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Despite playing a critical role in evolutionary processes and outcomes, relatively little is known about rates of recombination in the vast majority of species, including squamate reptiles—the second largest order of extant vertebrates, many species of which serve as important model organisms in evolutionary and ecological studies. This paucity of data has resulted in limited resolution on questions related to the causes and consequences of rate variation between species and populations, the determinants of within-genome rate variation, as well as the general tempo of recombination rate evolution on this branch of the tree of life. In order to address these questions, it is thus necessary to begin broadening our phylogenetic sampling. We here provide the first fine-scale recombination maps for two species of spiny lizards, Sceloporus jarrovii and Sceloporus megalepidurus, which diverged at least 12 Mya. As might be expected from similarities in karyotype, population-scaled recombination landscapes are largely conserved on the broad-scale. At the same time, considerable variation exists at the fine-scale, highlighting the importance of incorporating species-specific recombination maps in future population genomic studies.

Species with multimodal communication integrate information from social cues in different modalities into behavioral responses that are mediated by changes in gene expression in the brain. Differences in patterns of gene expression between signal modalities may shed light on the neuromolecular mechanisms underlying multisensory processing. Here, we use RNA-Seq to analyze brain transcriptome responses to either chemical or visual social signals in a territorial lizard with multimodal communication. Using an intruder challenge paradigm, we exposed 18 wild-caught, adult, male Sceloporus jarrovii to either male conspecific scents (femoral gland secretions placed on a small pebble), the species-specific push-up display (a programmed robotic model), or a control (an unscented pebble). We conducted differential expression analysis with both a de novo S. jarrovii transcriptome assembly and the reference genome of a closely related species, Sceloporus undulatus. Despite some inter-individual variation, we found significant differences in gene expression in the brain across signal modalities and the control in both analyses. The most notable differences occurred between chemical and visual stimulus treatments, closely followed by visual stimulus versus the control. Altered expression profiles could explain documented aggression differences in the immediate behavioral response to conspecific signals from different sensory modalities. Shared differentially expressed genes between visually- or chemically-stimulated males are involved in neural activity and neurodevelopment and several other differentially expressed genes in stimulus-challenged males are involved in conserved signal-transduction pathways associated with the social stress response, aggression and the response to territory intruders across vertebrates.

Behavioural responses to communicative signals combine input from multiple sensory modalities and signal compensation theory predicts that evolutionary shifts in one sensory modality could impact the response to signals in other sensory modalities. Here, we conducted two types of field experiments with 11 species spread across the lizard genus Sceloporus to test the hypothesis that the loss of visual signal elements affects behavioural responses to a chemical signal (conspecific scents) or to a predominantly visual signal (a conspecific lizard), both of which are used in intraspecific communication. We found that three species that have independently lost a visual signal trait, a colourful belly patch, responded to conspecific scents with increased chemosensory behaviour compared to a chemical control, while species with the belly patch did not. However, most species, with and without the belly patch, responded to live conspecifics with increased visual displays of similar magnitude. While aggressive responses to visual stimuli are taxonomically widespread in Sceloporus , our results suggest that increased chemosensory response behaviour is linked to colour patch loss. Thus, interactions across sensory modalities could constrain the evolution of complex signalling phenotypes, thereby influencing signal diversity.

Most animal signals across sensory modalities are multicomponent traits that can be broken down into discrete elements. If different elements are perceived as unique, independent units (elemental perception), instead of as integrated percepts (configural perception), single changes in the presence/absence or the abundance of specific elements of a multicomponent signal may be enough to impact communication. Here, we found that male Yarrow’s spiny lizards (Sceloporus jarrovii) can discriminate single compounds of a multicomponent chemical signal (femoral gland secretions), different concentrations of a signaling compound, and a single compound from a mixture of compounds. In addition, one chemical compound elicited a response similar to that evoked by the complete natural scent. We conclude that perception of chemical signals in S. jarrovii lizards is elemental but also configural. The elemental perception of signaling compounds seems to occur with high sensitivity and narrow resolution, so that minor changes in single key elements may affect chemical communication. Given the multicomponent nature of most animal signals, hypotheses regarding signal function and evolution would be enhanced if researchers could determine whether these results apply to signals in other sensory modalities and identify the key elements of complex signals, from a receiver’s perspective. Most signals in animal communication are quite complex. For example, odors are mixtures of multiple volatile chemical compounds, and the way in which receivers perceive and process these mixtures to extract relevant information influences the structure and evolution of chemical signals. In a series of behavioral trials, we investigated how male Sceloporus jarrovii lizards may perceive conspecific odors by testing their response to individual and combined mixtures of two compounds present in femoral gland secretions at two different concentrations. We demonstrate that lizards can discriminate structurally similar compounds and that the response to a compound changes when said compound is part of a larger mixture. Compound concentration affected the perception of individual compounds but not complex mixtures. Deciphering what elements and/or configurations are perceived in an odor mixture is the only way to understand the role of mixture composition and its impact on communication.

Single substances within complex vertebrate chemical signals could be physiologically or behaviourally active. However, the vast diversity in chemical structure, physical properties and molecular size of semiochemicals makes identifying pheromonally active compounds no easy task. Here, we identified two volatile cyclic dipeptides, cyclo(L-Leu-L-Pro) and cyclo(L-Pro-L-Pro), from the complex mixture of a chemical signal in terrestrial vertebrates (lizard genus Sceloporus), synthesised one of them and investigated their biological activity in male intra-specific communication. In a series of behavioural trials, lizards performed more chemosensory behaviour (tongue flicks, lip smacks and substrate lickings) when presented with the synthesised cyclo(L-Pro-L-Pro) chemical blend, compared to the controls, the cyclo(L-Leu-L-Pro) blend, or a combined blend with both cyclic dipeptides. The results suggest a potential semiochemical role of cyclo(L-Pro-L-Pro) and a modulating effect of cyclo(L-Leu-L-Pro) that may depend on the relative concentration of both compounds in the chemical signal. In addition, our results stress how minor compounds in complex mixtures can produce a meaningful behavioural response, how small differences in structural design are crucial for biological activity, and highlight the need for more studies to determine the complete functional landscape of biologically relevant compounds.

Animal signals evolve by striking a balance between the need to convey information through particular habitats and the limitations of what types of signals can most easily be produced and perceived. Here, we present new results from field measures of undisturbed behavior and biochemical analyses of scent marks from 12 species of Sceloporus lizards to explore whether evolutionary changes in chemical composition are better predicted by measures of species behavior, particularly those associated with visual displays, chemoreception, and locomotion, or by measures of habitat climate (precipitation and temperature). We found that more active lizard species used fewer compounds in their volatile scent marks, perhaps conveying less specific information about individual and species identity. Scent marks from more active lizard species also had higher proportions of saturated fatty acids, and the evolution of these compounds has been tracking the phylogeny closely as we would expect for a metabolic byproduct. In contrast, the proportions of unsaturated fatty acids were better explained by evolutionary shifts in habitat temperature (and not precipitation), with species in warmer climates using almost no volatile unsaturated fatty acids. The proportion of aldehydes was explained by both behavior and environment, decreasing with behavioral activity and increasing with habitat temperature. Our results highlight the evolutionary flexibility of complex chemical signals, with different chemical compounds responding to different elements of the selective landscape over evolutionary time.

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.

