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

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.

Colouration matters in dull toads: ultraviolet adornment for ladies and agrochemicals fading effects

Integument colouration can influence many aspects of fitness, and is under strong sexual selection. Amphibians often express sexual dichromatism, and ultra-violet (UV) colouration is usually biased toward males as a sexual signal. As an honest signal, colouration is related to several individual traits, but can also be related to environmental factors such as anthropogenic pollutants, to which amphibians are highly sensitive. In this study, we investigated sexual dichromatism and UV reflectance covering a large visual spectrum (wavelength ranging from 300 to 700 nm) on different body areas (throat, ventral and dorsal areas), in a widespread amphibian species, the spiny toad (Bufo spinosus). Then, we tested the impact of chronic exposure to two widespread herbicides (glyphosate's primary metabolite [AMPA] and Nicosulfuron) on their colouration. We found a strong but unexpected sexual dichromatism with females reflecting more in the UV spectrum (throat and ventral area) than males, suggesting these body parts might be critical in intra-specific signalling. Females with higher ventral UV reflectance were in better body condition, suggesting an honest signal role of UV reflectance which could influence male choice. Throat colouration was further differentially influenced by agrochemicals according to sexes. In AMPA-exposed males, throat was more saturated in yellow-orange than in control males, and Nicosulfuron exposure decreased the throat's reflectance hue in females, which can bear consequences on mate attractiveness. Future studies need to investigate the underlying mechanisms that are altered by agrochemical exposure.

Relocation to avoid costs: A hypothesis on red carotenoid-based signals based on recent CYP2J19 gene expression data

In many vertebrates, the enzymatic oxidation of dietary yellow carotenoids generates red keto-carotenoids giving color to ornaments. The oxidase CYP2J19 is here a key effector. Its purported intracellular location suggests a shared biochemical pathway between trait expression and cell functioning. This might guarantee the reliability of red colorations as individual quality signals independent of production costs. We hypothesize that the ornament type (feathers vs. bare parts) and production costs (probably CYP2J19 activity compromising vital functions) could have promoted tissue-specific gene relocation. We review current avian tissue-specific CYP2J19 expression data. Among the ten red-billed species showing CYP2J19 bill expression, only one showed strong hepatic expression. Moreover, a phylogenetically-controlled analysis of 25 red-colored species shows that those producing red bare parts are less likely to have strong hepatic CYP2J19 expression than species with only red plumages. Thus, both production costs and shared pathways might have contributed to the evolution of red signals.

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.

Molecular parallelism in signaling function across different sexually selected ornaments in a warbler

Extravagant ornaments are thought to signal male quality to females choosing mates, but the evidence linking ornament size to male quality is controversial, particularly in cases in which females prefer different ornaments in different populations. Here, we use whole-genome sequencing and transcriptomics to determine the genetic basis of ornament size in two populations of a widespread warbler, the common yellowthroat (Geothlypis trichas). Within a single subspecies, females in a Wisconsin population prefer males with larger black masks as mates, while females in a New York population prefer males with larger yellow bibs. Despite being produced by different pigments in different patches on the body, the size of the ornament preferred by females in each population was linked to numerous genes that function in many of the same core aspects of male quality (e.g., immunity and oxidative balance). These relationships confirm recent hypotheses linking the signaling function of ornaments to male quality. Furthermore, the parallelism in signaling function provides the flexibility for different types of ornaments to be used as signals of similar aspects of male quality. This could facilitate switches in female preference for different ornaments, a potentially important step in the early stages of divergence among populations.

Colourful traits in female birds relate to individual condition, reproductive performance and male-mate preferences: a meta-analytic approach

Colourful traits in females are suggested to have evolved and be maintained by sexual selection. Although several studies have evaluated this idea, support is still equivocal. Evidence has been compiled in reviews, and a handful of quantitative syntheses has explored cumulative support for the link between condition and specific colour traits in males and females. However, understanding the potential function of females' colourful traits in sexual communication has not been the primary focus of any of those previous studies. Here, using a meta-analytic approach, we find that evidence from empirical studies in birds supports the idea that colourful female ornaments are positively associated with residual mass and immune response, clutch size and male-mate preferences. Hence, colourful traits in female birds likely evolved and are maintained by sexual selection as condition-dependent signals.

