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Producing and maintaining sexually selected ornaments often hinders survival. Because viability-related traits dictate the survival costs conferred by sexual ornaments, the evolution of viability-related traits can limit and/or compensate for ornament evolution. Here, we examine how the ornamental coloration of male dragonflies co-adapts with thermal physiology--a key suite of viability-related traits that influences nearly all reproductive and ecological interactions. Males of many dragonfly species produce dark color patches on their wings to attract potential mates and intimidate reproductive rivals. However, dark coloration also subjects male dragonflies to heat stress in warm climates by absorbing excess solar radiation. Our phylogenetic comparative analyses revealed that dragonfly species with dark sexual coloration have also evolved increased critical thermal maxima, which may allow them to compensate for ornament-induced heating. This pattern of correlated evolution was especially strong for species that inhabit tropical climates, where the heating costs of dark coloration are most severe. Given that darkened sexual coloration is taxonomically widespread and consistently elevates body temperatures, the pattern of co-adaptation between sexual ornaments and thermal physiology found here could represent a key process driving eco-physiological divergence in the past and influencing how populations respond to the changing climates of the future. [ABSTRACT FROM AUTHOR]

Background and Aim: Colour polymorphic populations constitute excellent model systems for testing ecological and evolutionary hypotheses, as alternate colourations are associated with differences in various biological, behavioural and life‐history traits. Melanism is the most common and most obvious form of polymorphism. In reptiles in general (and snakes in particular), thermal melanism has been a popular hypothesis used to explain the appearance of black individuals. It suggests that a trade‐off exists between superior thermoregulation and reduced protection through the loss of crypsis. Surprisingly, despite the growing body of literature available, to date, there are only two qualitative reviews on the evolutionary significance of melanism in reptiles and no quantitative synthesis. We conducted the first systematic review and meta‐analysis summarising the current knowledge on melanism in polymorphic terrestrial snakes and synthesised the evidence for an adaptive advantage of the melanistic morph. Location: Northern Hemisphere. Taxon: Polymorphic terrestrial snakes. Results: Mean prevalence of melanism is 31% with no significant differences between species. Annual precipitation was a significant moderator of melanism prevalence. Our results revealed no significant differences in odds ratios of melanism between sexes or in mean body size. Main Conclusions: Environmental plasticity can be considered the primary cause of melanism, possibly as a result of seasonal climatic variations, a result corroborated by the formal meta‐analysis conducted. Conclusions from the meta‐analysis are that melanism in snakes follows Gloger's rule, as is the case in birds and mammals, and as opposed to the thermal melanism hypothesis. Further, our findings do not lend support for other predictions from the thermal melanism hypothesis, such as skewed frequency of melanistics in favour of males or females, or larger body sizes in black individuals. Our results hold implications for the future diversity of animal populations, as climate change is predicted to decrease the degree of colour variation. [ABSTRACT FROM AUTHOR]

Producing and maintaining sexually selected ornaments often hinders survival. Because viability-related traits dictate the survival costs conferred by sexual ornaments, the evolution of viability-related traits can limit and/or compensate for ornament evolution. Here, we examine how the ornamental coloration of male dragonflies co-adapts with thermal physiology—a key suite of viability-related traits that influences nearly all reproductive and ecological interactions. Males of many dragonfly species produce dark color patches on their wings to attract potential mates and intimidate reproductive rivals. However, dark coloration also subjects male dragonflies to heat stress in warm climates by absorbing excess solar radiation. Our phylogenetic comparative analyses revealed that dragonfly species with dark sexual coloration have also evolved increased critical thermal maxima, which may allow them to compensate for ornament-induced heating. This pattern of correlated evolution was especially strong for species that inhabit tropical climates, where the heating costs of dark coloration are most severe. Given that darkened sexual coloration is taxonomically widespread and consistently elevates body temperatures, the pattern of co-adaptation between sexual ornaments and thermal physiology found here could represent a key process driving eco-physiological divergence in the past and influencing how populations respond to the changing climates of the future.

Animal colour is a highly adaptive phenotypic trait that can respond to several selection pressures, including those facilitated by variations in climate. The thermal melanism hypothesis predicts that for ectotherms, selection for darker phenotypes should be highest in cooler areas because darker pigmentation should provide thermoregulatory advantages. We tested whether intraspecific variation in colour distribution of Cape Cobras (Naja nivea) conforms to this hypothesis using a dataset of ∼800 georeferenced photographs of cobras available from community science platforms. We scored the dorsal colouration of snakes in each photograph and tested for associations between snake colours and climate variables at those locations. Our results provide strong evidence to suggest that temperature and solar radiation are important predictors of N. nivea colour occurrence, whereas elevation and precipitation are not. Overall, darker snakes have a significantly higher probability of occurrence in colder areas with low solar radiation than lighter snakes, which are more likely to occur in hotter areas with more solar radiation. Our study is the first to provide evidence for thermal melanism in a sub-Saharan African snake species, which was made possible by the availability of community science data. [ABSTRACT FROM AUTHOR]

Seasonal differences in insect pigmentation are attributed to the influence of ambient temperature on pigmentation development. This thermal plasticity is adaptive and heritable, and thereby capable of evolving. However, the specific genes contributing to the variation in plasticity that can drive its evolution remain largely unknown. To address this, we analysed pigmentation and pigmentation plasticity in Drosophila melanogaster. We measured two components of pigmentation in the thorax and abdomen: overall darkness and the proportion of length covered by darker pattern elements (a trident in the thorax and bands in the abdomen) in females from two developmental temperatures (17 or 28°C) and 191 genotypes. Using a GWAS approach to identify the genetic basis of variation in pigmentation and its response to temperature, we identified numerous dispersed QTLs, including some mapping to melanogenesis genes (yellow, ebony, and tan). Remarkably, we observed limited overlap between QTLs for variation within specific temperatures and those influencing thermal plasticity, as well as minimal overlap between plasticity QTLs across pigmentation components and across body parts. For most traits, consistent with selection favouring the retention of plasticity, we found that lower plasticity alleles were often at lower frequencies. The functional analysis of selected candidate QTLs and pigmentation genes largely confirmed their contributions to variation in pigmentation and/or pigmentation plasticity. Overall, our study reveals the existence and underlying basis of extensive and trait‐specific genetic variation for pigmentation and pigmentation plasticity, offering a rich reservoir of raw material for natural selection to shape the evolution of these traits independently. [ABSTRACT FROM AUTHOR]