Multiple and multimodal signals can evolve because they convey different information to different receivers or in different contexts. From the perspective of display receivers, however, multimodal signals may pose a challenge since evolutionary changes in any one aspect of the signal may require shifts in other aspects of receiver physiology and behavior. Here, we use field experiments with four species of Sceloporus lizards to test whether evolutionary loss of one element of a complex signal (a colorful belly patch) has led to a change in the behavioral response to a live conspecific. Instead, we found that males of three species (S. merriami, cozumelae, and siniferus) responded to the live conspecific with increased visual and decreased chemical behavior, supporting a Sensory Isolation hypothesis in which animals minimize interference by isolating a single sensory modality, for example, closing eyes to pay closer attention to a sound or smell. In an exception that offers additional support, males of the fourth species, S. parvus, also showed a trade-off in their response, but responded to the live stimulus with more chemical and less visual behavior. We found little evidence that lizards that have lost production of one signal element (belly color) have also altered their response behavior as a consequence. These results emphasize the potentially important role of receiver response in maintaining complex and multimodal signals. Animals use all of their sensory systems to communicate with each other, but using more than one sense at a time can be a challenge. Here, we presented male lizards in the field to a tethered intruder to ask whether lizards that have lost one element of the signal (a color patch) over evolutionary time have also evolved their response to communicative signals. Instead, we found that males of three species responded primarily with visual behavior and decreased their use of chemical behavior, as if focusing their attention entirely on the visual sensory modality. Males of the fourth species responded primarily with chemical behavior, and decreased their use of visual behavior. These results suggest that there may be mechanical constraints limiting communication signals that make use of more than one sensory system.

Testosterone, in addition to promoting the expression of sexual ornaments can negatively affect immune function, leaving individuals more susceptible to parasites (immunocompetence handicap hypothesis). Immunosuppressive effects of testosterone also can occur indirectly, through increased glucocorticoid hormones (corticosterone, cortisol). Therefore, the expression of sexual ornaments and the ability to respond to parasites and diseases may be influenced by the interaction between testosterone and corticosterone. In this study we examined correlations of both testosterone and corticosterone with ectoparasite load and with expression of colorful sexual ornaments (patches on belly and throat) in three species of Sceloporus lizards (S. grammicus, S. megalepidurus, S. torquatus). In addition, we evaluated contributions of sex, body condition, and reproductive season. We expected that individuals with higher testosterone and lower corticosterone levels would have more colorful ornaments than individuals with higher corticosterone levels. In addition, if testosterone has negative effects on immune function but only at higher corticosterone levels, individuals with higher levels of testosterone and corticosterone should have higher ectoparasite loads. Contrary to these expectations, we did not detect an interaction between testosterone and corticosterone statistically affecting either ectoparasite load or the expression of colorful ornaments. Further, we did not find a positive association of either testosterone or corticosterone on ectoparasite loads in any of the three study species. Only in S. grammicus males was a hormone statistically associated with ectoparasite load, but it was a negative association with testosterone. The relationships between both hormones and different aspects of the colorful patches (brightness and chroma) varied drastically among the three species as well as between sexes. Hence, even among congeneric species, we did not observe consistent patterns among color, steroid hormones and ectoparasites. Different associations between these variables may reflect different physiological strategies for the production of colorful signals and immune defense.

Mechanisms underlying production of animal coloration can affect key traits besides coloration. Melanin, and molecules regulating melanin, can directly and indirectly affect other phenotypic traits, such as immune function. We asked whether melanization and a whole-organism measure of immune function are associated with wound healing. Working with two populations of adult male western fence lizards, Sceloporus occidentalis, we compared one high-elevation and one low-elevation population in California where individuals are increasingly darker at higher elevations, measuring wound healing rates. Because of potential interactions of steroid hormones and immune function, we also measured plasma levels of testosterone and corticosterone. Mean healing rates differed significantly, with males in the darker high-elevation population healing more quickly compared to lighter low-elevation males. Males in the low-elevation population had significantly higher mean baseline steroid concentrations. These steroid hormones were also negatively associated with wound healing. We discuss potential differences in selective regime that could produce different patterns. These data also suggest that hormonal pleiotropy does not constrain phenotypic variation.

Fossils and other paleontological information can improve phylogenetic comparative method estimates of phenotypic evolution and generate hypotheses related to species diversification. Here, we use fossil information to calibrate ancestral reconstructions of suitable climate for Sceloporus lizards in North America. Integrating data from the fossil record, general circulation models of paleoclimate during the Miocene, climate envelope modeling, and phylogenetic comparative methods provides a geographically and temporally explicit species distribution model of Sceloporus-suitable habitat through time. We provide evidence to support the historic biogeographic hypothesis of Sceloporus diversification in warm North American deserts and suggest a relatively recent Sceloporus invasion into Mexico around 6 Ma. We use a physiological model to map extinction risk. We suggest that the number of hours of restriction to a thermal refuge limited Sceloporus from inhabiting Mexico until the climate cooled enough to provide suitable habitat at approximately 6 Ma. If the future climate returns to the hotter climates of the past, Mexico, the place of highest modern Sceloporus richness, will no longer provide suitable habitats for Sceloporus to survive and reproduce.

Adaptive behavior is shaped by the type and intensity of selection pressures coming from the environment, such as predation risk and resource availability, and can be modulated by individual's neuroendocrine profile involving steroid hormones and the brain-stem monoaminergic circuits projecting to forebrain structures. Boldness when faced with a predator and exploration/activity when confronted with a new environment reflect the degree of cautiousness and/or “risk-taking” of an individual. In this study we have explored to which extent two populations of Podarcis siculus occupying different ecological niches: mainland (ML) and an islet (ISL) differ in the level of cautiousness and whether these differences are paralleled by differences in their monoaminergic profiles. Boldness was tested in the field as antipredator behavior, while novel space and object explorations were tested in a laboratory setting in an open field apparatus. Finally, serotonin, dopamine, noradrenaline (NA) and adrenaline (ADR) concentrations were measured in whole brain samples by ELISA. Lizards from ML population spent significantly more time hiding after a predator encounter in the field, displayed lower intensity of novel space exploration in a laboratory setting, and contained significantly higher whole-brain concentrations of NA and ADR than their ISL counterparts. Parallelism between the level of risk-taking behavior and concentrations of neurotransmitters mediating alertness and reaction to stress suggests that the differing environmental factors on ML and ISL may have shaped the degree of cautiousness in the residing lizard populations by affecting the activity of NA/ADR neural circuits.

Hormones can mediate suites of correlated traits. Melanocortins regulate melanin synthesis and elements of the melanocortin system can directly, and indirectly, affect a number of other traits, such as stress reactivity. Trait correlations within the melanocortin system have been studied mainly in birds and mammals but less so in reptiles. We examined adult male western fence lizards (Sceloporus occidentalis) and if melanization was correlated with plasma levels of three hormones, including peptide hormone α-melanocyte stimulating hormone (α-MSH), testosterone and corticosterone, and ectoparasite loads. This lizard is darker at higher elevations in California, and we compared five high-elevation and four low-elevation populations during comparable periods of the breeding season at each site. We first validated use of an α-MSH assay kit with lizard plasma. Since Anolis carolinensis is one of the few species with published values for α-MSH plasma levels, we assayed both Anolis and Sceloporus plasma and compared hormone values to those we generated for Anolis to the publish values. We also evaluated effects of different methods of storing spiked plasma pools on resulting α-MSH concentrations. Plasma levels of α-MSH did not differ significantly, but some populations differed significantly in mean corticosterone and mean testosterone. Combining all individuals from the nine populations, we found that individual variation in α-MSH was not associated with individual variation in melanization, but levels of α-MSH were positively associated with plasma testosterone and negatively associated with corticosterone. The lack of association between individual levels of melanization and expression of most other traits differs from a growing number of within-population studies of melanization, and we discuss what differences in physiological mechanisms could produce different hypothetical patterns. Circulating levels of -MSH are only one element of the melanocortin system; in situ synthesis of α-MSH by the skin and the diversity of melanocortin receptors could also contribute to variation in traits mediated by the melanocortin system and should be examined.

