The interacting effects of total light intensity and chromatic contrast on visual signal visibility in an Anolis lizard

Effects of Urbanization on Ventral Patch Size and Phenotypic Correlates of Patch Expression in Male Western Fence Lizards (Sceloporus occidentalis)

In some animals, males use colorful ornaments or badges to visually communicate with conspecifics. These traits can be condition-dependent, suggesting that environmental changes could impact the intensity of male sexual signals. Drastic habitat changes caused by urbanization can act as physiological stressors, potentially affecting male signaling traits through changes to condition or immune function. Here, we quantified the effects of urbanization on ventral patch size and correlates of patch expression, namely body size, body condition, corticosterone concentrations, and ectoparasites in male Western Fence Lizards (Sceloporus occidentalis). We compared three aspects of male ventral color patches between urban and natural populations: area of the throat patch, total area of the paired belly patches, and total area of the black borders of the belly patches. All three area measurements across both habitat types were positively related to body size, and total belly patch area was positively related to body condition, indicating that these traits may signal male competitive ability and/or quality. Males from urban populations had larger throat patches than those from natural populations after controlling for body size. This difference in patch size was associated with a difference in probability of ectoparasite infection, but not with differences in corticosterone concentrations or body condition between urban and natural populations. Our results may indicate an effect of urbanization on immune function affecting male patch expression, although this idea remains untested. Overall, we show that urbanization can impact male sexual traits, which may have repercussions for visual communication in urban environments.

Predator perception of aposematic and cryptic color morphs in two Oophaga species

Animals that are toxic often advertise their unprofitability to potential predators through bright aposematic colors while cryptic ones blend in with their natural background to avoid predators. In the poison dart frogs, Oophaga pumilio and O. granulifera, some populations in Costa Rica and Panama display cryptic green and aposematic red color morphs. We herein used reflectance spectra from the dorsum of red and green morphs of these frogs to estimate their perception by the visual systems of three potential predators (birds, lizards, and crabs) against three natural backgrounds (leaves, trunks and leaf litter). Statistical analyses revealed no strong differences in color contrast against backgrounds between the two frog species. However, strong effects of frog morph, predator, background, and their interactions were observed. When viewed against diverse backgrounds, red frogs of both Oophaga species are more color conspicuous to birds and Anoline lizards than to crabs. A strong effect of species was observed on luminance contrast. Concerning the latter, green frogs particularly in O. granulifera appear more conspicuous than red frogs, while birds perceive higher brightness contrasts than lizards or crabs. Our results further support the importance of birds and lizards as Oophaga predators and provide a first quantitative comparison of conspicuousness between these two frog species.

Environmental conditions and male quality traits simultaneously explain variation of multiple colour signals in male lizards

Male lizards often display multiple pigment‐based and structural colour signals which may reflect various quality traits (e.g. performance, parasitism), with testosterone (T) often mediating these relationships. Furthermore, environmental conditions can explain colour signal variation by affecting processes such as signal efficacy, thermoregulation and camouflage. The relationships between colour signals, male quality traits and environmental factors have often been analysed in isolation, but simultaneous analyses are rare. Thus, the response of multiple colour signals to variation in all these factors in an integrative analysis remains to be investigated.

Here, we investigated how multiple colour signals relate to their information content, examined the role of T as a potential mediator of these relationships and how environmental factors explain colour signal variation.

We performed an integrative study to examine the covariation between three colour signals (melanin‐based black, carotenoid‐based yellow–orange and structural UV), physiological performance, parasitism, T levels and environmental factors (microclimate, forest cover) in male common lizards Zootoca vivipara from 13 populations.

We found that the three colour signals conveyed information on different aspects of male condition, supporting a multiple message hypothesis. T influenced only parasitism, suggesting that T does not directly mediate the relationships between colour signals and their information content. Moreover, colour signals became more saturated in forested habitats, suggesting an adaptation to degraded light conditions, and became generally brighter in mesic conditions, in contradiction with the thermal melanism hypothesis.

We show that distinct individual quality traits and environmental factors simultaneously explain variations of multiple colour signals with different production modes. Our study therefore highlights the complexity of colour signal evolution, involving various sets of selective pressures acting at the same time, but in different ways depending on colour production mechanism.

Evolutionary history limits species' ability to match colour sensitivity to available habitat light

The spectrum of light that an animal sees-from ultraviolet to far red light-is governed by the number and wavelength sensitivity of a family of retinal proteins called opsins. It has been hypothesized that the spectrum of light available in an environment influences the range of colours that a species has evolved to see. However, invertebrates and vertebrates use phylogenetically distinct opsins in their retinae, and it remains unclear whether these distinct opsins influence what animals see, or how they adapt to their light environments. Systematically using published visual sensitivity data from across animal phyla, we found that terrestrial animals are more sensitive to shorter and longer wavelengths of light than aquatic animals and that invertebrates are more sensitive to shorter wavelengths of light than vertebrates. Using phylogenetically controlled analyses, we found that closed and open canopy habitat species have different spectral sensitivities when comparing across the Metazoa and excluding habitat generalists, while deepwater animals are no more sensitive to shorter wavelengths of light than shallow-water animals. Our results suggest that animals do adapt to their light environment; however, the invertebrate-vertebrate evolutionary divergence may limit the degree to which animals can perform visual tuning.

