Melanin- and carotenoid-dependent signals of great tits (Parus major) relate differently to metal pollution

Avian colouration in a polluted world: a meta-analysis

Brilliant, diverse colour ornaments of birds were one of the crucial cues that led Darwin to the idea of sexual selection. Although avian colouration plays many functions, including concealment, thermoregulation, or advertisement as a distasteful prey, a quality-signalling role in sexual selection has attracted most research attention. Sexually selected ornaments are thought to be more susceptible to external stressors than naturally selected traits, and as such, they might be used as a test for environmental quality. For this reason, the last two decades have seen numerous studies on the impact of anthropogenic pollution on the expression of various avian colour traits. Herein, we provide the first meta-analytical summary of these results and examine whether there is an interaction between the mechanism of colour production (carotenoid-based, melanin-based and structural) and the type of anthropogenic factor (categorised as heavy metals, persistent organic pollutants, urbanisation, or other). Following the assumption of heightened condition dependence of ornaments under sexual selection, we also expected the magnitude of effect sizes to be higher in males. The overall effect size was close to significance and negative, supporting a general detrimental impact of anthropogenic pollutants on avian colouration. In contrast to expectations, there was no interaction between pollution types and colour-producing mechanisms. Yet there were significant differences in sensitivity between colour-producing mechanisms, with carotenoid-based colouration being the most affected by anthropogenic environmental disturbances. Moreover, we observed no significant tendency towards heightened sensitivity in males. We identified a publication gap on structural colouration, which, compared to pigment-based colouration, remains markedly understudied and should thus be prioritised in future research. Finally, we call for the unification of methods used in colour quantification in ecological research to ensure comparability of results among studies.

A review of the impacts of air pollution on terrestrial birds

Air pollution has a ubiquitous impact on ecosystem functioning through myriad processes, including the acidification and eutrophication of soil and water, deposition of heavy metals and direct (and indirect) effects on flora and fauna. Describing the impacts of air pollution on organisms in the field is difficult because levels of exposure do not occur in a uniform manner across space and time, and species responses tend to be nuanced and difficult to isolate from other environmental stressors. However, given its far-reaching effects on human and ecosystem health, the impacts of air pollution on species are expected to be substantial, and could be direct or indirect, acting via a range of mechanisms. Here, we expand on previous reviews, to evaluate the existing evidence for the impacts of air pollution on avian species in the field, and to identify knowledge gaps to guide future research. We identified 203 studies that have investigated the impacts of air pollution (including nitrogen and heavy metal deposition) on wild populations of birds, considering 231 species from ten feeding guilds. The majority of studies (82 %) document at least one species trait leading to an overall fitness value that is negatively correlated with pollution concentrations, including deleterious effects on reproductive output, molecular (DNA) damage and overall survival, and effects on foraging behaviour, plumage colouration and body size that may show adaptation. Despite this broad range of trait effects, biases in the literature towards certain species (Parus major and Ficeluda hypoleuca), geographical regions (Western Europe) and pollutants (heavy metal deposition), mean that many unknowns remain in our current understanding of the impacts of air pollution on avian species. We discuss these findings in context of future work, and propose research approaches that could help to provide a more holistic understanding of how avian species are impacted by air pollution.

Reduced reflectance and altered color: The potential cost of external particulate matter accumulation on urban Rock Pigeon (Columba livia) feathers

Feather quality in birds is considered an honest signal of individual health as feather appearance and function depend on an individual’s ability to maintain them. In addition to flight and insulation, feathers are essential for social interactions and sexual selection in the form of visual signals. Airborne particulate matter (PM) can accumulate on feather surfaces and alter feather appearance. We quantified PM accumulation on Rock Pigeon (Columba livia) feathers and analyzed the spectral properties of extracted particulates. Feathers were sampled from two pigeon populations, one rural and one urban, in the Dallas-Fort Worth area, with 47 and 93% developed land cover, respectively, within 2 km of the populations. We determined accumulated PM gravimetrically after rinsing feathers and then measured the visible-near-infrared diffuse reflectance and color properties of extracted particulates. The rate of PM accumulation on rural birds was higher than on urban birds. However, feather particulates collected from urban pigeons had significantly lower total reflectance, reflectance in the visible portion of the spectrum, lightness, and hue angle compared to those of rural pigeons. The hue angle of rural feather particulates displayed a negative relationship with PM accumulation while total reflectance, reflectance in the visible range, and lightness of urban feather particulates were negatively related to PM accumulation. Our findings suggest that wild birds could incur an urban pollution penalty as PM accumulation has the potential to alter feather properties. Further research is needed to better understand the influence of external PM accumulation on the physiological and behavioral health of birds.

Animal Coloration in the Anthropocene

Natural habitats are increasingly affected by anthropogenically driven environmental changes resulting from habitat destruction, chemical and light pollution, and climate change. Organisms inhabiting such habitats are faced with novel disturbances that can alter their modes of signaling. Coloration is one such sensory modality whose production, perception and function is being affected by human-induced disturbances. Animals that acquire pigment derivatives through diet are adversely impacted by the introduction of chemical pollutants into their environments as well as by general loss of natural habitat due to urbanization or logging leading to declines in pigment sources. Those species that do manage to produce color-based signals and displays may face disruptions to their signaling medium in the form of light pollution and turbidity. Furthermore, forest fragmentation and the resulting breaks in canopy cover can expose animals to predation due to the influx of light into previously dark environments. Global climate warming has been decreasing snow cover in arctic regions, causing birds and mammals that undergo seasonal molts to appear conspicuous against a snowless background. Ectotherms that rely on color for thermoregulation are under pressure to change their appearances. Rapid changes in habitat type through severe fire events or coral bleaching also challenge animals to match their backgrounds. Through this review, we aim to describe the wide-ranging impacts of anthropogenic environmental changes on visual ecology and suggest directions for the use of coloration both as an indicator of ecological change and as a tool for conservation.