Hormones mediate the expression of suites of correlated traits and hence may act either to facilitate or constrain adaptive evolution. Selection on one trait within a hormone-mediated suite of traits may lead to a change in the strength of the hormone signal, causing changes in correlated traits. Growing evidence suggests that melanization, which is in part regulated by hormonal signals, is tightly linked to other traits, such as aggression and stress physiology. Here, we examine six populations of Sceloporus occidentalis lizards differing in degree of melanization (three dark higher-elevation populations and three less-melanized lower-elevation populations) to investigate potential correlations between behavior, hormones, and parasites. We measured aggression by recording behavioral responses of males to staged territorial intrusions; behavior was summarized by two principal components. Analysis revealed that males in the three darker populations signaled aggression less often and made more physical contact than males in the lighter populations. Analyses of plasma steroid hormones (corticosterone and testosterone) revealed significant population differences, but counter to expectation higher aggression was associated with lower testosterone compared across populations. Finally, the three darker populations had higher mean mite loads than the three lighter populations. Overall, this array of phenotypic correlations does not parallel patterns of within-population differences in melanization found in other vertebrates, suggesting that hormonal correlations do not constrain phenotype variation across populations in this species. Given this contradiction between population- and individual-level variation, we urge more study at both levels of variation in traits potentially associated with melanization in other vertebrates.

Acute glucocorticoid elevations can be adaptations to short-term stressors. The breeding season hypothesis predicts reduced glucocorticoid responsiveness to acute stressors in populations or species with short breeding seasons. The striped plateau lizard (Sceloporus virgatus) has a short breeding season in Arizona. We measured plasma corticosterone and total androgen levels (dihydrotestosterone and testosterone) following one of the four stress-handling treatments (0, 10, 60, or 180 min). In both sexes, longer handling stress yielded higher corticosterone; females had higher corticosterone than males at all time points. Androgens did not vary with handling duration, in either sex. Combining treatments, plasma androgens correlated positively with corticosterone (CORT) in females but not in males; plasma CORT and body mass residuals were negatively correlated in both sexes, suggesting lizards in poor body condition and/or not investing heavily in reproduction (follicle mass) have higher acute corticosterone. Total plasma androgens and body mass residuals were positively associated in males, but showed no association in females. The maximal CORT elevation after handling stress in this single-clutching species was of comparable magnitude to responses in related multi-clutching lizard species with longer breeding seasons. Using data from studies of multiple populations of three Sceloporus species, we found no relationship between the relative magnitude of the CORT increase and either latitude or elevation, two variables in the literature correlated with duration of the breeding season, and only weak relationships with geographic elevation and actual (not relative) stress-elevated CORT values in this multi-population comparison.

Long-term signal evolution is shaped by a variety of selective pressures including the need to convey additional information or to improve message transfer to specific receivers or through multiple environments. Here, we test the relative importance of information and sensory modality in shaping the long-term evolution of multimodal signals in Sceloporus lizards. To broadcast identity at territorial boundaries, male Sceloporus use both visual motion (headbob) and chemical signals, whereas they use color (blue belly patches) to signal aggression. Using modern phylogenetic comparative methods, we found a negative correlation between evolutionary changes in visual motion (headbobs) and chemical (femoral pore) signals, but only indirect ties between the evolution of color and motion signals (both of which are perceived visually) through viviparity, and no evidence of an evolutionary link between color and chemical signals. We also find a negative correlation between arboreality and chemical signals. Thus, information content (in this case, broadcasting individual identity versus signaling aggression) appears to play a more important role than sensory modality or physical distance in guiding long-term signal evolution. Additional insights into the underlying evolutionary processes are described, illustrating the utility of a phylogenetic approach.

Mechanisms underlying production of animal coloration can affect key traits besides coloration. Melanin, and molecules regulating melanin, can directly and indirectly affect other phenotypic traits, such as immune function. We asked whether melanization and a whole-organism measure of immune function are associated with wound healing. Working with two populations of adult male western fence lizards, Sceloporus occidentalis, we compared one high-elevation and one low-elevation population in California where individuals are increasingly darker at higher elevations, measuring wound healing rates. Because of potential interactions of steroid hormones and immune function, we also measured plasma levels of testosterone and corticosterone. Mean healing rates differed significantly, with males in the darker high-elevation population healing more quickly compared to lighter low-elevation males. Males in the low-elevation population had significantly higher mean baseline steroid concentrations. These steroid hormones were also negatively associated with wound healing. We discuss potential differences in selective regime that could produce different patterns. These data also suggest that hormonal pleiotropy does not constrain phenotypic variation.

Many evolutionary forces can shape the evolution of communicative signals, and the long-term impact of each force may depend on relative timing and magnitude. We use a phylogenetic analysis to infer the history of blue belly patches ofSceloporuslizards, and a detailed spectrophotometric analysis of four species to explore the specific forces shaping evolutionary change. We find that the ancestor ofSceloporushad blue patches. We then focus on four species; the first evolutionary shift (captured by comparison ofS. merriamiandS. siniferus) represents an ancient loss of the belly patch byS. siniferus, and the second evolutionary shift, bounded byS. undulatusandS. virgatus, represents a more recent loss of blue belly patch byS. virgatus. Conspicuousness measurements suggest that the species with the recent loss (S. virgatus) is the least conspicuous. Results for two other species (S. siniferusandS. merriami) suggest that over longer periods of evolutionary time, new signal colours have arisen which minimize absolute contrast with the habitat while maximizing conspicuousness to a lizard receiver. Specifically, males of the species representing an ancient loss of blue patch (S. siniferus) are more conspicuous than are females in the UV, whereasS. merriamimales have evolved a green element that makes their belly patches highly sexually dimorphic but no more conspicuous than the white bellies ofS. merriamifemales. Thus, our results suggest that natural selection may act more immediately to reduce conspicuousness, whereas sexual selection may have a more complex impact on communicative signals through the introduction of new colours.

We studied neuroendocrine correlates of aggression differences in adults of two Sceloporus lizard species. These species differ in the degree of sex difference in aggressive color signals (belly patches) and in aggression: Sceloporus undulatus (males blue, high aggression; females white, low aggression) and Sceloporus virgatus (both sexes white, lower aggression). We measured plasma testosterone and counted cells expressing androgen receptor-like immunoreactivity to the affinity-purified polyclonal AR antibody, PG-21, in three brain regions of breeding season adults. Male S. undulatus had the highest mean plasma testosterone and differed significantly from conspecific females. In contrast, there was no sex difference in plasma testosterone concentrations in S. virgatus . Male S. undulatus also had the highest mean number of AR-positive cells in the preoptic area: the sexes differed in S. undulatus but not in S. virgatus , and females of the two species did not differ. In the ventral medial hypothalamus, S. undulatus males had higher mean AR cell counts compared to females, but again there was no sex difference in S. virgatus . In the habenula, a control brain region, the sexes did not differ, and although the sex by species interaction significant was not significant, there was a trend ( p = 0.050) for S. virgatus to have higher mean AR cell counts than S. undulatus . Thus hypothalamic AR cell counts paralleled sex and species differences in aggression, as did mean plasma testosterone levels in these breeding-season animals.