Antibiotic treatment increases yellowness of carotenoid feather coloration in male greenfinches (Chloris chloris)

Carotenoid plumage coloration is an important sexually selected trait in many bird species. However, the mechanisms ensuring the honesty of signals based on carotenoid pigments remain unclear. It has recently been suggested that intestinal integrity, which is affected by gut parasites and microbiota and influences nutrient absorption and acquisition, mediates the relationship between carotenoid ornamentation and individual quality. Here, we test whether carotenoid plumage coloration in greenfinches (Chloris chloris) is affected by the treatment of an antibiotic or an antiparasitic drug. We captured wild greenfinches (N = 71) and administered anticoccidial medication toltrazuril (TOLTRA) to one group, antibiotic metronidazole (METRO) to the second group to target trichomonosis, and the third group received no medication. In the METRO group, feathers grown during the experiment had significantly higher chroma of yellow parts, but there was no effect of TOLTRA on feather chroma. The results suggest that METRO increased the efficiency of carotenoid modification or deposition to the feathers rather than nutrient acquisition and/or freed energy resources that could be invested in coloration. Alternatively, though not measured, METRO might have affected microbial community and host physiology as microbial metabolites can modulate mitochondrial and immune function.

Biogeographic history moulds population differentiation in ageing of oxidative status in an amphibian

Regulation of oxidative status plays a substantial role in physiological ageing. However, we know little about age-related changes of oxidative status in wild animals, and even less about the role of population history in moulding ageing rates. We addressed these questions by means of a common garden experiment, using the Tyrrhenian tree frog Hyla sarda as the study species. This species underwent a range expansion from northern Sardinia (source) up to Corsica (newly founded) during the Late Pleistocene, and then the two populations became geographically isolated. We found that, at the beginning of the experiment, Sardinian and Corsican frogs had similar concentrations of all oxidative status markers analysed. One year later, Corsican frogs had higher oxidative stress and suffered higher mortality than Sardinian frogs. Our results suggest the intriguing scenario that population differentiation in rates of physiological ageing owing to oxidative stress might be an overlooked legacy of past biogeographic processes.

Wasteful carotenoid coloration and its effects on territorial behavior in a cichlid fish

The famously diverse body coloration of cichlid fish serves communicative functions in mating and social interactions including competition for resources. Here, we examined the effects of a color pattern trait-the width of a yellow bar on a black body-on territorial competition in males and females of a color variant ("Ikola") of the Lake Tanganyika cichlid Tropheus. First, measuring integumentary carotenoid concentrations in the yellow and black body regions, we established that wider yellow bars require more carotenoids allocated to body coloration. However, we also detected high carotenoid concentrations in the black body regions (> 100 µg/g fresh skin), raising questions about the function of non-displayed integumentary carotenoids. Behavioral experiments showed that fish with wider bars were quicker to explore an unfamiliar area of the tank. In experiments including presentations of fish dummies, the bar width of 'territorial' dummies had no effect on the latency time which test fish took to intrude into the dummies' territories. However, male test fish performed fewer aggressive acts against wide-barred than against narrow-barred dummy competitors. Our results suggest that intimidation by wide bars as well as correlations between bar width and explorative behavior may contribute to mediating success in territorial Tropheus "Ikola".

Organic selenium supplement partially alleviated diquat-induced oxidative insults and hepatic metabolic stress in nursery pigs

The study investigated antioxidant effects of Se on resilience to diquat-induced oxidative stress in nursery pigs. Thirty-five weaned pigs were individually housed and randomly assigned to one of the five treatments. Pigs were (1) fed a basal diet and intraperitoneally injected with sterile saline (negative control), (2) fed the basal diet and injected with diquat solution (positive control, PC), or fed the basal diet supplemented with 0·3 mg Se/kg as (3) sodium selenite (SS), (4) soyabean protein-chelated Se (SC) or (5) selenised yeast (SY) and intraperitoneally injected with diquat. Pigs were fed the experimental diets for 17 d and injected with diquat at 10 mg/kg body weight or saline on the 11th day of the study (day 0 post-injection (PI)). Diquat exposure induced acute stress and innate immune activation (P < 0·05) at 6 h PI and compromised (P < 0·05) plasma glutathione peroxidase activity on day 2 PI, which was accompanied by an increase in plasma malondialdehyde at 6 h and day 2 PI (P < 0·10). Organic Se, particularly SY, enhanced (P < 0·05) endogenous antioxidant activity in various aspects compared with the PC group. The growth rate and feed intake from day 0 to day 7 PI were significantly lower in the PC, SS and SC groups than the NC group (P < 0·05). Untargeted metabolomics analysis revealed that twenty-two hepatic metabolites (false discovery rate < 0·15) associated with lipid and cellular antioxidant metabolism were altered by diquat. SY restored hepatic metabolic profiles in some but not all samples.