In Lepidoptera, as an explanation for darker phenotypes occurring in colder areas, wing melanism has been proposed to increase solar thermal gain. Alternatively, trade-offs with aposematic signalling and ultraviolet protection have been proposed as explanations for variation in melanism. To investigate the roles of temperature, humidity, solar radiation and predation in driving melanism in the Ranchman's tiger moth (Arctia virginalis), we characterized wing melanism in 23 populations across the range. We also conducted predation experiments using artificial moths and carried out genetic analyses to examine population structure and to test whether wing coloration was hereditary. We found that wing melanism was positively associated with mean temperature during the flight season, which was the best predictor of melanism rates. Wing melanism also exhibited a negative association with humidity and a weak positive association with insolation. We also found two loci weakly associated with wing melanism and showed that melanism is likely to be highly hereditary but not closely associated with population differentiation. Our results contrast with previous findings that melanism is associated with colder conditions and higher predation risk and suggest that humidity and protection against ultraviolet radiation are potential drivers of variation in wing melanism that have been overlooked. [ABSTRACT FROM AUTHOR]

One of the main aspects associated with the diversity in animal colour is the variation in melanization levels. In ectotherms, melanism can be advantageous in aiding thermoregulation through heat absorption. Darker bodies may also serve as a shield from harmful UV-B radiation. Melanism may also confer protection against parasites and predators through improving immunity responses and camouflage in regions with high precipitation, with complex and shaded vegetations and greater diversity of pathogens and parasites. We studied melanism evolution in the globally distributed ant genus Pheidole under the pressures of temperature, UV-B radiation and precipitation, while considering the effects of body size and nest habit, traits that are commonly overlooked. More importantly, we account for worker caste polymorphism, which is marked by distinct roles and behaviours. We revealed for the first time distinct evolutionary trajectories for each worker subcaste. As expected, major workers from species inhabiting locations with lower temperatures and higher precipitation tend to be more melanised. Curiously, we show a slight trend where minor workers of larger species also tend to have darker bodies when inhabiting regions with higher precipitation. Lastly, we did not find evidence for the effects of UV-B radiation and nest habit in the lightness variation of workers. Our paper explores the evolution of ant melanization considering a marked ant worker polymorphism and a wide range of ecological factors. We discuss our findings under the light of the Thermal Melanism Hypothesis, the Photoprotection Hypothesis and the Gloger's Rule. [ABSTRACT FROM AUTHOR]

Many aposematic species show variation in their color patterns even though selection by predators is expected to stabilize warning signals toward a common phenotype. Warning signal variability can be explained by trade‐offs with other functions of coloration, such as thermoregulation, that may constrain warning signal expression by favoring darker individuals. Here, we investigated the effect of temperature on warning signal expression in aposematic Amata nigriceps moths that vary in their black and orange wing patterns. We sampled moths from two flight seasons that differed in the environmental temperatures and also reared different families under controlled conditions at three different temperatures. Against our prediction that lower developmental temperatures would reduce the warning signal size of the adult moths, we found no effect of temperature on warning signal expression in either wild or laboratory‐reared moths. Instead, we found sex‐ and population‐level differences in wing patterns. Our rearing experiment indicated that ~70% of the variability in the trait is genetic but understanding what signaling and non‐signaling functions of wing coloration maintain the genetic variation requires further work. Our results emphasize the importance of considering both genetic and plastic components of warning signal expression when studying intraspecific variation in aposematic species. [ABSTRACT FROM AUTHOR]

Abstract Many aposematic species show variation in their color patterns even though selection by predators is expected to stabilize warning signals toward a common phenotype. Warning signal variability can be explained by trade‐offs with other functions of coloration, such as thermoregulation, that may constrain warning signal expression by favoring darker individuals. Here, we investigated the effect of temperature on warning signal expression in aposematic Amata nigriceps moths that vary in their black and orange wing patterns. We sampled moths from two flight seasons that differed in the environmental temperatures and also reared different families under controlled conditions at three different temperatures. Against our prediction that lower developmental temperatures would reduce the warning signal size of the adult moths, we found no effect of temperature on warning signal expression in either wild or laboratory‐reared moths. Instead, we found sex‐ and population‐level differences in wing patterns. Our rearing experiment indicated that ~70% of the variability in the trait is genetic but understanding what signaling and non‐signaling functions of wing coloration maintain the genetic variation requires further work. Our results emphasize the importance of considering both genetic and plastic components of warning signal expression when studying intraspecific variation in aposematic species.

Color polymorphism offers rich opportunities for studying the eco‐evolutionary mechanisms that drive the adaptations of local populations to heterogeneous and changing environments. We explored the color morph diversity and composition in a Chrysomela lapponica leaf beetle across its entire distribution range to test the hypothesis that environmental and climatic variables shape spatiotemporal variation in the phenotypic structure of a polymorphic species. We obtained information on 13 617 specimens of this beetle from museums, private collections, and websites. These specimens (collected from 1830–2020) originated from 959 localities spanning 33° latitude, 178° longitude, and 4200 m altitude. We classified the beetles into five color morphs and searched for environmental factors that could explain the variation in the level of polymorphism (quantified by the Shannon diversity index) and in the relative frequencies of individual color morphs. The highest level of polymorphism was found at high latitudes and altitudes. The color morphs differed in their climatic requirements; composition of colour morphs was independent of the geographic distance that separated populations but changed with collection year, longitude, mean July temperature and between‐year temperature fluctuations. The proportion of melanic beetles, in line with the thermal melanism hypothesis, increased with increasing latitude and altitude and decreased with increasing climate seasonality. Melanic morph frequencies also declined during the past century, but only at high latitudes and altitudes where recent climate warming was especially strong. The observed patterns suggest that color polymorphism is especially advantageous for populations inhabiting unpredictable environments, presumably due to the different climatic requirements of coexisting color morphs. [ABSTRACT FROM AUTHOR]