Relationships between soil pollution by heavy metals and melanin-dependent coloration of a fossorial amphisbaenian reptile

Melanin is the basis of coloration in many animals, and although it is often used in communication, thermoregulation, or camouflage, melanin has many other physiological functions. For example, in polluted habitats, melanin can have a detoxifying function. Melanic coloration would help to sequester in the skin the heavy metal contaminants from inside the body, which will be expelled to the exterior when the skin is sloughed. Moreover, animals should have evolved more melanic colorations in more polluted habitats ("industrial melanism" hypothesis). We examined whether the fossorial amphisbaenian reptile, Trogonophis wiegmanni, is able to eliminate heavy metals, derived from soil pollution by seagull depositions, through sloughing its skin. Our results suggest a covariation between levels of soil pollution by heavy metals and the concentration of heavy metals in the sloughed skins of amphisbaenians. This suggests that amphisbaenians may expel heavy metals from their bodies when they slough the skins. We also tested whether amphisbaenians inhabiting soils with higher levels of heavy metal pollution had darker (melanin-dependent) body colorations. However, contrary to predictions from the "industrial melanization" hypothesis, we found a negative relationship between soil pollution and proportions of melanic coloration. This contradictory result could, however, be explained because heavy metals have endocrine disruption effects that increase physiological stress, and higher stress levels could result in decreased melanogenesis. We suggest that although amphisbaenians might have some detoxifying mechanism linked to melanin in the skin, this process might be negatively affected by stress and result ineffective under conditions of high soil pollution.

Male zebra finches exposed to lead (Pb) during development have reduced volume of song nuclei, altered sexual traits, and received less attention from females as adults

Lead (Pb) is a pervasive global contaminant that interferes with sensitive windows for neurological development and causes oxidative damage to tissues. The effects of moderate and high exposure to Pb have been well-studied in birds, but whether low-level early-life exposure to Pb influences adult phenotype remains unclear. Female songbirds use a male's song and coloration to discriminate between high- and low-quality males. Therefore, if early-life exposure to Pb disrupts song learning ability or shifts the allocation of antioxidant pigments away from colorful secondary sexual traits, male birds exposed to Pb may be less attractive to females. We exposed developing zebra finches (Taeniopygia guttata) to Pb-contaminated drinking water (100 or 1000 parts per billion [ppb]) after hatching (days 0-100). Once male finches reached adulthood (120-150 days post hatch), we measured song learning ability, coloration of bill and cheek patches, and volume of song nuclei in the brain. We also measured female preference for Pb-exposed males relative to control males. Finally, we measured motoric and spatial cognitive performance in male and female finches to assess whether cognitive traits differed in their sensitivity to Pb exposure. Male zebra finches exposed to 1000 ppb Pb had impaired song learning ability, reduced volume of song nuclei, bills with less redness and received less attention from females. Additionally, Pb exposure impaired motoric performance in both male and female finches but did not affect performance in a spatial cognitive task. Adult finches exposed to Pb-contaminated water had higher blood-Pb levels, though in all cases blood-Pb levels were below 7.0 µg dL^-1. This study suggests that low-level exposure to Pb contributes to cognitive deficits that persist into adulthood and may indirectly influence fitness by altering secondary sexual traits and reducing male attractiveness.

A review of metal-induced effects on vitamins A, E and D3 in birds

Birds have been historically affected by metals and are considered powerful sentinels of environmental pollution. Some toxic elements (Pb, Cd, Hg, As) have been broadly studied in avian species and are well known for their accumulation capacity and deleterious effects, including alterations in vitamin levels. Vitamins A, E and D3 are fat-soluble nutrients involved in multiple physiological functions (e.g., immune function, vision, reproduction, growth and development). Publications reporting metal-induced effects on vitamins in birds are growing and, in some cases, results seem contradictory, making them difficult to interpret. Therefore, a clear view of the overall picture is needed. This mini-review article aims to compile relevant data and describe current knowledge on the effects of the most toxic elements (i.e., Pb, Cd, Hg, As) on vitamins A, E and D3 in birds. Although vitamins are diet dependent, they are strongly regulated and transformed in the organism, and metal-related disruption in their homeostasis may provoke alterations in different directions. Moreover, vitamin status and form in vivo is the result of complex interacting processes in the organism and metal exposure may produce cascade effects. Different factors that may contribute to the variable response of vitamins to metals in birds are discussed. Some final remarks and recommendations are provided for future studies. This mini-review shows an overview of the current knowledge in metal-induced alterations in vitamins of special concern for avian ecotoxicology, a research discipline facing important challenges in the coming years.