Carotenoid-based coloration predicts both longevity and lifetime fecundity in male birds, but testosterone disrupts signal reliability

Sexual selection promotes the evolution of conspicuous animal ornaments. To evolve as signals, these traits must reliably express the “quality” of the bearer, an indicator of individual fitness. Direct estimates of individual fitness may include the contribution of longevity and fecundity. However, evidence of a correlation between the level of signal expression and these two fitness components are scarce, at least among vertebrates. Relative fitness is difficult to assess in the wild as age at death and extra-pair paternity rates are often unknown. Here, in captive male red-legged partridges, we show that carotenoid-based ornament expression, i.e., redness of the bill and eye rings, at the beginning of reproductive life predicts both longevity (1–7 years) and lifetime breeding output (offspring number and hatching success). The recently proposed link between the individual capacity to produce red (keto) carotenoid pigments and the efficiency of cell respiration could, ultimately, explain the correlation with lifespan and, indirectly, fecundity. Nonetheless, in males of avian species, carotenoid-based coloration in bare parts is also partially controlled by testosterone. We also manipulated androgen levels throughout life by treating males with testosterone or antiandrogen compounds. Treatments caused correlations between signal levels and both fitness components to disappear, thus making the signals unreliable. This suggests that the evolution of carotenoid-based sexual signals requires a tightly-controlled steroid metabolism.

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.

Dietary canthaxanthin reduces xanthophyll uptake and red coloration in adult red-legged partridges

Carotenoids give color to conspicuous animal signals that are often the product of sexual selection. Knowledge of the mechanisms involved in carotenoid-based signaling is critical to understanding how these traits evolve. However, these mechanisms remain only partially understood. Carotenoids are usually viewed as scarce dietary antioxidants whose allocation to ornaments may trade off against health. This trade-off would ensure its reliability as a signal of individual quality. In the case of red (keto)carotenoids, the literature suggests that some species may show constraints in their uptake. Canthaxanthin is one of the most common ketocarotenoids in red ornaments of animals. It is often commercially used as a dietary supplement to obtain redder birds (e.g. poultry). We increased the dietary canthaxanthin levels in captive red-legged partridges (Alectoris rufa). This species shows red non-feathered parts mostly pigmented by another common ketocarotenoid: astaxanthin. We studied the impact on the uptake of carotenoids and vitamins and, finally, on coloration. We also tested the potential protective effect of canthaxanthin when exposing birds to a free radical generator (diquat). Canthaxanthin did not apparently protect birds from oxidative stress, but interfered with the absorption of yellow carotenoids (lutein and zeaxanthin). Zeaxanthin is a precursor of astaxanthin in enzymatic pathways, and their levels in tissues and eggs were lower in canthaxanthin-supplied birds. This led to lower astaxanthin levels in ornaments and paler coloration. As far as we know, this is the first report of a carotenoid supplementation decreasing animal coloration. The results have implications for understanding carotenoid-based signaling evolution, but also for improving husbandry/experimental procedures.

Morphological and biological characterization of a light-colored mutant in the multicolored Asian lady beetle, Harmonia axyridis

Insect cuticle color formed with melanin pigments has numerous types of mutants which usually cause pleiotropic effects. Melanism has been widely studied, but mutants with light-colored phenotype as well as the consequent fitness changes have rarely been reported.Here, in the laboratory strain of Harmonia axyridis, we found a novel mutant gr and confirmed that the mutation was inherited in a simple Mendelian autosomal recessive manner. This mutant (HAM) continuously displayed a light-colored pigmentation versus dark blackish in the wild phenotype (HAW). L-DOPA and dopamine are melanin precursors, and less L-DOPA was present in the cuticle of larval and adult HAM mutants compared to HAW wild type, but more dopamine was detected in the larval cuticle of HAM (p ≤ 0.0235). For the orange background of elytra, the composition as well as total concentration of carotenoids was different between HAM and HAW, which resulted in significantly lower saturation value but significantly higher hue value in HAM than in HAW (p < 0.0001).Extensive fitness changes were detected in HAM. (a) HAM larvae had similar predation capacity and preimaginal development time as HAW, but the newly emerged adults were much smaller (p < 0.0001). (b) Both fecundity and egg hatch rate in cross ♀(HAM) × ♂(HAM) were significantly lower than those in ♀(HAW) × ♂(HAW) (p ≤ 0.0087), but were not different with those in ♀(HAW) × ♂(HAM).