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Climatic gradients frequently predict large‐scale ecogeographical patterns in animal coloration, but the underlying causes are often difficult to disentangle. We examined ecogeographical patterns of reflectance among 343 European butterfly species and isolated the role of selection for thermal benefits by comparing animal‐visible and near‐infrared (NIR) wavebands. NIR light accounts for ~50% of solar energy but cannot be seen by animals so functions primarily in thermal control. We found that reflectance of both dorsal and ventral surfaces shows thermally adaptive correlations with climatic factors including temperature and precipitation. This adaptive variation was more prominent in NIR than animal‐visible wavebands and for body regions (thorax‐abdomen and basal wings) that are most important for thermoregulation. Thermal environments also predicted the reflectance difference between dorsal and ventral surfaces, which may be due to modulation between requirements for heating and cooling. These results highlight the importance of climatic gradients in shaping the reflectance properties of butterflies at a continent‐wide scale. [ABSTRACT FROM AUTHOR]

Ecogeographic rules provide a framework within which to test evolutionary hypotheses of adaptation. Gloger's rule predicts that endothermic animals should have darker colors in warm/rainy climates. This rule also predicts that animals should be more rufous in warm/dry climates, the so-called complex Gloger's rule. Empirical studies frequently demonstrate that animals are darker in cool/wet climates rather than in warm/wet climates. Furthermore, sensory ecology predicts that, to enhance crypsis, animals should be darker in darker light environments. We aimed to disentangle the effects of climate and light environments on plumage color in the large Neotropical passerine family Furnariidae. We found that birds in cooler and rainier climates had darker plumage even after controlling for habitat type. Birds in darker habitats had darker plumage even after controlling for climate. The effects of temperature and precipitation interact so that the negative effect of precipitation on brightness is strongest in cool temperatures. Finally, birds tended to be more rufous in warm/dry habitats but also, surprisingly, in cool/wet locales. We suggest that Gloger's rule results from complementary selective pressures arising from myriad ecological factors, including crypsis, thermoregulation, parasite deterrence, and resistance to feather abrasion. [ABSTRACT FROM AUTHOR]

Traits that are significant to the thermal ecology of temperate or montane species are expected to prominently co‐vary with the thermal environment experienced by an organism. The Himalayan Pieris canidia butterfly exhibits considerable variation in wing melanisation. We investigated: (i) whether variation in wing melanisation and (ii) activity period of this montane butterfly was influenced by the seasonally and elevationally changing thermal landscape.We discovered that wing melanisation varied across elevation, seasons, sex, and wing surfaces, with the variation strongly structured in space and time: colder seasons and higher elevations produced more melanic individuals. Notably, melanisation did not vary uniformly across all wing surfaces: (i) melanisation of the ventral hindwing co‐varied much more prominently with elevation, but (ii) melanisation on all other surfaces varied with seasonal changes in the thermal environment.Observed wing surface‐specific patterns indicated thermoregulatory function for this variation in melanisation. Such wing surface‐specific responses to seasonal and elevational variation in temperature have rarely been reported in montane insects.Moreover, daily and seasonal thermal cycles were found to strongly influence activity periods of this species, suggesting the potential limits to wing melanisation plasticity.Overall, these results showed that the seasonal and elevational gradients in temperature influence the thermal phenotype as well as activity periods of this Himalayan butterfly. It will be critical to study the phenotypic evolution of such montane insects in response to the ongoing climate change, which is already showing significant signs in this iconic mountain range. [ABSTRACT FROM AUTHOR]

Insect colours assist in body protection, signalling, and physiological adaptations. Colours also convey multiple channels of information. These channels are valuable for species identification, distinguishing individual quality, and revealing ecological or evolutionary aspects of animals' life. During recent years, the emerging interest in colour research has been raised in social hymenopterans such as ants, wasps, and bees. These insects provide important ecosystem services and many of those are model research organisms. Here we review benefits that various colour types give to social insects, summarize practical applications, and highlight further directions. Ants might use colours principally for camouflage, however the evolutionary function of colour in ants needs more attention; in case of melanin colouration there is evidence for its interrelation with thermoregulation and pathogen resistance. Colours in wasps and bees have confirmed linkages to thermoregulation, which is increasingly important in face of global climate change. Besides wasps use colours for various types of signalling. Colour variations of well chemically defended social insects are the mimetic model for unprotected organisms. Despite recent progress in molecular identification of species, colour variations are still widely in use for species identification. Therefore, further studies on variability is encouraged. Being closely interconnected with physiological and biochemical processes, insect colouration is a great source for finding new ecological indicators and biomarkers. Due to novel digital imaging techniques, software, and artificial intelligence there are emerging possibilities for new advances in this topic. Further colour research in social insects should consider specific features of sociality. [ABSTRACT FROM AUTHOR]

1. When the breeding environment fluctuates across generations, reproductive traits may evolve plasticity that optimises the balance between survival and mating success for the prevailing environment. 2. For sexually selected colouration, this balance can depend on environmental temperatures. Accordingly, breeding colouration often co‐varies with temperature through space and time. However, whether such traits exhibit plasticity in response to environmental temperatures is poorly understood. 3. In the present study, a dragonfly (Pachydiplax longipennis) was reared under ambient or experimentally warmed conditions and tested for plasticity in its intrasexually selected wing colouration. Although wing colouration improves male territorial success, these advantages are smaller under warmer conditions than cooler conditions. It was therefore predicted that males reared under the ambient thermal conditions of the study site (Cleveland, Ohio) would develop more wing colouration than those reared under experimentally warmed conditions. 4. Contrary to this prediction, males reared in warm larval temperatures produced more wing colouration. Thus, although the secondary sexual colouration of this species displays some thermal plasticity, it does not appear to be adaptive relative to the known thermal variation of intrasexual selection in this population. 5. Given that the environment often determines the strength and direction of sexual selection, future studies should consider the potential for non‐adaptive, and even maladaptive, developmental plasticity in the sexually selected traits of insects. [ABSTRACT FROM AUTHOR]