Mechanisms underlying production of animal coloration can affect key traits besides coloration. Melanin, and molecules regulating melanin, can directly and indirectly affect other phenotypic traits including aggression, stress-reactivity, and immune function. We studied correlation of melanization with these other traits, comparing within- and between-population differences of adult male western fence lizards, Sceloporus occidentalis. We compared one high- and one low-elevation population in California where individuals are increasingly darker at higher elevations, working during comparable periods of the breeding season at each site (first egg clutch). We measured agonistic behaviors of free-ranging males in response to staged territorial intrusions (STIs). In other sets of males we measured baseline testosterone and corticosterone levels, and hormonal-reactivity to a stress handling paradigm. We counted ectoparasite loads for all males. There were no significant associations between individual variation in melanization and individual variation in any of the variables measured. However, analysis of behavior from the STIs revealed that males in the darker high-elevation population responded with more aggressive behavior compared to males in the lighter low-elevation population. Males in the low-elevation population had significantly higher mean baseline testosterone, but the two populations did not differ in adrenal function (baseline corticosterone or corticosterone after 1-h confinement stress). Males in the darker high-elevation population had higher mean mite loads compared to males in the lighter population. This array of phenotypic differences between the two populations, and the absence of trait associations when assessing individual variation, do not parallel the patterns in other vertebrates. We describe potential differences in selective regimes that could produce these different patterns across vertebrates. These data suggest that hormonal pleiotropy does not constrain phenotypic variation.

Animals rely on multimodal signals to obtain information from conspecifics through alternative sensory systems, and the evolutionary loss of a signal in one modality may lead to compensation through increased use of signals in an alternative modality. We investigated associations between chemical signaling and evolutionary loss of abdominal color patches in males of four species (two plain-bellied and two colorful-bellied) of Sceloporus lizards. We conducted field trials to compare behavioral responses of male lizards to swabs with femoral gland (FG) secretions from conspecific males and control swabs (clean paper). We also analyzed the volatile organic compound (VOC) composition of male FG secretions by stir bar extraction and gas chromatography-mass spectrometry (GC-MS) to test the hypothesis that loss of the visual signal is associated with elaboration of the chemical signal. Males of plain-bellied, but not colorful-bellied species exhibited different rates of visual displays when exposed to swabs of conspecific FG secretions relative to control swabs. The VOC composition of male Sceloporus FG secretions was similar across all four species, and no clear association between relative abundances of VOCs and evolutionary loss of abdominal color patches was observed. The emerging pattern is that behavioral responses to conspecific chemical signals are species- and context-specific in male Sceloporus, and compensatory changes in receivers, but not signalers may be involved in mediating increased responsiveness to chemical signals in males of plain-bellied species.

Colonization of novel habitats is often associated with differences in ecological community composition. For small diurnal animals, differences in predator diversity and abundance can lead to behavioural shifts in the novel habitat. The eastern fence lizard Sceloporus undulatus (Bosc and Daudin, 1801) recently colonized the gypsum dunes of White Sands, a predator-poor community relative to the predator-rich community of the surrounding Chihuahuan dark-soil habitat. We used field experiments to assess S. undulatus anti-predator behaviour in white-sand versus dark-soil habitats, and used laboratory assays to determine whether behavioural differences could be mediated by hormonal regulation. Overall, we found that white-sand lizards were less vigilant but more wary than their dark-soil counterparts; it took them longer to detect a simulated predator, but once detected they were more likely to retreat from their perches than dark-soil lizards. At the proximate level, differences in anti-predator behaviour could not be explained by differences in plasma hormone levels (corticosterone and testosterone); we detected elevated corticosterone for lizards in our stress treatment relative to control treatment, but found no differences between habitats in baseline or acute corticosterone levels. At the evolutionary level, we suggest that differences in anti-predator behaviour may be explained by differences across habitats in predation environment, habituation, and/or the cost of retreating. Our study implicates changes in predator community composition in mediating ecological divergence in behaviour.

Lizards have communicative displays involving primarily vision and chemicals, and recent work suggests trade-offs between these two modalities. In reptiles, little work assesses effects of conspecific chemicals on subsequent signaling behavior. Here, we studied responses to conspecific secretions in two Sceloporus species differing in visual signaling: male Sceloporus undulatus have blue abdominal patches used in aggressive territorial encounters, while Sceloporus virgatus males have evolutionarily lost the patches and have low rates of aggressive display. We measured behavior of free-ranging males following presentation of swabs with conspecific male chemicals from femoral glands and the cloaca, or of clean swabs. For male S. undulatus (blue), neither likelihoods nor rates of behavioral responses differed between swab treatments. In contrast, male S. virgatus (white) presented with secretions had significantly higher likelihood of performing head bob, push-up, and shudder displays, than males in control swab trials. Rates of behavior also differed for S. virgatus, with higher rates of push-up display and tongue flick in trials with conspecific chemicals, but rates of other displays, number of moves, and mean total distance moved did not differ between treatments. Male S. undulatus moved significantly greater distances than S. virgatus, independent of treatment. In sum, male S. virgatus responded to conspecific male chemicals by increasing their low rates of display behavior, whereas male S. undulatus did not alter their already high rates of display or movement following chemical exposure. Chemical signals may play a different role in social signaling in the species with the loss of the abdominal color signal.

Many lizards have brightly colored body parts thought to be signals, but color perception in lizards has not been well studied. We present data documenting differential response to colors in an iguanid lizard. Free-living Uta palmeri differentially approached and bit at a red-colored object when simultaneously presented objects of four different colors (red, blue, yellow, black). This response to red was statistically significant for all three age-sex classes of lizards tested (adult males, adult females, juveniles). In addition, juveniles and adult females exhibited intermediate levels of responses (approaching and sniffing, and licking) to the yellow object but adult males did not. Seasonally important food items of this species are red (fish regurgitated by nesting seabirds) and yellow (yolk from broken seabird eggs), and response behavior (biting versus licking) was similar to that exhibited during feeding. Thus, as presented in our experimental paradigm, red and yellow elicited feeding responses. The spectral reflectance curves of the red and yellow stimuli were found to exhibit peaks at wavelengths of 630 nm (range 570–670), and at 600 nm (range 550–700), respectively. These values overlap with the spectral range of the gravid coloration of female U. palmeri (peak 615 nm, range 550–690) that is thought to be a signal to conspecifics. Furthermore, the reflectance curves are similar in shape and height (intensity). Thus, the similarities of the reflectance curves of the red paint and of the natural coloring suggest that palmeri can detect the female gravid coloration. Whether the response to red objects is due to discriminating differences in hue or in relative brightness remains to be examined.

The study of multicomponent signals in the context of social systems has generated interesting results demonstrating that complex signals are used in many communication systems. The multicomponent signal in the majority of Sceloporus lizards consists of a color signal (blue abdominal coloration) and the behavioral display of the color signal (fullshow behavior). In a small number of species, males have lost the color signal. We staged outdoor trials between conspecific males in two closely related Sceloporus lizards that differ in the presence of blue abdominal coloration. In the species with the evolutionary loss of blue abdominal coloration (Sceloporus virgatus), fullshow behavior is present but reduced compared with that of the species with male abdominal coloration (Sceloporus undulatus consobrinus). In comparison, the mean rates of other behaviors that do not display abdominal skin (push-up, head-bob) did not differ between these species. We also found that S. virgatus males were more likely to show a neutral response following the first fullshow during the 60-min trial. While, S. u. consobrinus males were more likely to respond to the first fullshow with an aggressive response. Thus, in this case, the color signal and the behavioral signal are evolutionarily decoupled because in S. virgatus the loss of the color signal is not coincident with the loss of the behavioral signal.