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.

The fractal dimension of a conspicuous ornament varies with mating status and shows assortative mating in wild red-legged partridges (Alectoris rufa)

Complex body designs, such as plumage ornaments in birds, can be described by fractal geometry. These complex patterns could have a role as visual signals during courtship and social interactions, but an empirical validation in the wild is currently lacking. Here, we investigated whether the fractal dimension (FD) of a complex plumage pattern displayed by red-legged partridges Alectoris rufa could function as a potential sexual signal. We captured wild birds early in the breeding season and tested if mated and unmated birds differed in the FD of their conspicuous melanin-based black bib. We also tested if the FD of the black bib was correlated within the pair, looking for evidence of assortative mating based on the expression of this trait. We simultaneously assessed similar effects in other ornamental traits (black bib size, white throat patch and black flank band surface, redness of the eye rings and bill). Mated birds showed higher black bib FD values than unmated ones. Mated males, but not females, also displayed a larger black bib. Moreover, the black bib FD (but not the trait size) and the white throat patch surface showed assortative mating. Finally, females with higher black bib FD showed smaller black flank band surface, suggesting a trade-off in the expression of the two melanin-pigmented plumage traits. This provides unique and novel indication for the shape complexity of a pigmented trait, here described by its fractal dimension, to be potentially under sexual selection in a wild animal.

Trade-off between carotenoid-based sexual ornamentation and sperm resistance to oxidative challenge

It has been hypothesized that carotenoid-based sexual ornamentation signals male fertility and sperm competitive ability as both ornamentation and sperm traits may be co-affected by oxidative stress, resulting in positive covariation (the 'redox-based phenotype-linked fertility hypothesis'; redox-based PLFH). On the other hand, the 'sperm competition theory' (SCT) predicts a trade-off between precopulatory and postcopulatory traits. Here, we manipulate oxidative status (using diquat dibromide) and carotenoid availability in adult zebra finch (Taeniopygia guttata) males in order to test whether carotenoid-based beak ornamentation signals, or is traded off against, sperm resistance to oxidative challenge. Initial beak colouration, but not its change during the experiment, was associated with effect of oxidative challenge on sperm velocity, such that more intense colouration predicted an increase in sperm velocity under control conditions but a decline under oxidative challenge. This suggests a long-term trade-off between ornament expression and sperm resistance to oxidative challenge. Shortening of the sperm midpiece following oxidative challenge further suggests that redox homeostasis may constrain sperm morphometry. Carotenoid supplementation resulted in fewer sperm abnormalities but had no effect on other sperm traits. Overall, our data challenge the redox-based PLFH, partially support the SCT and highlight the importance of carotenoids for normal sperm morphology.

Are long-term widespread avian body size changes related to food availability? A test using contemporaneous changes in carotenoid-based color

Recent changes in global climate have been linked with changes in animal body size. While declines in body size are commonly explained as an adaptive thermoregulatory response to climate warming, many species do not decline in size, and alternative explanations for size change exist. One possibility is that temporal changes in animal body size are driven by changes in environmental productivity and food availability. This hypothesis is difficult to test due to the lack of suitable estimates that go back in time. Here, we use an alternative, indirect, approach and assess whether continent-wide changes over the previous 100 years in body size in 15 species of Australian birds are associated with changes in their yellow carotenoid-based plumage coloration. This type of coloration is strongly affected by food availability because birds cannot synthesize carotenoids and need to ingest them, and because color expression depends on general body condition. We found significant continent-wide intraspecific temporal changes in body size (wing length) and yellow carotenoid-based color (plumage reflectance) for half the species. Direction and magnitude of changes were highly variable among species. Meta-analysis indicated that neither body size nor yellow plumage color showed a consistent temporal trend and that changes in color were not correlated with changes in size over the past 100 years. We conclude that our data provide no evidence that broad-scale variation in food availability is a general explanation for continent-wide changes in body size in this group of species. The interspecific variability in temporal changes in size as well as color suggests that it might be unlikely that a single factor drives these changes, and more detailed studies of museum specimens and long-term field studies are required to disentangle the processes involved.