Gradients in cuticle lightness of ectotherms have been demonstrated across latitudes and elevations. Three key hypotheses have been used to explain these macroecological patterns: the thermal melanism hypothesis (TMH), the melanism‐desiccation hypothesis (MDH) and the photo‐protection hypothesis (PPH). Yet the broad abiotic measures, such as temperature, humidity and UV‐B radiation, typically used to detect these ecogeographical patterns, are a poor indication of the microenvironment experienced by small, cursorial ectotherms like ants.We tested whether these macroecological hypotheses explaining cuticle lightness held at habitat and microclimatic level by using a vertical gradient within a tropical rainforest.We sampled 222 ant species in lowland, tropical rainforest across four vertical strata: subterranean, ground, understory and canopy. We recorded cuticle lightness, abundance and estimated body size for each species and calculated an assemblage‐weighted mean for cuticle lightness and body size for each vertical stratum. Abiotic variables (air temperature, vapour pressure deficit and UV‐B radiation) were recorded for each vertical stratum.We found that cuticle lightness of ant assemblages was vertically stratified: ant assemblages in the canopy and understory were twice as dark as assemblages in ground and subterranean strata. Cuticle lightness was not correlated with body size, and there was no support for the TMH. Rather, we attribute this cline in cuticle lightness to a combination of the MDH and the PPH.Our findings indicate that broad macroecological patterns can be detected at much smaller spatial scales and that microclimatic gradients can shape trait variation, specifically the cuticle lightness of ants. These results suggest that any changes to microclimate that occur due to land‐use change or climate warming could drive selection of ants based on cuticle colour, altering assemblage structure and potentially ecosystem functioning. [ABSTRACT FROM AUTHOR]

Ectothermic animals depend on external heat sources for pursuing their daily activities. However, reaching sufficiently high temperature can be limiting at high altitudes, where nights are cold and seasons short. We focus on the role of a green‐brown color polymorphism in grasshoppers from alpine habitats. The green‐brown polymorphism is phylogenetically and spatially widespread among Orthopterans and the eco‐evolutionary processes that contribute to its maintenance have not yet been identified.We here test whether green and brown individuals heat up to different temperatures under field conditions. If they do, this would suggest that thermoregulatory capacity might contribute to the maintenance of the green‐brown polymorphism.We recorded thorax temperatures of individuals sampled and measured under field conditions. Overall, thorax temperatures ranged 1.7–42.1°C. Heat up during morning hours was particularly rapid, and temperatures stabilized between 31 and 36°C during the warm parts of the day. Female body temperatures were significantly higher than body temperatures of males by an average of 2.4°C. We also found that brown morphs were warmer by 1.5°C on average, a pattern that was particularly supported in the polymorphic club‐legged grasshopper Gomphocerus sibiricus and the meadow grasshopper Pseudochorthippus parallelus.The difference in body temperature between morphs might lead to fitness differences that can contribute to the maintenance of the color polymorphism in combination with other components, such as crypsis, that functionally trade‐off with the ability to heat up. The data may be of more general relevance to the maintenance of a high prevalence polymorphism in Orthopteran insects. [ABSTRACT FROM AUTHOR]

Gloger's rule is an ecogeographical rule that links animal colouration with climatic variation. This rule is named after C.W.L. Gloger who was one of the first to summarise the associations between climatic variation and animal colouration, noting in particular that birds and mammals seemed more pigmented in tropical regions. The term 'Gloger's rule' was coined by B. Rensch in 1929 and included different patterns of variation from those described by Gloger. Rensch defined the rule in two ways: a simple version stating that endothermic animals are predicted to be darker in warmer and humid areas due to the increased deposition of melanin pigments; and a complex version that includes the differential effects of humidity and temperature on both main types of melanin pigments – eu‐ and phaeo‐melanin. The blackish eu‐melanins are predicted to increase with humidity, and decrease only at extreme low temperatures, while the brown‐yellowish phaeomelanins prevail in dry and warm regions and decrease rapidly with lower temperatures. A survey of the literature indicates that there is considerable variation/confusion in the way Gloger's rule is understood (based on 271 studies that define the rule). Whereas the complex version is hardly mentioned, only a quarter of the definitions are consistent with the simple version of Gloger's rule (darker where warm and wet), and most definitions mention only the effects of humidity (darker where wet). A smaller subset of studies define the rule based on other correlated climatic and environmental variables such as vegetation, latitude, altitude, solar radiation, etc., and a few even contradict the original definition (darker where cold). Based on the literature survey, I synthesised the qualitative (N = 124 studies) and quantitative (meta‐analytically, N = 38 studies, 241 effects) evidence testing the simple version of Gloger's rule (I found no tests of the complex version). Both lines of evidence supported the predicted effects of humidity (and closely linked variables) on colour variation, but not the effects of temperature. Moreover, humidity effects are not restricted to birds and mammals, as the data indicate that these effects also apply to insects. This suggests that the simple version of Gloger's rule as originally defined may not be valid, and possibly that the rule should be re‐formulated in terms of humidity effects only. I suggest, however, that more data are needed before such a reformulation, due to potential publication biases. In conclusion, I recommend that authors cite Rensch when referring to Gloger's rule and that they make clear which version they are referring to. Future research should concentrate on rigorously testing the validity and generality of both versions of Gloger's rule and establishing the mechanism(s) responsible for the patterns it describes. Since humidity seems to be the core climatic variable behind Gloger's rule, I suggest that the two most plausible mechanisms are camouflage and protection against parasites/pathogens, the latter possibly through pleiotropic effects on the immune system. Understanding the processes that lead to climatic effects on animal colouration may provide insights into past and future patterns of adaptation to climatic change. [ABSTRACT FROM AUTHOR]