Sexually selected traits as bioindicators: exposure to mercury affects carotenoid-based male bill color in zebra finches

To examine whether sexually selected traits are particularly sensitive bioindicators of environmental toxicants, we assessed the effects of exposure to environmentally relevant dietary concentrations of the pollutant methylmercury on pigment coloration in zebra finches (Taeniopygia guttata). First, we tested whether effects of methylmercury on coloration were influenced by timing of exposure. Birds were either exposed developmentally (up to 114 days after hatching), as adults (after reaching sexual maturity), or for their entire life. Bill coloration, which is a carotenoid-based, sexually selected trait, was less red in males with lifetime exposure to methylmercury, compared to controls. Neither adult, nor developmental exposure influenced bill color in adult males, with the possible exception of early exposure of nestlings. Among females, where bill color is not under strong sexual selection, neither lifetime nor adult exposure to methylmercury affected bill color. For males and females, there was no effect of either lifetime or adult methylmercury exposure on coloration of back feathers, which is a non-sexually-dimorphic, melanin-based trait that is not likely the result of sexual selection. This study is a comprehensive experimental test of the proposal that sexually selected traits may be particularly useful bioindicators of the stress imposed by environmental toxins such as methylmercury.

Blood concentrations of 50 elements in Eagle owl (Bubo bubo) at different contamination scenarios and related effects on plasma vitamin levels

Some metals and metalloids (e.g. Pb, Hg, Cd and As) are well-known for their bioaccumulation capacity and their toxic effects on birds, but concerns on other minor elements and rare earth elements (ME and REE) are growing due to their intensive use in modern technology and potential toxicity. Vitamins and carotenoids play essential roles in nestling growth and proper development, and are known to be affected by the metals classically considered as toxic. However, we are unaware of any attempts to evaluate the exposure to 50 elements and related effects in plasma vitamins and carotenoids in raptor species. The main goals of this study are: (i) to assess the exposure to 50 elements (i.e. classic toxic elements, trace elements, REE and ME) in nestling Eagle owls (Bubo bubo) inhabiting three differently polluted environments (mining, industrial and control areas) in southeastern Spain, and (ii) to evaluate how element exposure affects plasma vitamin and carotenoid levels, hematocrit and body measurements (mass and wing length) of the individuals. Our results show that local contamination in the mining area contributes to increased blood concentrations of Pb, As and Tl in nestlings, while diet differences between control and mining/industrial areas may account for the different levels of Mn, Zn, and Sr in blood, and lutein in plasma. Plasma tocopherol levels were increased in the mining-impacted environment, which may be a mechanism of protection to prevent toxic element-related oxidative stress. Plasma α-tocopherol was enhanced by 20% at blood Pb concentrations ≥8 ng/ml, and nestlings exhibited up to 56% increase in α-tocopherol levels when blood Pb concentrations reached 170 ng/ml. Tocopherol seems to be a sensitive biomarker under an exposure to certain toxic elements (e.g. Pb, As, Tl).

Anthropogenic noise is associated with telomere length and carotenoid-based coloration in free-living nestling songbirds

Growing evidence suggests that anthropogenic noise has deleterious effects on the behavior and physiology of free-living animals. These effects may be particularly pronounced early in life, when developmental trajectories are sensitive to stressors, yet studies investigating developmental effects of noise exposure in free-living populations remain scarce. To elucidate the effects of noise exposure during development, we examined whether noise exposure is associated with shorter telomeres, duller carotenoid-based coloration and reduced body mass in nestlings of a common urban bird, the great tit (Parus major). We also assessed how the noise environment is related to reproductive success. We obtained long-term measurements of the noise environment, over a ∼24-h period, and characterized both the amplitude (measured by LA_eq, LA_90, LA₁₀, LA_max) and variance in noise levels, since more stochastic, as well as louder, noise regimes might be more likely to induce stress. In our urban population, noise levels varied substantially, with louder, but less variable, noise characteristic of areas adjacent to a highway. Noise levels were also highly repeatable, suggesting that individuals experience consistent differences in noise exposure. The amplitude of noise near nest boxes was associated with shorter telomeres among smaller, but not larger, brood members. In addition, carotenoid chroma and hue were positively associated with variance in average and maximum noise levels, and average reflectance was negatively associated with variance in background noise. Independent of noise, hue was positively related to telomere length. Nestling mass and reproductive success were unaffected by noise exposure. Results indicate that multiple dimensions of the noise environment, or factors associated with the noise environment, could affect the phenotype of developing organisms, that noise exposure, or correlated variables, might have the strongest effects on sensitive groups of individuals, and that carotenoid hue could serve as a signal of early-life telomere length.

Carotenoid- but not melanin-based plumage coloration is negatively related to metal exposure and proximity to the road in an urban songbird

Rapid urbanization is a global phenomenon that is increasingly exposing organisms to novel stressors. These novel stressors can affect diverse aspects of organismal function, including development of condition-dependent ornaments, which play critical roles in social and sexual selection. We investigated the relationship between metal pollution, proximity to roads, and carotenoid- and melanin-based plumage coloration in a common songbird, the great tit (Parus major). We studied populations located across a well-characterized metal pollution gradient and surrounded by roadway networks. Metal exposure and road-associated pollution could reduce carotenoid-based pigmentation by inducing oxidative stress or affecting habitat quality, but metals could also enhance melanin-based pigmentation, through effects on melanogenesis and testosterone concentrations. Using a large sample size (N > 500), we found that birds residing close to a point source for metals had reduced ultraviolet chroma, a component of carotenoid-based pigmentation. Moreover, birds with high feather metal concentrations had lower carotenoid chroma, hue, and ultraviolet chroma, with effects modified by age class. Birds residing closer to roads also had lower carotenoid chroma and hue. Melanin-based pigmentation showed high between-year repeatability, and no association with anthropogenic pollution. Results suggest that carotenoid-, but not melanin-, based pigmentation is negatively affected by multiple anthropogenic stressors. We are the first to demonstrate a negative association between roads and a plumage-based signaling trait, which could have important implications for sexual signaling dynamics in urban landscapes.