Oxidative stress affects sperm performance and ejaculate redox status in subordinate House Sparrows

Oxidative stress (OS) is the result of random cellular damage caused by reactive oxygen species that leads to cell death, ageing, or illness. Most physiological processes can result in OS, which in turn has been identified as a major cause of infertility. In promiscuous species, the fertilizing ability of the ejaculate partly determines the male reproductive success. When dominance determines access to fertile females, theory predicts that lower ranking males should increase resource investment into enhancing ejaculate quality. We hypothesized that subordinate males should thus prioritize antioxidant protection of their ejaculates to protect them from OS. We put this hypothesis to the test, by chronically dosing wild House Sparrows with diquat (∼1mg/kg), an herbicide that increases pro-oxidant generation. We found that, although they increased their antioxidant levels in the ejaculate, diquat-treated males produced sperm with reduced velocity. Importantly, and contrary to our hypothesis, males at the bottom of the hierarchy suffered the largest reduction in sperm velocity. We suggest that resource access hinders individuals' ability to cope with environmental hazards. Our results point at OS as a likely physiological mechanism mediating ejaculate quality, while individual ability to access resources may play a role in constraining the extent to which such resources can be allocated into the ejaculate.

Specific carotenoid pigments in the diet and a bit of oxidative stress in the recipe for producing red carotenoid-based signals

Colorful ornaments have been the focus of sexual selection studies since the work of Darwin. Yellow to red coloration is often produced by carotenoid pigments. Different hypotheses have been formulated to explain the evolution of these traits as signals of individual quality. Many of these hypotheses involve the existence of a signal production cost. The carotenoids necessary for signaling can only be obtained from food. In this line, carotenoid-based signals could reveal an individual’s capacity to find sufficient dietary pigments. However, the ingested carotenoids are often yellow and became transformed by the organism to produce pigments of more intense color (red ketocarotenoids). Biotransformation should involve oxidation reactions, although the exact mechanism is poorly known. We tested the hypothesis that carotenoid biotransformation could be costly because a certain level of oxidative stress is required to correctly perform the conversion. The carotenoid-based signals could thus reveal the efficiency of the owner in successfully managing this challenge. In a bird with ketocarotenoid-based ornaments (the red-legged partridge; Alectoris rufa), the availability of different carotenoids in the diet (i.e. astaxanthin, zeaxanthin and lutein) and oxidative stress were manipulated. The carotenoid composition was analyzed and quantified in the ornaments, blood, liver and fat. A number of oxidative stress biomarkers were also measured in the same tissues. First, we found that color and pigment levels in the ornaments depended on food levels of those carotenoids used as substrates in biotransformation. Second, we found that birds exposed to mild levels of a free radical generator (diquat) developed redder bills and deposited higher amounts of ketocarotenoids (astaxanthin) in ornaments. Moreover, the same diquat-exposed birds also showed a weaker resistance to hemolysis when their erythrocytes were exposed to free radicals, with females also enduring higher oxidative damage in plasma lipids. Thus, higher color production would be linked to higher oxidative stress, supporting the biotransformation hypothesis. The recent discovery of an avian oxygenase enzyme involved in converting yellow to red carotenoids may support our results. Nonetheless, the effect could also depend on the abundance of specific substrate carotenoids in the diet. Birds fed with proportionally higher levels of zeaxanthin showed the reddest ornaments with the highest astaxanthin concentrations. Moreover, these birds tended to show the strongest diquat-mediated effect. Therefore, in the evolution of carotenoid-based sexual signals, a biotransformation cost derived from maintaining a well-adjusted redox machinery could coexist with a cost linked to carotenoid acquisition and allocation (i.e. a resource allocation trade-off).

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.