Most vertebrates respond to acute stressors with rapid plasma glucocorticoid elevations. Variation within species in this hormonal response should correlate with differences in physiological responses and behavioural tactics, yet this is rarely documented. We measured behavioural and hormonal responses of free-ranging male tree lizards (Urosaurus ornatus) during staged encounters with a collared lizard predator (Crotaphytus nebrius) or a control (stick). In our study population, males express alternate reproductive tactics that correlate with throat colour pattern: territorial males have a central blue patch on an orange background (OB males) and nonterritorial males lack the blue patch (O males). We also studied a previously uncharacterized male type with a mottled orange and blue throat (M males). A single encounter with an approaching predator initiated an elevation in plasma corticosterone that was similar among the male types (mean ± SE = 9.64 ± 3.17 (OB), 9.72 ± 2.49 (O), 10.19 ± 2.82 (M)). Despite this similarity, male types differed behaviourally. Compared to OB and M males, O males initiated flight when the predator was further away and stayed hidden longer after seeking refuge. The M males showed responses similar to OB males and may functionally be the same morph. Among individuals of all morphs, total corticosterone levels after the predator encounter correlated strongly with their flight initiation distance and hiding duration during the encounter. We conclude that male tree lizards with alternative reproductive tactics show alternative antipredator behavioural tactics, and that individual variation in escape responses is directly correlated with corticosterone levels.

Male secondary sexual traits purportedly facilitate female choice of a potential mate's quality. One example of male quality is the ability to resist detrimental infections by parasites; sexual traits that reflect parasite infection can allow females to select for parasite resistance in future offspring. High parasite numbers can constrain the exaggeration of male traits, but few studies have examined the effects of parasites on female secondary traits. Female Mexican boulder spiny lizards Sceloporus pyrocephalus, undergo a change in secondary sexual coloration over their breeding season and express red gular regions and gray gular stripes in association with late stages of follicle maturation during the reproductive cycle, as well as blue ventral stripes. We examined if color change associated with the female reproductive cycle varied in hue, saturation and brightness in relation to high nematode loads. Our results suggest that high nematode loads of particular areas of the body are correlated with dull, as opposed to bright, secondary sexual coloration in females of S. pyrocephalus. One reason females could be most vulnerable to nematodes during late reproductive stages is because of corresponding high concentrations of circulating testosterone known to occur in this species at this time. Testosterone has demonstrated immunosuppressive effects in many species; however, results have varied and conclusive evidence for its role in immune function remains controversial. Using a subset of individuals from this study, we found that high concentrations of plasma testosterone were significantly related to high nematode loads. These findings prompt further studies examining the physiological attributes of parasite infestation, the honest signaling capabilities of nematode loads and their potential role in sexual selection for coloration in females.

Signals that convey related information may impose selection on each other, creating evolutionary links between different components of the communicative repertoire. Here, we ask about the consequences of the evolutionary loss of one signal (a colour patch) on another (a motion display) in Sceloporus lizards. We present data on male lizards of four species: two pairs of sister taxa representing two independent evolutionary losses of the static colour patch (Sceloporus cozumelae and Sceloporus parvus; Sceloporus siniferus and Sceloporus merriami). Males of the two species that have undergone an evolutionary loss of blue-belly patches (S. cozumelae, S. siniferus) were less active than their blue-bellied sister taxa (S. parvus, S. merriami), consistent with the suggestion that the belly patches were lost to reduce conspicuousness of species with high predation pressure. In contrast, the headbob display appears to have become more, rather than less, conspicuous over evolutionary time. The colour patch is exhibited primarily during aggressive encounters, whereas headbob displays are multifunction signals used in several different contexts, including aggressive encounters. Males of species that have lost the colour patch produced more motion displays, and the structure of those motion displays were more similar to those produced during combat. In both evolutionary episodes, a static colour signal appears to have been replaced by dynamic motion displays that can be turned off in the presence of predators and other unwanted receivers. The predominant pattern is one of evolutionary compensation and interactions between multiple signals that convey related information.

Although many lizard species detect and respond to conspecific chemical cues, variation in this ability among populations of the same species from different habitat types is less well understood. We compared chemoreceptive behaviors and head-bobs in response to conspecific chemical cues by Ornate Tree Lizards (Urosaurus ornatus) between populations from an upland forested habitat and a lowland desert habitat. Under this experimental design, U. ornatus from different habitats performed chemoreceptive behaviors and head-bobs at similar rates. Rate of chemoreceptive behaviors and head-bobs in response to chemical cues deposited by same- and opposite-sex conspecifics was higher than that of cues deposited by the subject in their home tank, demonstrating that U. ornatus has the ability to detect cues from conspecifics. Head-bob rate was also higher in same- and opposite-sex tanks compared to home tanks, demonstrating that chemical cues alter the rate of a social display behavior. Urosaurus ornatus exh...

A small but taxonomically diverse number of vertebrates have a left-eye preference for aggression, but this has been shown only in males. Here we present data consistent with a left-eye preference for aggressive rejection display in females of the striped plateau lizard, Sceloporus virgatus. Already-mated females perform this display, which contains key elements of male–male aggressive displays, to courting males. Lateral eye placement reduces binocular vision of such displays. In a field study, we recorded the responses of individual gravid females to one of three possible stimulus presentations: a tethered conspecific male presented so the female viewed him (1) from a head-on (HO) position, (2) with her left visual field (LVF) or (3) with her right visual field (RVF). Although a small proportion of all trials yielded no response, females were significantly less likely to show any response in RVF presentations compared with HO or LVF presentations. In a large majority of trials, females responded to males, and almost all responding females displayed aggressively. When females displayed after RVF presentations, a female was equally likely to use her LVF or RVF to view the male when she first performed an aggressive rejection display. In contrast, in LVF and HO presentations where females displayed, females were significantly more likely than expected by chance to use their LVF to view the males when they displayed. Charge, a very aggressive act, also was significantly more likely to occur when females were viewing males with their LVF than their RVF or HO. Finally, in a small number of unmanipulated natural encounters we observed, all females used their LVF to display to courting males. Our results suggest that, as in males of several vertebrate species, females show a left-eye bias for conspecific aggression.

Phrynosomatid lizards show considerable variation among species in the occurrence of a secondary sexual trait, blue abdominal coloration. The production of blue skin may be controlled by at least two cellular components, melanin in melanophores, and guanine in iridophores. To examine the hypothesis that a mechanism producing variation in abdominal coloration is variation in the presence of melanin in the melanophores within the dermal layer of skin, we used light microscopy to compare melanin density of five species of phrynosomatid lizards with ancestral and derived abdominal coloration. Our results show that the skin of adults with blue abdominal coloration has more dermal melanin than white skin regardless of species or sex. We experimentally tested this relationship by examining the dermal melanin in skin from female Sceloporus undulatus consobrinus with exogenously elevated levels of testosterone or 5α-dihydrotestosterone. These females displayed malelike abdominal coloration and malelike me...