Colour variation across climatic gradients is a common ecogeographical pattern; yet there is long-standing contention over underlying causes, particularly selection for thermal benefits. We tested the evolutionary association between climate gradients and reflectance of near-infrared (NIR) wavelengths, which influence heat gain but are not visible to animals. We measured ultraviolet (UVA), visible (Vis) and NIR reflectance from calibrated images of 372 butterfly specimens from 60 populations (49 species, five families) spanning the Australian continent. Consistent with selection for thermal benefits, the association between climate and reflectance was stronger for NIR than UVA-Vis wavelengths. Furthermore, climate predicted reflectance of the thorax and basal wing, which are critical to thermoregulation; but it did not predict reflectance of the entire wing, which has a variable role in thermoregulation depending on basking behaviour. These results provide evidence that selection for thermal benefits has shaped the reflectance properties of butterflies. [ABSTRACT FROM AUTHOR]

The delimitation of two ichneumonid species, Spilopteron apicale (Matsumura) and S. tosaense (Uchida), was investigated using DNA sequences of the mitochondrial COI and nuclear 28S rRNA genes, as well as adult morphology. The two species have long been confused and were until recently speculated to be the same species with continuous colour variation. Our molecular and morphological studies reveal that there are two distinct species: a dark species confined to northern or high latitude localities (S. apicale) and a widely distributed species with a marked geographical gradient in body colour (S. tosaense) across the Japanese Archipelago. In the latter species, female body colour became significantly darker with latitude and altitude. A lectotype of Chorischizus apicalis Matsumura, 1912 is designated.

Aim: Geographic distributions are driven by a combination of species sensitivity and exposure to climate. We quantified colour lightness, a trait that mediates the interaction between sensitivity and exposure, of diverse butterfly communities to test whether colour lightness is associated with community assembly across climate‐elevation gradients. Location: Ecuadorian Andes. Methods: We used a long‐term dataset of museum specimens for two of the most species‐rich genera of Pieridae butterflies in Ecuador, Catasticta and Leptophobia. Within a phylogenetic framework, we examined how communities assemble based on four traits across elevation: colour lightness, species‐specific heating rate, maximum temperature (under experimental solar exposure), and elevation breadth. Results: We found that colour lightness in both genera was related to elevation, but the two genera exhibited opposite patterns; Catasticta are darker and Leptophobia are lighter with increasing elevation. The two genera have opposite configurations of body and body + wings colour lightness but achieve comparable thermoregulation, assessed via their rates of heating under experimental solar exposure. Additionally, we found that the phylogenetic signal for colour lightness was strong, and that patterns between traits and elevation held after correction for phylogeny in Catasticta but not in Leptophobia. Main conclusions: The two genera exhibit divergent relationships between elevational and colour lightness patterns, with evidence that these relationships evolved multiple times in Catasticta. Communities from these two genera have likely been shaped by selection on different traits, with Catasticta colour lightness more responsive to temperature than Leptophobia. The observed geographic patterns of colour lightness in both body + wings (Catasticta) and body (Leptophobia) correspond strikingly with the distribution of montane cloud forests. Habitat fragmentation and cloud lifting from climate change documented across the Andes may therefore significantly impact communities through increased exposure to solar radiation, highlighting the complexity of conserving these diverse montane communities. [ABSTRACT FROM AUTHOR]

Abstract: Aim: The observed decrease in insect colour lightness with increasing latitude has been explained in earlier macroecological studies by the increased solar absorption of dark coloration, which allows extended periods of activity. However, melanin‐based dark coloration also protects against pathogens and ultraviolet radiation, which select for dark coloration at low latitudes where these selection pressures intensify. In nocturnal insects, the relative importance of these protective functions of dark coloration is expected to surpass thermoregulatory functions, as nocturnal species cannot benefit from extended periods of activity during the daytime. Hence, we expected that diurnal and nocturnal insects show contrasting geographical patterns of colour lightness. We tested these predictions using geometrid moths, which comprise both diurnal and nocturnal species. Location: Western Palearctic. Time period: Contemporary. Major taxa studied: Geometridae (Lepidoptera). Methods: We used digital image analysis to assess the colour lightness of 637 species, compiled their distribution across 3,777 grid cells of 50 km × 50 km and calculated the assemblage‐based average colour lightness. We used multiple regressions, autoregressive error models and randomizations to test for relationships between colour lightness and environmental variables associated with the thermal environment, putative pathogen pressure and ultraviolet radiation. Results: We found a clear decrease in colour lightness of assemblages of both diurnal and nocturnal moth species with increasing latitude. In every model, solar radiation was the most important predictor of colour lightness; that is, colour lightness consistently increased with increasing solar radiation. Main conclusions: These results indicate that the thermal environment is the most influential climatic driver of insect colour lightness—independent of thermoregulatory strategy and nocturnal or diurnal activity. This challenges the view that extended periods of activity are the main selection pressure for geographical variation in insect colour lightness. Consequently, the relationship between insect colour lightness and the thermal environment might be more general than previously thought. [ABSTRACT FROM AUTHOR]

Morphological traits can determine the ecological niches and performance of ectotherms and structure their distributions along environmental gradients. The thermal melanism hypothesis and Bergmann’s rule describe patterns of body colour luminance and body size along environmental gradients shaped by thermal influences on morphology. However, these patterns have rarely been investigated at the interspecific level for subtropical and tropical mountain environments. In this study, we sampled butterfly assemblages along elevations across three subtropical and tropical locations in China and examined how environmental factors affected body colour luminance and body size. We additionally reconstructed phylogenetic relationships among the sampled butterfly species and investigated morphology– elevation relationships within an evolutionary framework. Butterfly assemblages were consistently darker and larger at higher elevations across three replicate locations. Furthermore, based on a phylogenetic comparative analysis, we found that body colour luminance and body size of butterfly assemblages responded to elevation through both long-term processes and more recent environmental influences. Our findings support the thermal melanism hypothesis and Bergmann’s rule from diverse subtropical and tropical butterfly assemblages, indicating elevation may structure the distributions of tropical species through morphology. The thermal functions of morphology should therefore be considered when investigating species distribution patterns and responses to environmental changes. [ABSTRACT FROM AUTHOR]