Use of plumage and gular pouch color to evaluate condition of oil spill rehabilitated California brown pelicans (Pelecanus occidentalis californicus) post-release

Sublethal effects of oil spills may dampen seabird rehabilitation success due to lingering negative impacts of contamination and stress on reproduction and long-term survival. These effects can be difficult to measure while birds are in care as well as once birds are released. Expression of sexually selected traits that are sensitive to condition can provide information on physiological status of birds. We evaluated plumage molt and gular pouch skin color of California brown pelicans (Pelecanus occidentalis californicus) following oil contamination and rehabilitation to test for differences between previously oiled and rehabilitated (post-spill) and presumably uncontaminated pelicans. Post-spill pelicans released with either color leg bands alone, or bands plus harness-mounted satellite GPS tags, were relocated and visually assessed in the field at non-breeding communal roosts and compared to surrounding unmarked pelicans in the general population. Non-oiled pelicans bearing GPS tags were also included in the study. Post-spill pelicans lagged the general population in molt of ornamental yellow crown feathers but hind neck transition into white plumage was not significantly different. Both post-spill and non-oiled pelicans wearing GPS tags had lower gular redness scores than the unmarked, non-oiled population. Pre-breeding gular pouch redness of post-spill pelicans was more strongly influenced by wearing of a GPS tag than a history of oil contamination and rehabilitation. Gular pouch redness of post-spill pelicans in the first 18 months after release was positively correlated with long term survivorship. If gular pouch color is a condition-dependent sexual signal and overall health influences plumage molt progression, our results indicate that many post-spill pelicans marked with bands alone were in relatively good condition going into the next breeding season, but those released with electronic tags experienced additional stress due to wearing the equipment, introducing a confounding variable to the post-release study.

Ants and their nests as indicators for industrial heavy metal contamination

Ants accumulate heavy metals and respond to pollution with modification in species composition, community structure, altered behaviour and immunity. However, the levels of heavy metals in ants' nests and explicit individual-level responses towards heavy metals have not been revealed. We found that red wood ants Formica lugubris accumulate high and correlated values of such heavy metals as Al, Cd, Co, Cu, Fe, Ni, Pb and Zn both in ants and nest material near cobalt smelter in Finland. Relative differences in metal concentrations were higher in nests than in ants. The highest values were obtained for elements such as Co (36.6), Zn (14.9), Cd (9.7), Pb (8.5), Cu (7.4), Ni (6.4), As (4.7), Cr (2.9) and Fe (2.4) in nest material, and Co (32.7), Cd (6.3), Pb (6), Fe (2.8), Ni (2.9) and Zn (2.1) in ants. In industrial and reference areas, ants have no differences in size, but differed in dry and residual body mass. In polluted areas, F. lugubris had less melanised heads, but not thoraxes. The sensitivity of cuticular colouration in red wood ants subjected to heavy metal pollution might be related to metal-binding properties of melanins. The overall results are useful for the improvement of biomonitoring techniques using ants as indicators of industrial contamination and for further discovery of novel ecotoxicological biomarkers.

Biological responses to environmental contamination. How can metal pollution impact signal honesty in avian species?

Environmental pollution, for example with metals, can significantly affect the ecosystem balance leading to severe changes. Biologically active pigments are relevant for the appearance and condition of birds. Melanin and carotenoid particles are the most frequently deposited pigments in avian integument. They are responsible for the majority of colors of bird plumage. The phenotypic expression can be affected by metal contamination. It can be manifested as color bleaching or differences in the size of plumage badges. In this study, we performed a comprehensive review of related studies in order to estimate the underlying population effect of this potential dependency. The study is based on the review of the literature regarding several avian species. It was designed to identify an area where the effect of the exposure is still poorly known. The analysis was specifically conducted to investigate the correlation between trace element concentration and eumelanin deposition. Moreover, we searched for factors that could affect spectral properties of feathers with carotenoid-based pigmentation. As a result, we found carotenoid-based pigmentation to be of a good use in terms of visual condition assessment. Changes in melanin-based pattern should be analyzed separately for eu- and pheomelanin as well as for a range of essential and toxic elements. Comprehensive studies on the subject are still scarce. Therefore, the issue requires further investigation.

Age and sex differences in niche use at molt and its effect on plumage coloration characteristics in a bird

Bird plumage is often very colorful and can communicate the quality of the bearer to conspecifics. These plumage-based signals of quality are composed of multiple pigments (e.g., melanin and carotenoids). Therefore, sex and age classes, which often show marked differences in plumage coloration, may have different dietary needs for the different plumage components and this might promote preferences for different dietary niches at different molting stages. However, no study has addressed the role that changes in niche use play in the expression of multiple component plumage signals in birds. We used stable isotope analysis to test the hypothesis that niche use is related to age and sex and to differently cultured plumage patches, yellow carotenoid-based and black melanin-based, in great tits Parus major. We recorded high niche overlap between plumage patches, although δ¹⁵N was higher in black than yellow plumage. Niche overlap was relatively low for age classes and relatively high for sex classes, and age classes showed a contrasting pattern of niche overlap between carotenoid- and melanin-based plumages. Moreover, δ¹³C, but not δ¹⁵N, had a significant negative relationship with carotenoid-based plumage, which was only apparent in juveniles. Taken together, our results demonstrate that niche use had a moderate influence on plumage coloration characteristics of great tit individuals, mostly associated with δ¹³C rather than with δ¹⁵N and with age rather than with sex. Therefore, our study is significant because it confirms the relevance of niche use during ornament production in free-living birds.