Communication in one sensory modality can influence communication in others. Lizards in many phrynosomatid species use primarily visual but also chemical signals. The striped plateau lizard, Sceloporus virgatus, exhibits evolutionary loss of a male color signal that in many species is used during aggressive postural displays towards conspecific males. These patches are used similarly in Urosaurus, the sister genus to Sceloporus. We compared a species in which a color signal has been lost, S. virgatus, to a species retaining the ancestral character state of blue abdominal display patches, Urosaurus ornatus, the common tree lizard, to test two hypotheses: (i) conspicuous postural displays that reveal the abdominal patch location are used less in the species that has lost the color patches; and (ii) potential chemical signals are used more in the species with the color loss. We analyzed both visual display behavior (push-up, full-show) and chemosensory behavior (tongue flick and nose tap) of male lizards following their introduction to a resident conspecific male in his home terrarium. Resident males performed very low rates of all behaviors, but intruders exhibited sufficient behavior for analysis. Supporting the first hypothesis, S. virgatus were less likely than U. ornatus to perform full-show, a display that reveals abdominal skin. Male S. virgatus were more likely to perform push-up than U. ornatus, although S. virgatus performed push-up infrequently. Push-up is a postural display that does not specifically reveal the abdominal patch location. Supporting the second hypothesis, S. virgatus were more likely to perform chemosensory behaviors and performed them at a greater rate than did U. ornatus. Work comparing more closely related species is warranted to determine whether a negative association between conspicuous visual displays and chemosensory behavior is a general pattern.

Two sympatric Delphinium species, D. barbeyi and D. nuttallianum, are ecologically and morphologically similar. However, D. barbeyi has multiple, large inflorescences while D. nuttallianum has a single, small inflorescence. These differences in floral display should result in greater intraplant pollen transfer in D. barbeyi, leading to higher rates of self-pollination through geitonogamy. Reduced gene flow by pollen should in turn produce greater population differentiation among populations of D. barbeyi relative to D. nuttallianum. We tested these predictions by comparing pollinator behavior, breeding systems, outcrossing rates, and population genetic structure of sympatric populations of the two species in Colorado. Bumble bee and hummingbird pollinators visit more flowers and inflorescences per foraging bout in D. barbeyi than in D. nuttallianum. The species differed in breeding system; D. barbeyi produced more seeds by autogamy (9 vs. 2%) than D. nuttallianum and suffered no reduction in seed set in hand-self vs. outcross pollinations, in contrast to a 41% decline in D. nuttallianum. The outcrossing rate in one D. barbeyi population was 55%, but ranged from 87 to 97% in four D. nuttallianum populations. Genetic differentiation among population subdivisions estimated by hierarchical F statistics was .10 times greater in D. barbeyi ( 5 0.055‐0.126) than D. nuttallianum ( 5 0.004‐0.009) at spatial scales ranging from metres u u to 3.5 km. Spatial autocorrelation analysis also indicated more pronounced local genetic structure in D. barbeyi than D. nuttallianum populations. Fixation indices (FIS )o fD. barbeyi adults were much lower than expected based on mating system equilibrium and suggest that differences in the degree of self-compatibility and/or the timing of postpollination selection/inbreeding depression between the two species further contribute to the genetic differences between them.

In a field study on male tree lizards (Urosaurus ornatus), we tested the hypothesis that aggression is lateralized, and predicted that resident males should preferentially use the left-visual field (LVF)/right hemisphere in aggressive interactions with other males. This prediction was based on results for a small but taxonomically diverse number of vertebrates indicating a left-eye preference/ right hemisphere specialization for aggression. Many lizards use postural displays to signal aggression and lateral eye placement prevents binocular vision of such displays. The fullshow is an aggressive display that is typically performed with the body held perpendicular to that of the opponent. We staged male-male encounters by introducing stimulus males to free-ranging males (hereafter called focal males), and then recorded both the focal male’s behavior and the visual field that he used to view the intruder while performing fullshow displays. When intruders were introduced in the right visual field (RVF), focal males were equally likely to use either the LVF or RVF to perform their first fullshow. However, in these RVF introductions focal males were more likely to turn and use the LVF if the intruder was smaller. In contrast to RVF introductions, in LVF introductions focal males were significantly more likely than expected by chance to continue to use the LVF to view the intruder when performing the first fullshow. Furthermore, if focal males performed their first fullshow with the intruder in their LVF, they subsequently gave more aggressive displays than focal males that performed their first display with the intruder in their RVF. Additionally, charge (a highly aggressive behavior that usually leads to direct physical contact and a bite) was significantly more likely to occur while the LVF was being used to view the opponent. These results are consistent with the hypothesis that aggression is lateralized to the right hemisphere. This study is one of only a few examples of laterality of brain function demonstrated in a free-ranging vertebrate.

SYNOPSIS. An organism's phenotype results from an interaction of environment and genotype. Sex steroids play a role in translating sexual genotype into phenotype. The focus of this research has been to extend the model of sex steroid hormone action in sexual differentiation to individual variation in reproductive phenotype. The hypothesis generated, called the relative plasticity hypothesis, has been tested in a species with alternative phenotypes, tree lizards ( Urosaurus ornatus ). Such species are useful models for tests of these ideas because variation is extreme and easily studied. These tests have shown that permanent differentiation of the territorial and nonterritorial phenotypes is accomplished by hormonal mechanisms operating during early development. These are similar to organizational actions of hormones classically described for sexual differentiation. A unique finding of this work is that the adrenal hormone progesterone regulates differentiation of the two male types. Furthermore, the nonterritorial male tree lizard appears to conditionally switch tactics between sedentary satellite and nomadic behavior. Nomadic behavior is triggered under stressful environmental conditions. Nonterritorial tree lizards show greater inhibition of reproductive hormones following stress than do territorial tree lizards, suggesting that a differential sensitivity of reproductive hormones to stress is the endocrine basis of conditional tactic switching in this morph. This mechanism is similar to the classical activational effects of hormones. Together, these findings indicate that individual variation in sexual phenotype is produced by mechanisms similar to sexual differentiation. Further refinement of the models and integration with some evolutionary ideas is explored.

Tree lizards (Urosaurus ornatus) vary in throat fan (dewlap) color. Earlier, we described five dewlap types (Orange, Orange-Blue, Yellow, Yellow-Blue, and Blue), and reported that only males had blue in the dewlap and that presence or absence of a discrete blue patch was correlated with male alternative reproductive phenotypes in a central Arizona population. Here, with a modified scheme characterizing two dewlap elements, background color (orange, yellow, blue) and blue patch occurrence, we assessed: (1) sexual, annual, and geographic variation in the frequencies of dewlap elements; (2) simple habitat correlates; and (3) the effects of laboratory rearing regime on dewlap type. Within a population, frequencies of males and females expressing orange or yellow backgrounds did not differ, suggesting that control of background is similar in the sexes. Within several populations, frequencies of the dewlap elements did not differ across years (and probably generations), indicating that phenotype frequencies are relatively stable. Among five populations frequencies of background colors varied, as did frequencies of male types (blue patch present or absent). Dewlap frequencies did not correlate with habitat (boulders or mesquite trees), although few populations were sampled. In male and female offspring reared from eggs to sexual maturity in a common-garden laboratory study, background color frequencies in both sexes and blue patch frequencies in males differed among offspring from different populations. Offspring frequencies matched respective parental population frequencies. Results suggest that among-population variation in frequencies of the two dewlap elements are mediated by differences in genetics, in maternal effects, or both. Thus, differences in male behavior functionally linked to the blue patch also may be controlled by genetic or maternal effects.