The importance of colour for temperature regulation in animals remains controversial. Colour can affect an animal's temperature because all else being equal, dark surfaces absorb more solar energy than do light surfaces, and that energy is converted into heat. However, in reality, the relationship between colour and thermoregulation is complex and varied because it depends on environmental conditions and the physical properties, behaviour and physiology of the animal. Furthermore, the thermal effects of colour depend as much on absorptance of near-infrared ((NIR), 700-2500 nm) as visible (300-700 nm) wavelengths of direct sunlight; yet the NIR is very rarely considered or measured. The few available data on NIR reflectance in animals indicate that the visible reflectance is often a poor predictor of NIR reflectance. Adaptive variation in animal coloration (visible reflectance) reflects a compromise between multiple competing functions such as camouflage, signalling and thermoregulation. By contrast, adaptive variation in NIR reflectance should primarily reflect thermoregulatory requirements because animal visual systems are generally insensitive to NIR wavelengths. Here, we assess evidence and identify key research questions regarding the thermoregulatory function of animal coloration, and specifically consider evidence for adaptive variation in NIR reflectance. This article is part of the themed issue 'Animal coloration: production, perception, function and application'. [ABSTRACT FROM AUTHOR]

Melanins are ancient biological pigments found in all kingdoms of life. In fungi, their role in microbial pathogenesis is well established; however, these complex biomolecules also confer upon fungal microorganisms the faculty to tolerate extreme environments such as the Earth's poles, the International Space Station and places contaminated by toxic metals and ionizing radiation. A remarkable property of melanin is its capacity to interact with a wide range of electromagnetic radiation frequencies, functioning as a protecting and energy harvesting pigment. Other roles of fungal melanin include scavenging of free radical, thermo-tolerance, metal ion sequestration, cell development, and mechanical-chemical cellular strength. In this review, we explore the various functions ascribed to this biological pigment in fungi and its remarkable physicochemical properties. [ABSTRACT FROM AUTHOR]

ABSTRACT Aim In ectotherms, the colour of an individual's cuticle may have important thermoregulatory and protective consequences. In cool environments, ectotherms should be darker, to maximize heat gain, and larger, to minimize heat loss. Dark colours should also predominate under high UV-B conditions because melanin offers protection. We test these predictions in ants (Hymenoptera: Formicidae) across space and through time based on a new, spatially and temporally explicit, global-scale combination of assemblage-level and environmental data. Location Africa, Australia and South America. Methods We sampled ant assemblages ( n = 274) along 14 elevational transects on three continents. Individual assemblages ranged from 250 to 3000 m a.s.l. (minimum to maximum range in summer temperature of 0.5-35 °C). We used mixed-effects models to explain variation in assemblage cuticle lightness. Explanatory variables were average assemblage body size, temperature and UV-B irradiation. Annual temporal changes in lightness were examined for a subset of the data. Results Assemblages with large average body sizes were darker in colour than those with small body sizes. Assemblages became lighter in colour with increasing temperature, but darkened again at the highest temperatures when there were high levels of UV-B. Through time, temperature and body size explained variation in lightness. Both the spatial and temporal models explained c. 50% of the variation in lightness. Main conclusions Our results are consistent with the thermal melanism hypothesis, and demonstrate the importance of considering body size and UV-B radiation exposure in explaining the colour of insect cuticle. Crucially, this finding is at the assemblage level. Consequently, the relative abundances and identities of ant species that are present in an assemblage can change in accordance with environmental conditions over elevation, latitude and relatively short time spans. These findings suggest that there are important constraints on how ectotherm assemblages may be able to respond to rapidly changing environmental conditions. [ABSTRACT FROM AUTHOR]

Mimicry is considered a classic example of the elaborate adaptations that natural selection can produce, yet often similarity between Batesian (harmless) mimics and their unpalatable models is far from perfect. Variation in mimetic accuracy is a puzzle, as natural selection should favour mimics that are hardest to distinguish from their models. Numerous hypotheses exist to explain the persistence of inaccurate mimics, but most have rarely or never been tested against empirical observations from wild populations. One reason for this is the difficulty in measuring pattern similarity, a key aspect of mimicry. Here, we use a recently developed method, based on the distance transform of binary images, to quantify pattern similarity both within and among species for a group of hoverflies and their hymenopteran models. This allowed us to test three key hypotheses regarding inaccurate mimicry. Firstly, we tested the prediction that selection should be more relaxed in less accurate mimics, but found that levels of phenotypic variation are similar across most hoverfly species. Secondly, we found no evidence that mimics have to compromise between accuracy to multiple model species. However, we did find that darker-coloured hoverflies are less accurate mimics, which could lead to a trade-off between mimicry and thermoregulation in temperate regions. Our results shed light on a classic problem concerning the limitations of natural selection. [ABSTRACT FROM AUTHOR]

Color and pigmentation patterns of the integument can facilitate crypsis, thermoregulation, and social signaling. According to the “thermal melanism hypothesis”, cold environmental temperature should increase the quantity of melanin that is deposited in the integument thereby facilitating radiative warming. We studied the influences of water temperature (26 °C or 31 °C) and substrate color (black or white) on the degree of melanization in the red-eared slider, Trachemys scripta elegans , under laboratory conditions. Turtles reared on a black substrate, or in 26 °C water, for 120 days were darker than those reared on a white substrate or in 31 °C water. A potential tradeoff between the fitness benefits of crypsis and the benefits of radiative warming through melanism was detected because turtles reared in 26 °C water and on a white substrate were darker than those reared on a white substrate and in 31 °C water. Low temperatures limited metabolic processes because turtles reared in 26 °C water grew more slowly than those reared in 31 °C water. However, histological analyses revealed that melanization was a dynamic process in all treatments confirming that the degree of melanization in the cool water treatment was not influenced by the initial and relatively dark hatchling coloration in individuals that grew relatively slowly. [ABSTRACT FROM AUTHOR]