Associations of urban environmental pollution with health-related physiological traits in a free-living bird species

Urban environmental pollution results in contamination of the tissues of synanthropic organisms by toxic trace elements with potential impacts on human health. Passerine birds may serve as convenient indicators of such contamination. In this study we investigated the effect of blood and plumage contamination with heavy metals (lead Pb, cadmium Cd, copper Cu, chromium Cr) and arsenic metalloid (As) on condition, health and ornamental colour in free-living great tit (Parus major) males from 13 cities across the Czech Republic (EU), mist netted during the early breading season (April-May). Our results showed a significant association of heavy metal tissue contamination with immune function, namely leukocyte composition in the avian blood circulation. High heavy metal contamination in bird feathers was linked to a high heterophil/lymphocyte (H/L) ratio, indicating long-term stress in individuals inhabiting heavily polluted environments. In contrast, males with higher concentrations of heavy metals in blood had a lower H/L ratio, assumingly due to the direct toxicity of heavy metals in certain cell types. This is also supported by traits indicative of anaemia-like haemolytic conditions (decreased absolute erythrocyte count) and increased haematopoiesis (a tendency for increased frequencies of immature erythrocytes). We did not find any association of heavy metal contamination with the bacteriolytic activity of plasma complement, feather growth or ornamentation (black breast stripe area and yellow colouration). There was no significant relationship between heavy metal contamination in blood or feathers and PM₁₀ pollution at the study sites. Our correlational study is the first to show on a large geographic scale that despite strict European air pollution regulations and regular monitoring that have allowed general improvements in atmospheric contamination, non-degradable heavy metals persistently contaminate animal blood and feathers in anthropogenic environments at levels that may have subclinical yet physiological effects with varied influence on health.

Ivermectin alters reproductive success, body condition and sexual trait expression in dung beetles

Ivermectin is a very common parasiticide used in livestock. It is excreted in the dung and has negative effects on survival and reproduction of dung-degrading organisms, including dung beetles. Here we exposed the dung beetle Euoniticellus intermedius to different concentrations of ivermectin in the food and evaluated reproductive success and the expression of traits associated with survival and reproduction under laboratory conditions. It is the first time the effects of ivermectin were evaluated on offspring physiological condition and the expression of a secondary sexual trait. We also registered the number of emerged beetles, sex ratio and body size of emerged adult beetles. Besides reducing the number of emerged beetles and body size, as found in the same and other insects, ivermectin at high doses reduced muscle mass while at intermediate doses it increased lipid mass. Ivermectin changed offspring sex ratio and at high doses increased the size of male horn, which is an important trait defining the male mating success. Our results highlight the importance of regulating parasiticide usage in livestock in order to maintain ecosystem services provided by dung beetles and confirm that contaminants impose new environmental conditions that not only impact on wild animal survival, but also on evolutionary processes such as sexual selection.

Evolution of Sex Differences in Trait- and Age-Specific Vulnerabilities

Traits that facilitate competition for reproductive resources or that influence mate choice generally have a heightened sensitivity to stressors. They have evolved to signal resilience to infectious disease and nutritional and social stressors, and they are compromised by exposure to man-made toxins. Although these traits can differ from one species or sex to the next, an understanding of the dynamics of competition and choice can in theory be used to generate a priori predictions about sex-, age-, and trait-specific vulnerabilities for any sexually reproducing species. I provide a review of these dynamics and illustrate associated vulnerabilities in nonhuman species. The age- and sex-specific vulnerability of such traits is then illustrated for stressor-related disruptions of boys’ and girls’ physical growth and play behavior, as well as for aspects of boys’ and girls’ and men’s and women’s personality, language, and spatial abilities. There is much that remains to be determined, but enough is now known to reframe trait sensitivity in ways that will allow scientists and practitioners to better identify and understand vulnerable human traits, and eventually ameliorate or prevent their expression.

Opposing effects of oxidative challenge and carotenoids on antioxidant status and condition-dependent sexual signalling

Several recent hypotheses consider oxidative stress to be a primary constraint ensuring honesty of condition-dependent carotenoid-based signalling. The key testable difference between these hypotheses is the assumed importance of carotenoids for redox homeostasis, with carotenoids being either antioxidant, pro-oxidant or unimportant. We tested the role of carotenoids in redox balance and sexual signalling by exposing adult male zebra finches (Taeniopygia guttata) to oxidative challenge (diquat dibromide) and manipulating carotenoid intake. As the current controversy over the importance of carotenoids as antioxidants could stem from the hydrophilic basis of commonly-used antioxidant assays, we used the novel measure of in vivo lipophilic antioxidant capacity. Oxidative challenge reduced beak pigmentation but elicited an increase in antioxidant capacity suggesting resource reallocation from signalling to redox homeostasis. Carotenoids counteracted the effect of oxidative challenge on lipophilic (but not hydrophilic) antioxidant capacity, thereby supporting carotenoid antioxidant function in vivo. This is inconsistent with hypotheses proposing that signalling honesty is maintained through either ROS-induced carotenoid degradation or the pro-oxidant effect of high levels of carotenoid-cleavage products acting as a physiological handicap. Our data further suggest that assessment of lipophilic antioxidant capacity is necessary to fully understand the role of redox processes in ecology and evolution.