Dewlap colour variation in Anolis sagrei is maintained among habitats within islands of the West Indies

Animal signals evolve in an ecological context. Locally adapting animal sexual signals can be especially important for initiating or reinforcing reproductive isolation during the early stages of speciation. Previous studies have demonstrated that dewlap colour in Anolis lizards can be highly variable between populations in relation to both biotic and abiotic adaptive drivers at relatively large geographical scales. Here, we investigated differentiation of dewlap coloration among habitat types at a small spatial scale, within multiple islands of the West Indies, to test the hypothesis that similar local adaptive processes occur over smaller spatial scales. We explored variation in dewlap coloration in the most widespread species of anole, Anolis sagrei, across three characteristic habitats spanning the Bahamas and the Cayman Islands, namely beach scrub, primary coppice forest and mangrove forest. Using reflectance spectrometry paired with supervised machine learning, we found significant differences in spectral properties of the dewlap between habitats within small islands, sometimes over very short distances. Passive divergence in dewlap phenotype associated with isolation-by-distance did not seem to explain our results. On the other hand, these habitat-specific dewlap differences varied in magnitude and direction across islands, and thus, our primary test for adaptation-parallel responses across islands-was not supported. We suggest that neutral processes or selection could be involved in several ways, including sexual selection. Our results shed new light on the scale at which signal colour polymorphism can be maintained in the presence of gene flow, and the relative role of local adaptation and other processes in driving these patterns of dewlap colour variation across islands.

Habitat complexity and complex signal function – exploring the role of ornamentation

Animals often communicate in complex, heterogeneous environments, leading to hypothesized selection for increased detectability or discriminability in signaling traits. The extent to which secondary sexual ornaments have evolved to overcome the challenges of signaling in complex environments, however, remains understudied, especially in comparison to their role as indicator traits. This study tested the hypothesis that the condition-dependent secondary sexual ornamentation in the wolf spider Rabidosa rabida functions to increase detectability/discriminability in visually complex environments. We predicted that male ornamentation would interact with the complexity of the signaling environment to affect male mating success. In particular, we expected ornaments to confer a greater mating advantage when males courted in visually complex environments. To test this, we artificially manipulated male foreleg ornamentation (present/absent) and ran repeated-measures mating trials across laboratory microcosms that represented simple versus complex visual signaling environments. Microcosm visual complexity differed in their background pattern, grass stem color, and grass stem placement. We found that ornamented males mated more often and more quickly than unornamented males across both environments, but we found no support for an ornament-by-environment interaction. Male courtship rate, however, did interact with the signaling environment. Despite achieving the same level of mating success across signaling environments, ornamented males courted less rapidly in complex versus simple environments, although environmental complexity had no influence on unornamented male courtship rates. Our results suggest that the visual complexity of the signaling environment influences the interactive influence of ornamentation and dynamic visual courtship on female mate choice.

Ecological Opportunity from Innovation, not Islands, Drove the Anole Lizard Adaptive Radiation

Islands are thought to facilitate adaptive radiation by providing release from competition and predation. Anole lizards are considered a classic example of this phenomenon: different ecological specialists ('ecomorphs') evolved in the Caribbean Greater Antilles (Cuba, Hispaniola, Jamaica, and Puerto Rico), resulting in convergent assemblages that are not observed in mainland Latin America. Yet, the role of islands in facilitating adaptive radiation is more often implied than directly tested, leaving uncertain the role of biogeography in stimulating diversification. Here, we assess the proposed "island effect" on anole diversification using Bayesian phylogenetic comparative methods that explicitly incorporate rate heterogeneity across the tree and demonstrate two cases of would-be false positives. We discovered that rates of speciation and morphological evolution of island and mainland anoles are equivalent, implying that islands provide no special context for exceptionally rapid diversification. Likewise, rates of evolution were equivalent between island anoles that arose via in situ versus dispersal-based mechanisms, and we found no evidence for island-specific rates of speciation or morphological evolution. Nonetheless, the origin of Anolis is characterized by a speciation pulse that slowed over time - a classic signature of waning ecological opportunity. Our findings cast doubt on the notion that islands catalyzed the anole adaptive radiation and instead point to a key innovation, adhesive toe pads, which facilitated the exploitation of many arboreal niches sparsely utilized by other iguanian lizards. The selective pressures responsible for arboreal niche diversification differ between islands and the mainland, but the tempo of diversification driven by these discordant processes is indistinguishable.