1. Melanism - the occurrence of dark morphs - in insects has been attributed to differences in, among other things, thermoregulation and immune defence. Dark individuals are hypothesised to perform better in colder areas, and to exhibit stronger melanin-based immune defence. 2. In the present study, the geographical distribution of two colour morphs in Aphodius depressus ( Kugelann), its climatic correlates, and temporal stability was described. Underlying mechanisms were then targeted through experiments: the inheritance of colour through controlled crosses, heating rates by thermal imaging, physiological tolerance by critical thermal limits, and immune efficiency by melanisation of implants. 3. In A. depressus, colour appears inherited by simple Mendelian principles, with red dominating over black. The frequency of two colour morphs forms a large-scale cline. In the South West of Finland, all individuals are black, whereas, in the North East, most are red. This pattern has remained constant over 13 years (1996-2008). 4. The geographical pattern was not attributable to thermoregulation: black morphs were more abundant in warmer rather than colder parts of the country. In experiments, we found no differences in the heating rate of the two morphs, or in their upper temperature maxima. Neither did the morphs differ in their response to artificial objects inserted in their haemolymph. 5. Overall, colour variation in A. depressus occurs as a stable, genetically determined dimorphism, governed by Mendelian inheritance. Yet, no support for prevailing theory of factors sustaining melanism was found. The reasons for colour polymorphism in insects may thus be complex, and should be sought on a case-by-case basis. [ABSTRACT FROM AUTHOR]

Orientation with respect to the sun has been observed in a wide range of species and has generally been interpreted in terms of thermoregulation and/or ultraviolet (UV) protection. For countershaded animals, orientation with respect to the sun may also result from the pressure to exploit the gradient of coloration optimally to enhance crypsis., Here, we use computational modelling to predict the optimal countershading pattern for an oriented body. We assess how camouflage performance declines as orientation varies using a computational model that incorporates realistic lighting environments., Once an optimal countershading pattern for crypsis has been chosen, we determine separately how UV protection/irradiation and solar thermal inflow fluctuate with orientation., We show that body orientations that could optimally use countershading to enhance crypsis are very similar to those that allow optimal solar heat inflow and UV protection., Our findings suggest that crypsis has been overlooked as a selective pressure on orientation and that new experiments should be designed to tease apart the respective roles of these different selective pressures. We propose potential experiments that could achieve this. [ABSTRACT FROM AUTHOR]

Abstract: The color-mediated thermoregulation hypothesis predicts that dark body color (low reflectance) allows organisms to gain heat more efficiently than does pale coloration (high reflectance). This prediction is intuitive and widely assumed to be true, but has poor empirical support. We used rare, captive-bred, mutant melanistic, albino and wild-type Australian bluetongue lizards, Tiliqua scincoides to measure the effects of skin reflectance on the heating and cooling rates. We measured heating under an artificial radiant heat source and cooling rates in an ice-cooled box using live lizards in a room with still air. The effect of skin reflectance on heat transfer was clear, despite the substantial influence of body size. Melanistic T. scincoides showed low reflectance and gained heat faster than highly reflective albinos. Melanistic lizards also lost heat faster than albinos. Wild-type lizards were intermediate in reflectance, gained heat at rates indistinguishable from melanistic lizards, and lost heat at rates indistinguishable from albino lizards. This study system allowed us to control for variables that were confounded in other studies and may explain the inconsistent support for the color-mediated thermoregulation hypothesis. Our results provide clear evidence that skin reflectance influences the rate of heating and cooling in ectotherms. [Copyright &y& Elsevier]

Altitudinal gradients offer a good opportunity to study organisms' adaptations to clinal environmental variables. Regarding altitude, the most influential variables on organisms are temperature and ultraviolet ( UV) solar radiation, the first decreasing and the second increasing with altitude. Both variables affect ectotherms' biology, as ectotherms depend on environmental temperature for thermoregulation, frequently being heliotherms. Here, we studied dorsal coloration in the lizard Psammodromus algirus ( Linnaeus, 1758) along a wide altitudinal gradient (2200 m) in Sierra Nevada ( south-east Spain). We hypothesize that the skin will be darker with altitude, i.e. in environments with lower temperatures and higher UV radiation intensity. Results show that individual dorsal colorations became darker at high altitude. We propose two non-mutually exclusive explanations for this result: (1) darker dorsal surface would favour faster warming at high altitudes, where temperature is lower, and (2) darker dorsal surface would protect against UV radiation, stronger at high altitudes. We found significant relationships between both temperature and UV radiation and population dorsal darkness, giving mixed support for the two explanations. Moreover, dorsal hue was positively correlated with substrate hue, suggesting that hue evolved to maximize crypsis. Our study therefore suggests that geographical variation in dorsal coloration in this lizard is adaptive, and darkness coloration might have evolved in response to adverse conditions (low temperature and high UV radiation) at high altitudes. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 112, 132-141. [ABSTRACT FROM AUTHOR]

The thermal melanin hypothesis posits that ectothermic individuals of larger size or from colder environments exhibit darker cuticles due to melanin's efficacy in absorbing solar radiation. However, melanin is also a crucial component of arthropod immunity. Thus, thermal selection for increased cuticle darkness may profoundly influence melanin-based immune function. In this study, we address the relationships between the thermal environment (season length), cuticular melanism and two aspects of melanin-based immunity across nine thermally distinct populations of the cricket Allonemobius socius. We found that season length (i.e. degree days) and body size had a positive association with cuticle melanism in both sexes across populations, supporting the thermal melanism hypothesis. Despite their smaller size, males were found to have darker cuticles and superior melanin-based immunity. This pattern may be the result of additional selection on males due to sex-specific temperature-dependent activities, such as male calling song. Perhaps most interestingly, we found that short season length populations (i.e. colder) exhibited a greater phenoloxidase activity (aspect of the melanin-based immune system) in addition to darker cuticles in both sexes. This pattern is consistent with direct thermal selection on cuticular color, coupled with indirect selection on melanin-based immunity due to pleiotropy. Thus, thermal selection on cuticle darkness appears to indirectly shape the evolution of pathogen resistance in this system, and potentially for other terrestrial arthropod systems whose ranges encompass a significant thermal gradient. [ABSTRACT FROM AUTHOR]