Coloured ornamental traits could be effective and non-invasive indicators of pollution exposure for wildlife

Growth in human populations causes habitat degradation for other species, which is usually gauged by physical changes to landscapes. Corresponding habitat degradation to air and water is also common, but its effects on individuals can be difficult to detect until they result in the decline or disappearance of populations. More proactive measures of pollution usually combine abiotic samples of soil, water or air with invasive sampling of expendable species, but this approach sometimes creates ethical dilemmas and has limited application for threatened species. Here, we describe the potential to measure the effects of pollution on many species of birds and fish by using ornamental traits that are expressed as coloured skin, feathers and scales. As products of sexual selection, these traits are sensitive to environmental conditions, thereby providing honest information about the condition of their bearers as ready-made biomarkers. We review the documented effects of several classes of pollutants, including pharmaceuticals, pesticides, industry-related compounds and metals, on two classes of colour pigments, namely melanins and carotenoids. We find that several pollutants impede the expression of both carotenoids and brown melanin, while enhancing traits coloured by black melanin. We also review some of the current limitations of using ornamental colour as an indicator of pollution exposure, suggest avenues for future research and speculate about how advances in robotics and remote imagery will soon make it possible to measure these traits remotely and in a non-invasive manner. Wider awareness of this potential by conservation managers could foster the development of suitable model species and comparative metrics and lay a foundation for pollution monitoring that is more generalizable and biologically relevant than existing standards.

Metal exposure influences the melanin and carotenoid-based colorations in great tits

Metals are naturally found in the environment but are also emitted through anthropogenic activities, raising some concerns about the potential deleterious effects of these elements on wildlife. The potential effects of metals on bird coloration have been the focus of several recent studies since animal colored-signals often reflect the physiology of their bearers and are thus used by animals to assess the quality of another individual as a mate or competitor. These studies have shown that the melanin pigmentation seems to be positively associated and the carotenoid-based coloration negatively associated with metal exposure in wild birds. Although these studies have been very useful to show the associations between metal exposure and coloration, only few of them have actually quantified the levels of metal exposure at the individual level; always focusing on one or two of them. Here, we measured the concentrations of eight metals in great tits' feathers and then assessed how these levels of metals were associated with the carotenoid and melanin-based colorations. We found that the melanin pigmentation was positively associated with the copper concentration and negatively correlated with the chromium concentration in feathers. In addition, we have shown that the carotenoid-based coloration was negatively associated with the feather's mercury concentration. This study is the first one to identify some metals that might affect positively and negatively the deposition of melanin and carotenoid into the plumage of wild birds.

Is oxidative status influenced by dietary carotenoid and physical activity after moult in the great tit (Parus major)?

In the context of sexual and natural selection, an allocation trade-off for carotenoid pigments may exist because of their obligate dietary origin and their role both in the antioxidant and immune systems and in the production of coloured signals in various taxa, particularly birds. When birds have expended large amounts of carotenoids to feather growth such as after autumn moult, bird health and oxidative status might be more constrained. We tested this hypothesis in a bird species with carotenoid-based plumage colour, by manipulating dietary carotenoids and physical activity, which can decrease antioxidant capacity and increase reactive oxygen metabolite (ROM) concentration. Great tits were captured after moult and kept in aviaries, under three treatments: physical handicap and dietary supplementation with carotenoids, physical handicap and control diet, and no handicap and control diet. We measured plasma composition (antioxidant capacity, ROM concentration, and vitamin A, vitamin E and total carotenoid concentrations), immune system activation (blood sedimentation) and stress response (heterophil/lymphocyte ratio) and predicted that handicap treatment should influence these negatively and carotenoid supplementation positively. Coloration of yellow feathers was also measured. Carotenoid supplementation increased total plasma carotenoid concentration, decreased feather carotenoid chroma and marginally increased ROM concentration. Handicap increased blood sedimentation only in males but had no clear influence on oxidative stress, which contradicted previous studies. Further studies are needed to investigate how physical activity and carotenoid availability might interact and influence oxidative stress outside the moult period, and their combined potential influence on attractiveness and reproductive investment later during the breeding season.

No delayed behavioral and phenotypic responses to experimental early-life lead exposure in great tits (Parus major)

Early-life exposure to pollutants, such as lead, may have long-lasting consequences on health, behavior, and cognition. However, experiments on delayed effects of specific pollutants are very rare in wild animals. We experimentally exposed wild nestling great tits (Parus major) to dietary lead (high, low, or control group) in levels relevant to exposure levels of wild populations in Europe and studied delayed effects on phenotypic and behavioral traits in captivity. We also included a group of birds from a vicinity of a copper smelter, exposed to a mixture of toxic metals and altered food supply during development. This experimental setup allowed us to compare the strength of direct (exposure to lead per se) and indirect (pollution-related changes in diet) effects of pollutants. Our experimental lead treatment significantly increased lead levels in bone and feces compared with controls. However, we found no carry-over effect of early-life dietary lead on morphology, plumage coloration, or heat shock proteins. Treatment did not affect activity, exploration, neophobia, or success in learning and spatial memory task. We conclude that with the exposure levels and relatively short exposure period used, delayed effects on the measured traits were not found. However, it is important to further study other types of behavioral traits and ultimately fitness effects.