Male tree lizards (Urosaurus ornatus) exhibit permanent adult differences in color and size that are functionally linked with behavioral differences: males with a central blue throat patch are territorial, more aggressive, and smaller than males lacking the patch, who are nomads or satellites foregoing territory defense. Gonadectomy or long-lasting hormone implants in hatchlings affect the development of these permanent adult differences, but similar manipulations in adults are ineffective. Analogous early hormone actions in vertebrate sexual differentiation typically must occur during a critical period. Whether there is a critical period for early hormone actions affecting development of male types in tree lizards is unclear. We followed a protocol identical to our previous experiments on hatchlings, but we manipulated testosterone (T) in later-aged juveniles to determine if they remained sensitive to such hormone manipulations. Testosterone implants given at 60 days posthatching (day 60) did not alter adult throat color, indicating that sensitivity of throat color to T declines by day 60. This contrasts to our earlier work where adult color was affected by T manipulations on day 30. However, size and throat color responded differently to exogenous T, as juveniles given T implants at day 60 grew less than control-implanted males.

Males of many species exhibit strongly dimorphic reproductive behavior and morphology associated with alternative reproductive tactics. Little is known about the physiological control of these individual differences. The relative plasticity hypothesis proposes that such within-sex differences arise from either organizational or activational actions of sex steroid hormones depending on whether adults can switch tactics or not. This hypothesis predicts that differences between individuals in a species where adults cannot switch between tactics (are "fixed") should be organized by early actions of hormones. Three experiments explored the possible organizational role of testosterone (T) on the development of male alternatives in the tree lizard (Urosaurus ornatus), which has two fixed male morphs. Orange (O) males have a mostly orange throat fan (dewlap), are non-territorial nomads, and are larger than orange-blue (OB) males which have an orange dewlap with a central blue patch and are territorial. In the first experiment intact males on the day of hatching were (1) sham-operated and implanted with empty capsules, (2) castrated, or (3) testosterone-implanted. As adults, the castration group had more O males than the control and the T-implanted group had more OB males than the control. Adult body size in castrated and T-implanted groups paralleled naturally occurring morph differences but both were smaller than controls. A second experiment with a lower dose yielded similar results for dewlap type and growth. In a third experiment, intact males were given an empty capsule or a T-implant at 30 days posthatching. Again, the frequency of OB males in the T-implanted group was significantly greater than that in the empty implant group, indicating that either the critical period extends past Day 30 or there is no well-defined critical period. Together, these results support the hypothesis that the organizational action of T or one of its metabolites contributes to the differentiation of these within-sex differences.

Abstract. A previous study found that food of the San Pedro side-blotched iguanian lizard was more abundant around seabird nest-sites (blue-footed booby, Sula nebouxii , and brown booby, S. leucogaster ), and that mating success of male lizards correlated with territory food quality, as measued by the cumulative number of booby chicks recorded in weekly censuses during the lizard breeding season. In the present study, descriptive data suggest that lizard density is correlated with the density of seabird nesting sites. Spatial distribution and social behaviour of adult male and female lizards differ, however. Females are not aggressive, lack territoriality and have highly overlapping home ranges, but males are aggressive, many are territorial, and male territories can contain several female home ranges. The number of females resident on a male's territory is a good index of mating success for the territorial male: 87% of matings observed involved a territorial male and a resident female. A food supplement experiment for half of the breeding season explored whether food distribution affects the placement of female home ranges. Experimental females shifted home ranges into supplemented areas, compared with control females, whose home ranges changed little. Females apparently responded to changes in food and not male distribution, because males did not alter territory locations during food addition. There was an increase in courtship rates for males in the food addition zone, suggesting that mating opportunities were affected by the food supplement. These results support the hypothesis, generated by a previous selection-gradient analysis, that variation in territory food quality contributes to variation in male mating success and indirect selection on male head size. In addition, contrasting these results with published information for Uta stansburiana supports the Emlen & Oring (1977, Science , 187, 215-223) hypothesis that ecological factors affecting the distribution of the limiting sex, and the ability of the other sex to monopolize mates are important mating system determinants.

Elevated plasma corticosterone during stressful events is linked to rapid changes in behavior in vertebrates and can mediate learning and memory consolidation. We tested the importance of acute corticosterone elevation in aversive learning of a novel stressor by wild male eastern fence lizards (Sceloporus undulatus). We found that inhibiting corticosterone elevation (using metyrapone, a corticosterone synthesis blocker) during an encounter with a novel attacker impaired immediate escape responses and limited learning and recall during future encounters. In the wild and in outdoor enclosures, lizards whose acute corticosterone response was blocked by an earlier metyrapone injection did not alter their escape behavior during repeated encounters with the attacker. Control-injected (unblocked) lizards, however, progressively increased flight initiation distance and decreased hiding duration during subsequent encounters. Aversive responses were also initially higher for control lizards exposed to a higher intensity first attack. Further, we demonstrate a role of corticosterone elevation in recollection, since unblocked lizards had heightened antipredator responses 24-28 h later. Exogenously restoring corticosterone levels in metyrapone-injected lizards maintained aversive behaviors and learning at control (unblocked) levels. We suggest that the corticosterone mediation of antipredator behaviors and aversive learning is a critical and general mechanism for the behavioral flexibility of vertebrate prey.

In response to stressful events, most vertebrates rapidly elevate plasma glucocorticoid levels. Corticosterone release stimulates physiological and behavioral responses that can promote survival while suppressing behaviors that are not crucial to immediate survival. Corticosterone also has preparatory effects for subsequent stressors. Using male tree lizards (Urosaurus ornatus), we tested our prediction that elevated corticosterone is important for mediating and enhancing antipredator behaviors. Male tree lizards express developmentally fixed polymorphisms that are mediated by early organizational actions of steroid hormones, and thus we also tested the hypothesis that morph-specific differences in antipredator behaviors of adults are independent of circulating corticosterone levels. Plasma corticosterone levels were elevated exogenously for 12-16 h using non-invasive dermal patches, and we then compared the behavioral responses of these corticosterone-patched males to control-patched males during a simulated encounter with a caged predator (collared lizard, Crotaphytus nebrius) in outdoor enclosures. Elevating corticosterone did not alter the antipredator behavioral repertoire of each male morph, but did enhance their responses during the predator encounter: all corticosterone-patched males responded more quickly, hid longer, and displayed more toward the predator than control-patched males. With the corticosterone patch, the non-territorial and wary orange morph was still behaviorally the most wary morph, responding more quickly and hiding longer than either the bolder orange-blue or mottled morphs. Smaller males were generally warier than larger males, regardless of the endocrine treatment or color morph type. In sum, elevated circulating corticosterone enhances antipredator responses for all male tree lizard morphs, without altering morph-specific or size-specific differences in their behavioral responses.

In a natural population, we document changes in the frequencies of alleles influencing body size and size-correlated alternative male reproductive tactics, and we examine the possible role of sexual selection in producing these changes. Male swordtails (Xiphophorus nigrensis, Rio Choy) exhibit three body size classes (small, intermediate, and large) that primarily derive from allelic variation (s, I, L) at the Y-linked pituitary (P) locus. Some XX individuals are male. They are small and can be fathered either by XX or XY males. We compared the frequencies of Y-linked P alleles across two generations in a natural population. There was a significant decrease in the Y-s genotype relative to Y-I and Y-L genotypes. Laboratory experiments suggest that the disadvantage of Y-s resulted, at least in part, from female preference for larger courting males relative to small chasing males. All larger courting males are of the Y-I or Y-L genotypes. Although the frequency of Y-linked P alleles changed across generations, there was no change in the distribution of male body size classes. The stability of the phenotypic distribution across generations, despite changes in P allele frequencies, was maintained by the production of small XX sons by Y-I and Y-L males.