Aim We explore geographic variation in body size within the wingless grasshopper, Phaulacridium vittatum, along a latitudinal gradient, and ask whether melanism can help explain the existence of clinal variation. We test the hypotheses that both male and female grasshoppers will be larger and lighter in colour at lower latitudes, and that reflectance and size will be positively correlated, as predicted by biophysical theory. We then test the hypothesis that variability in size and reflectance is thermally driven, by assessing correlations with temperature and other climatic variables. Location Sixty-one populations were sampled along the east coast of Australia between latitudes 27.63° S and 43.10° S, at elevations ranging from 10 to 2000 m a.s.l. Methods Average reflectance was used as a measure of melanism and femur length as an index of body size for 198 adult grasshoppers. Climate variables were generated by BIOCLIM for each collection locality. Hierarchical partitioning was used to identify those variables with the most independent influence on grasshopper size and reflectance. Results Overall, there was no simple relationship between size and latitude in P. vittatum. Female body size decreased significantly with latitude, while male body size was largest at intermediate latitudes. Rainfall was the most important climatic variable associated with body size of both males and females. Female body size was also associated with radiation seasonality and male body size with reflectance. The reflectance of females was not correlated with latitude or body size, while male reflectance was significantly higher at intermediate latitudes and positively correlated with body size. Analyses of climate variables showed no significant association with male reflectance, while female reflectance was significantly related to the mean temperature of the driest quarter. Main conclusions Geographic variation in the body size of the wingless grasshopper is best explained in terms of rainfall and radiation seasonality, rather than temperature. However, melanism is also a significant influence on body size in male grasshoppers, suggesting that thermal fitness does play a role in determining adaptive responses to local conditions in this sex. [ABSTRACT FROM AUTHOR]

Individuals can adapt to heterogeneity in their environment through either local adaptation or phenotypic plasticity. Colour forms of the ladybird Harmonia axyridis are a classic example of local adaptation, in which the frequency of melanic forms varies greatly between populations. In some populations, there are also large seasonal changes in allele frequency, with melanism being costly in summer and beneficial in winter. We report that the non-melanic morph of H. axyridis dramatically increases its degree of melanization at cold temperatures. Furthermore, there is genetic variation in reaction norms, with different families responding to temperature in different ways. Variation at different spatial and temporal scales appears to have selected for either genetic or phenotypically plastic adaptations, which may be important in thermoregulation. As melanism is known to have a large effect on fitness in H. axyridis, this plasticity of melanization may have hastened its spread as an invasive species. [ABSTRACT FROM AUTHOR]

1. Despite growing knowledge on the relationship between ecological variables and individual immune function, data on the spatial variability of immune defence in invertebrate natural populations are scarce. 2. Here, we use replicated populations of the butterfly Lycaena tityrus from different altitudes to investigate genetic variation in the melanin-based encapsulation response. As high- and low-altitude populations differ in cuticular pupal melanisation, we further tested for any associations between pupal melanisation and parasite resistance. 3. Although pupal melanisation was higher at higher compared with lower altitudes (and at a higher compared with a lower rearing temperature), any obvious relations to the encapsulation response were absent. Further phenotypic correlations within groups were significant in one out of four cases only, suggesting that in L. tityrus encapsulation operates largely independent of cuticular melanisation. 4. A significant interaction between altitude and temperature indicated that high-altitude animals show a stronger melanisation response than low-altitude ones at the lower temperature and vice versa, indicating local adaptation to different climates. [ABSTRACT FROM AUTHOR]

Evolution of conspicuous signals may be constrained if animal coloration has nonsignaling as well as signaling functions. In aposematic wood tiger moth ( Parasemia plantaginis) larvae, the size of a warning signal (orange patch on black body) varies phenotypically and genetically. Although a large warning signal is favored as an antipredator defense, we hypothesized that thermoregulation may constrain the signal size in colder habitats. To test this hypothesis, we conducted a factorial rearing experiment with two selection lines for larval coloration (small and large signal) and with two temperature manipulations (high and low temperature environment). Temperature constrained the size and brightness of the warning signal. Larvae with a small signal had an advantage in the colder environment, which was demonstrated by a faster development time and growth rate in the low temperature treatment, compared to larvae with a large signal. Interestingly, the larvae with a small signal were found more often on the plant than the ones with a large signal, suggesting higher basking activity of the melanic (small signal) individuals in the low temperature. We conclude that the expression of aposematic display is not only defined by its efficacy against predators; variation in temperature may constrain evolution of a conspicuous warning signal and maintain variation in it. [ABSTRACT FROM AUTHOR]

In the fell-field habitat at 140–270 m altitude on the sub-Antarctic Ile de la Possession, Iles Crozet, the dimorphic beetle Amblystogenium pacificum Putzeys (Coleoptera: Carabidae) was studied monthly throughout the southern year 1993–1994 by timed hand collections involving turning stones on the rocky fell-field. There were many mating couples and females containing well-developed eggs in every month and the species reproduced throughout the year, with no evidence of a winter cessation of egg production. However, the frequency of mating couples was about twice as great in the summer half-year (October–March) as in the winter half (April–September). Relatively small differences in monthly mean temperatures between the summer and winter at Iles Crozet render all-year reproduction not unexpected when comparisons are made with the temperatures and phenology of Carabidae in cool habitats in other parts of the world. Mating pairs were collected at unusually high frequencies for carabids. The frequency of such pairs, taken per unit effort by hand collecting, was highest in October–December, lower from January to May and intermediate from June to September. Comparison of beetles taken as solitary, non-mating individuals with those taken as mating couples showed that in summer, females of the black morph of this dimorphic species were more frequently involved in mating than expected, but no differences were detected in winter. Female beetles were divided into younger individuals (putative age 1–2 years) and older females (putative age 3–5 years or more) by the amount of claw-wear on individuals of this species. This division indicated that about 40% of the older females showed no egg development when captured, but only about 20% of the younger females were non-breeding. However, of the reproducing beetles, young and old females matured similar numbers of eggs. An altitude transect showed that the proportion of the black morph increased significantly with altitude. Studies at 140–270 m showed that a significantly smaller proportion of the older females of the brown morph were breeding than in the younger brown individuals, and this effect was particularly evident in the colder winter period. No such age differences existed amongst the black morph. [ABSTRACT FROM AUTHOR]