Selection based on the size of the black tie of the great tit may be reversed in urban habitats

A standard approach to model how selection shapes phenotypic traits is the analysis of capture–recapture data relating trait variation to survival. Divergent selection, however, has never been analyzed by the capture–recapture approach. Most reported examples of differences between urban and nonurban animals reflect behavioral plasticity rather than divergent selection. The aim of this paper was to use a capture–recapture approach to test the hypothesis that divergent selection can also drive local adaptation in urban habitats. We focused on the size of the black breast stripe (i.e., tie width) of the great tit (Parus major), a sexual ornament used in mate choice. Urban great tits display smaller tie sizes than forest birds. Because tie size is mostly genetically determined, it could potentially respond to selection. We analyzed capture/recapture data of male great tits in Barcelona city (N = 171) and in a nearby (7 km) forest (N = 324) from 1992 to 2008 using MARK. When modelling recapture rate, we found it to be strongly influenced by tie width, so that both for urban and forest habitats, birds with smaller ties were more trap-shy and more cautious than their larger tied counterparts. When modelling survival, we found that survival prospects in forest great tits increased the larger their tie width (i.e., directional positive selection), but the reverse was found for urban birds, with individuals displaying smaller ties showing higher survival (i.e., directional negative selection). As melanin-based tie size seems to be related to personality, and both are heritable, results may be explained by cautious personalities being favored in urban environments. More importantly, our results show that divergent selection can be an important mechanism in local adaptation to urban habitats and that capture–recapture is a powerful tool to test it.

Cryptic confounding compounds: A brief consideration of the influences of anthropogenic contaminants on courtship and mating behavior

Contaminants, like pesticides, polychlorinated biphenyls (PCBs), dioxins and metals, are persistent and ubiquitous and are known to threaten the environment. Traditionally, scientists have considered the direct physiological risks that these contaminants pose. However, scientists have just begun to integrate ethology and toxicology to investigate the effects that contaminants have on behavior. This review considers the potential for contaminant effects on mating behavior. Here we assess the growing body of research concerning disruptions in sexual differentiation, courtship, sexual receptivity, arousal, and mating. We discuss the implications of these disruptions on conservation efforts and highlight the importance of recognizing the potential for environmental stressors to affect behavioral experimentation. More specifically, we consider the negative implications for anthropogenic contaminants to affect the immediate behavior of animals, and their potential to have cascading and/or long-term effects on the behavioral ecology and evolution of populations. Overall, we aim to raise awareness of the confounding influence that contaminants can have, and promote caution when interpreting results where the potential for cryptic affects are possible.

Free radical exposure creates paler carotenoid-based ornaments: a possible interaction in the expression of black and red traits

Oxidative stress could be a key selective force shaping the expression of colored traits produced by the primary animal pigments in integuments: carotenoids and melanins. However, the impact of oxidative stress on melanic ornaments has only recently been explored, whereas its role in the expression of carotenoid-based traits is not fully understood. An interesting study case is that of those animal species simultaneously expressing both kinds of ornaments, such as the red-legged partridge (Alectoris rufa). In this bird, individuals exposed to an exogenous source of free radicals (diquat) during their development produced larger eumelanin-based (black) plumage traits than controls. Here, we show that the same red-legged partridges exposed to diquat simultaneously developed paler carotenoid-based ornaments (red beak and eye rings), and carried lower circulating carotenoid levels as well as lower levels of some lipids involved in carotenoid transport in the bloodstream (i.e., cholesterol). Moreover, partridges treated with a hormone that stimulates eumelanin production (i.e., alpha-melanocyte-stimulating hormone) also increased blood carotenoid levels, but this effect was not mirrored in the expression of carotenoid-based traits. The redness of carotenoid-based ornaments and the size of a conspicuous eumelanic trait (the black bib) were negatively correlated in control birds, suggesting a physiological trade-off during development. These findings contradict recent studies questioning the sensitivity of carotenoids to oxidative stress. Nonetheless, the impact of free radicals on plasma carotenoids seems to be partially mediated by changes in cholesterol metabolism, and not by direct carotenoid destruction/consumption. The results highlight the capacity of oxidative stress to create multiple phenotypes during development through differential effects on carotenoids and melanins, raising questions about evolutionary constraints involved in the production of multiple ornaments by the same organism.

Bird population declines due to radiation exposure at Chernobyl are stronger in species with pheomelanin-based coloration

Eumelanin and pheomelanin are the most common pigments providing color to the integument of vertebrates. While pheomelanogenesis requires high levels of a key intracellular antioxidant (glutathione, GSH), eumelanogenesis is inhibited by GSH. This implies that species that possess the molecular basis to produce large amounts of pheomelanin might be more limited in coping with environmental conditions that generate oxidative stress than species that produce eumelanin. Exposure to ionizing radiation produces free radicals and depletes antioxidant resources. GSH is particularly susceptible to radiation, so that species with large proportions of pheomelanic integument may be limited by the availability of GSH to combat oxidative stress and may thus suffer more from radiation effects. We tested this hypothesis in 97 species of birds censused in areas with varying levels of radioactive contamination around Chernobyl. After controlling for the effects of carotenoid-based color, body mass and similarity among taxa due to common phylogenetic descent, the proportion of pheomelanic plumage was strongly negatively related to the slope estimates of the relationship between abundance and radiation levels, while no effect of eumelanic color proportion was found. This represents the first report of an effect of the expression of melanin-based coloration on the capacity to resist the effects of ionizing radiation. Population declines were also stronger in species that exhibit carotenoid-based coloration and have large body mass. The magnitude of population declines had a relatively high phylogenetic signal, indicating that certain groups of birds, especially non-corvid passeriforms, are particularly susceptible to suffer from the effects of radioactive contamination due to phylogenetic inertia.