For species displaying plastic alternative reproductive tactics, the relative plasticity hypothesis (RPH) combined with the positive relationship between androgens and aggression predicts higher androgen levels in more aggressive socially dominant males relative to less aggressive subordinate males (directional RPH). We tested this prediction of the directional RPH by comparing plasma levels of testosterone, dihydrotestosterone, and corticosterone in 2y+-collared lizard males that defended territories with those of first-year males that were mature, but did not defend territories. As expected, 2y+-males exhibited higher rates of advertisement, aggression, and courtship than first-year males. Contrary to expectations of the directional RPH, levels of testosterone, dihydrotestosterone and corticosterone were similar in the males displaying these alternative tactics. Furthermore, whereas display by non-territorial males increased after a neighboring territorial male died, levels of testosterone and corticosterone decreased in these males, counter to prediction that the territorial tactic is activated by increased androgens. This result also suggests that high rates of aggression do not alter plasma steroids, although behavioral manipulations are needed to more fully test this hypothesis. Secretion of testosterone in non-territorial males may promote their high growth rates, and/or may prime them for the rapid behavioral changes that occur when opportunities for territory acquisition arise as a consequence of predation on territorial males. Relationships among hormones differed between these types of males: corticosterone was negatively correlated with both testosterone and dihydrotestosterone in territorial males, but androgens and corticosterone did not significantly covary in non-territorial males.

Conspicuous coloration in females is less well studied compared to that in males. Adult female Mexican boulder spiny lizards (Sceloporus pyrocephalus) have conspicuously colored throat, or gular, regions, ranging from red to yellow, while adult males only weakly express such color in their gular region. Both sexes have dark blue-black gular stripes and venter stripes. Understanding proximate mechanisms underlying trait expression can aid in understanding trait function. To characterize the proximate mechanisms potentially influencing color variation among field-captured male and female S. pyrocephalus, we quantified three aspects of color (hue, saturation, brightness) for three body locations (gular region, gular stripes, venter stripes) and then assessed how color was related to reproductive state and concentrations of the plasma steroid hormones testosterone (T) and corticosterone (CORT) in males and T, CORT, and 17-beta estradiol (E(2)) in females. Testes volume was not related to variation in color or in hormones, perhaps because most males were in peak reproductive condition. Large vitellogenic follicles as opposed to oviductal eggs were associated with higher E(2) in females. Males with more dull gular stripes and females with dull venter stripes had significantly higher CORT. Females with red gular regions and pale grey gular stripes had higher T and E(2) concentrations compared to females with a more yellow gular region and darker gular stripes. Thus, gular region color in females could communicate reproductive state; dull gular stripes in males and dull venter stripes in females could communicate stress status.

Animals often exhibit individual variation in their behavioral responses to the same stimuli in the biotic or abiotic environment. To elucidate the endocrine mechanisms mediating such behavioral variation, we have been studying a species of lizard with two distinct male phenotypes. Here we document behavioral variation across years in one of the two male phenotypes of the tree lizard, Urosaurus ornatus, and present hormone data that support an endocrine mechanism underlying this behavioral variation. Nonterritorial male tree lizards appear to be nomadic rovers in some years and sedentary satellites in others, whereas territorial males are always territorial. This behavioral variation by nonterritorial males was correlated with environmental conditions. In environmentally harsher years (as assessed by rainfall), nonterritorial males appear to behave as nomads, whereas in more benign years they are more site-faithful. A between-year comparison of levels of corticosterone and testosterone for the two male phenotypes supports a model for how hormones underlie the males' reproductive tactics, particularly the nonterritorial males' behavioral plasticity. In an environmentally harsher (drier) year, both types of males had higher corticosterone levels than in a milder (wetter) year, but only nonterritorial males had lower testosterone in the relatively harsher year. We propose that disruptive selection for individual variation in hormonal responses to environmental cues may be a common mechanism underlying the evolution of alternative male reproductive tactics in this and other species.

Male Sceloporus virgatus lack the blue abdominal patches which are used during aggressive encounters in other Sceloporus lizards. Herein we report that, despite having lost this signal, males have retained a behavioural response to experimentally restored blue abdominal patches. We tested two adaptive hypotheses: selection acted primarily upon signallers or selection acted upon both signallers and receivers. The first predicts that only the signal is lost and that male interactions should be affected by the restoration of blue patches. The latter predicts that both the signal and behavioural response are lost and the display of the restored blue patches should have no effect on male-male interactions. We compared the behaviour of receivers in paired encounters where one male (signaller) had blue-painted abdominal patches to a set of trials where both males had white-painted abdomens, unmanipulated abdomens or a novel-painted pattern. The receivers of the blue-painted signal were more likely to display submissive behaviour. The receivers in either the unmanipulated, white-painted or novel-painted signal trials were more likely to display neutral behaviour. These results support the hypothesis that receivers have retained a behavioural response and selection has acted primarily on the signaller. We believe this is the first documentation of males responding to an evolutionarily lost signal in conspecific males.

Behavioral ecologists and evolutionary biologists have become increasingly more focused on studying the physiological processes underlying phenotype expression. Greater attention to proximate mechanisms should improve our understanding of the physiological targets of selection (e.g., Finch and Rose, 1995) and of how physiological processes affect patterns of evolution (e.g., Endler and McLellan, 1988; Ryan, 1991). Workers interested in sexual selection have increasingly turned to questions about the hormonal mechanisms regulating expression of sex-specific secondary traits. Many organisms exhibit traits that differ between the sexes, and such traits often are thought to contribute to differential reproductive success. At the proximate level, these traits are examples of secondary sexual characters—sex-specific traits that develop after primary sexual differentiation and whose expression is dependent on the actions of sex steroid hormones (Van Tienhoven, 1983).

Vertebrates species vary in the degree to which the sexes differ in their expression of secondary sex characters, which can be expressed in one sex but not the other, fully expressed in both sexes, or expressed to different degrees in the two sexes. Sex steroid hormones contribute to the development of sex differences, either through action early in life (organization), following sexual maturation (activation), or both. However, relatively little is known about the contributions of sex steroid hormones to species-level variation in sexual dimorphism. We began to address this by assessing in tree lizards, Urosaurus ornatus, the effects of testosterone (T) and dihydrotestosterone (DHT) on expression of the male-typical traits: femoral pore secretions, accessory sex tissues, patches of ventrolateral blue, intensity and hue of throat color, and body size. We administered long-lasting hormone implants of these androgens to intact or ovariectomized adult females, intact hatchling females, and intact hatchling males to determine the relative contribution of organizational and activational influences of androgens on sexual differentiation of these traits. Waxy femoral pore secretions and full intensity of the orange background color of the throat fan (dewlap) required only activation and both androgens were equally effective. Both androgens caused hypertrophy of accessory tissues associated with the hemipenes, but only in hatchling males. Full expression of ventrolateral blue patches required organization by DHT. Androgens had complex organizational actions on growth. In the doses used in the experiment, DHT inhibited body-length growth but not mass growth of hatchling males. Only T inhibited the growth (length and mass) of hatchling females. Because earlier work found that castration also reduced the growth of hatchling males, the organizational effects of androgen on growth might be dose-related. Results for hormonal control of dewlap type were complex. In this population dewlap color is solid orange in females, but polymorphic in males with 50% expressing solid orange (O) and 50% orange-blue (orange with a central blue patch, OB). These color morphs represent permanent male alternatives: OB males are territorial and more aggressive than O males who do not defend territories. Results of the hormone manipulations are most consistent with the interpretation that dewlap morph type requires both organization and activation by androgen. T appears to be more important in organizing morph differences, whereas DHT appears to be more important in activating expression of underlying morph differences. Females only expressed male-typical OB dewlaps when given DHT, and only about 50% did so.