habitats differed from those inhabiting unfragmented habitats and examined the consistency of the pattern across species. Species exhibited both shared and unique patterns of phenotypic divergence between the two types of habitats, with shared patterns representing the stronger effect. For all species, populations in fragmented habitats had fewer dorsal-fin spots. In contrast, the magnitude and trajectory of divergence in dorsal-fin color, a sexually selected trait, differed among species. We identified fragmentation-mediated increased turbidity as a possible driver of these trait shifts. These results suggest that even closely related species can exhibit diverse phenotypic responses when encountering similar human-mediated selection regimes. This element of unpredictability complicates forecasting the phenotypic responses of wild organisms faced with anthropogenic change – an important component of biological conservation and ecosystem management.

Interpopulation Variation in a Condition-Dependent Signal: Predation Regime Affects Signal Intensity and Reliability

In many models of sexual selection, conspicuous ornaments are preferred by mates because they indicate heritable signaler viability. To function as indicators, ornaments must exhibit a proportional relationship between expression and viability. In cases where the evolutionary interests of signaler and receiver diverge, selection favors exploitative exaggeration by low-viability individuals producing unreliable signals. Theory suggests that the evolutionary stability of such communication systems requires costs that prevent low-viability males from expressing disproportionately intense signals. Therefore, given ecological variation in signaling cost, the reliability of signaling systems will vary concomitantly. In this study, we assess the effect of a variable signal cost, predation, on signal intensity and reliability among 16 populations of Bahamas mosquitofish (Gambusia hubbsi) that use colorful dorsal fins in courtship displays. We found that fin coloration was more intense in low-predation sites and could be used to predict body condition. However, this predictive relationship was apparent only in populations subject to predation risk. We demonstrate an important role for ecological signaling cost in communication and show that ecological heterogeneity drives interpopulation variation in both the intensity and the reliability of a sexual signal.