Haematological status of wintering great tits (Parus major) along a metal pollution gradient

In the long-term biomonitoring of wild populations inhabiting polluted areas, the use of non-destructive biomarkers as markers of condition is very important. We examined the possible effects of metal pollution on the haematological status of adult great tits (Parus major) along a well-established pollution gradient near a non-ferrous smelter in Belgium. We measured blood and feather metal concentrations and assessed the haematological status (amount of red blood cells, haemoglobin concentration, haematocrit, mean corpuscular volume and mean corpuscular haemoglobin) of adult great tits during winter at four study sites. Metal concentrations in blood and feathers indicated that cadmium and lead were the most important metals in the pollution gradient under study. Measurements of haematological parameters revealed that haemoglobin concentration, haematocrit, mean corpuscular volume and mean corpuscular haemoglobin were lower in great tits from the more polluted sites. These parameters were significantly negatively correlated with blood lead concentration. The amount of red blood cells, however, did not significantly differ among study sites. Our results indicate that the haematological status of great tits is negatively affected by metal pollution and may therefore be used as a successful biomarker for monitoring the negative impact of metal exposure in the wild.

Geographic patterns of genetic differentiation and plumage colour variation are different in the pied flycatcher (Ficedula hypoleuca)

The pied flycatcher is one of the most phenotypically variable bird species in Europe. The geographic variation in phenotypes has often been attributed to spatial variation in selection regimes that is associated with the presence or absence of the congeneric collared flycatcher. Spatial variation in phenotypes could however also be generated by spatially restricted gene flow and genetic drift. We examined the genetic population structure of pied flycatchers across the breeding range and applied the phenotypic Q(ST) (P(ST))-F(ST) approach to detect indirect signals of divergent selection on dorsal plumage colouration in pied flycatcher males. Allelic frequencies at neutral markers were found to significantly differ among populations breeding in central and southern Europe whereas northerly breeding pied flycatchers were found to be one apparently panmictic group of individuals. Pairwise differences between phenotypic (P(ST)) and neutral genetic distances (F(ST)) were positively correlated after removing the most differentiated Spanish and Swiss populations from the analysis, suggesting that genetic drift may have contributed to the observed phenotypic differentiation in some parts of the pied flycatcher breeding range. Differentiation in dorsal plumage colouration however greatly exceeded that observed at neutral genetic markers, which indicates that the observed pattern of phenotypic differentiation is unlikely to be solely maintained by restricted gene flow and genetic drift.

Sublethal effects on seabirds after the Prestige oil-spill are mirrored in sexual signals

It has been suggested that sexual signals may be a useful measure of environmental quality as they represent the sum of environmental pressures on the animal. Accordingly, it has been proposed that carotenoid-based coloration may be especially valuable in monitoring and detecting the sublethal effects of toxic pollutants in the environment. Here, we evaluate whether the carotenoid-based coloration in the bill of adult yellow-legged gulls reflects oil-induced sublethal effects in breeding colonies affected by the Prestige oil spill. In 2004, we took blood samples from 27 adult birds at four insular breeding colonies located in the pathway of the Prestige oil spill. We measured the size of the red bill spot area and analysed plasma biochemical parameters indicative of sublethal effects of oil contamination in gulls, including glucose, total protein, creatinine, inorganic phosphorus, aspartate aminotransferase (AST) and gamma-glutamyl transferase. We showed that the size of their red bill spot area was positively related to body condition, while negatively related with AST levels, an enzyme that is commonly used as an indication of hepatic damage in birds. Hence, the present study provides support for the idea that carotenoid-based colour integuments may be a useful measure of environmental quality.

Does anthropogenic metal pollution affect carotenoid colouration, antioxidative capacity and physiological condition of great tits (Parus major)?

Although several studies have shown that carotenoid-based signals are negatively affected by (metal) pollution, the underlying mechanisms are not well understood. There are two possible, not mutually exclusive, hypotheses to explain the metal-induced fading of carotenoid colouration. Metal pollution could affect oxidative stress levels and/or the diet. We assessed the expression of the yellow breast of adult and nestling great tits (Parus major) and related this to physiological parameters in plasma indicative of oxidative stress (total antioxidative capacity) and nutritional condition (albumin, triglyceride, total protein, cholesterol and uric acid concentrations). In four study sites along a metal pollution gradient, both adult and nestling great tits had significantly reduced carotenoid colouration at the most polluted sites. While nestlings' total antioxidative capacity was significantly affected by metal pollution, there was no significant effect on adults' total antioxidative capacity. Both for adult and nestling birds, no clear relation between total antioxidative capacity and carotenoid colouration was found. However, there were significant differences among sites in nutritional parameters, indicating that metal pollution might affect diet composition and quality. We found strong among brood variation in nestlings for all variables (except cholesterol), suggesting that there might be a considerable genetic and/or parental investment factor involved.