Oxidative stress during courtship affects male and female reproductive effort differentially in a wild bird with biparental care

Oxidative stress has been suggested as one of the physiological mechanisms modulating reproductive effort, including investment in mate choice. Here, we evaluated whether oxidative stress influences breeding decisions by acting as a cost of or constraint on reproduction in the brown booby (Sula leucogaster), a long-lived seabird with prolonged biparental care. We found that during courtship, levels of lipid peroxidation (LP) of males and females were positively associated with gular skin color, a trait presumably used in mate choice, while levels of reactive oxygen species (ROS) were higher as laying approached and in early breeding pairs. Evidence of a constraining effect of oxidative stress for females was suggested by the fact that females with higher ROS during courtship laid smaller first eggs and had chicks with lower rates of body mass gain, and higher female LP was associated with lower offspring attendance time. No evidence of an oxidative cost of parental effort was found; from courtship to parental care male and female' ROS decreased, and changes in LP levels were non-significant. Finally, using a cross-fostering experiment we found that offspring ROS was unrelated to rearing and genetic parents' ROS. Interestingly, offspring LP was positively associated with the LP during courtship of both the rearing parents and the genetic father, suggesting that offspring LP might have both a genetic and an environmental component. Hence, in the brown booby oxidative stress may be a cost of investment in reproductive traits before egg laying and constrain females' investment in eggs and parental care.

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.

Imidacloprid-treated seed ingestion has lethal effect on adult partridges and reduces both breeding investment and offspring immunity

The ingestion of imidacloprid treated seeds by farmland birds may result in exposure to toxic amounts of this insecticide. Here we report on the effects that the exposure to the recommended application rate and to 20% of that rate may produce on birds feeding on treated seeds. Experimental exposure to imidacloprid treated seeds was performed on red-legged partridges (Alectoris rufa) (n=15 pairs per treatment group: control, 20% or 100% of the recommended application rate) during two periods that corresponded to the autumn (duration of exposure: 25 days) and late winter (10 days) cereal sowing times in Spanish farmlands. We studied effects on the survival, body condition, oxidative stress biomarkers, plasma biochemistry, carotenoid-based coloration, T-cell mediated immune response and reproduction of exposed adult partridges, and on the survival and T-cell immune response of their chicks. The high dose (recommended application rate) killed all partridges, with mortality occurring faster in females than in males. The low dose (20% the recommended application rate) had no effect on mortality, but reduced levels of plasma biochemistry parameters (glucose, magnesium and lactate dehydrogenase), increased blood superoxide dismutase activity, produced changes in carotenoid-based integument coloration, reduced the clutch size, delayed the first egg lay date, increased egg yolk vitamins and carotenoids and depressed T-cell immune response of chicks. Moreover, the analysis of the livers of dead partridges revealed an accumulation of imidacloprid during exposure time. Despite the moratorium on the use of neonicotinoids in the European Union, birds may still be at high risk of poisoning by these pesticides through direct sources of exposure to coated seeds in autumn and winter.

An appraisal of how the vitamin A-redox hypothesis can maintain honesty of carotenoid-dependent signals

The vitamin A-redox hypothesis provides an explanation for honest signaling of phenotypic quality by carotenoid-dependent traits. A key aspect of the vitamin A-redox hypothesis, applicable to both yellow and red coloration, is the hypothesized negative feedback of tightly regulated Vitamin A plasma levels on the enzyme responsible for sequestering both Vitamin A and carotenoids from the gut. We performed a meta-analysis and find that vitamin A levels are positively related to carotenoid plasma levels (r = 0.50, P = 0.0002). On the basis of this finding and further theoretical considerations, we propose that the vitamin A-redox hypothesis is unlikely to explain carotenoid-dependent honest signaling.

Foraging for carotenoids: do colorful male hihi target carotenoid-rich foods in the wild?

Birds that color their feathers with dietary carotenoid pigments are expected to seek out these pigments when they are molting. We show that molting male hihi, who express carotenoid-based plumage, seek out naturally occurring foods that are rich in carotenoid pigments. Female hihi, who do not express carotenoid-based plumage, do not seek out carotenoid-rich foods. This lends strength to the idea that carotenoid-based plumage reveals an individual’s foraging ability.

Dietary access to carotenoids is expected to determine the strength of carotenoid-based signal expression and potentially to maintain signal honesty. Species that display carotenoid-based yellow, orange, or red plumage are therefore expected to forage selectively for carotenoid-rich foods when they are depositing these pigments during molt, but whether they actually do so is unknown. We set out to address this in the hihi (Notiomystis cincta), a New Zealand passerine where males, but not females, display yellow carotenoid-based plumage. We measured circulating carotenoid concentrations in male and female hihi during breeding and molt, determined the nutritional content of common foods in the hihi diet, and conducted feeding observations of male and female hihi during molt. We found that although male and female hihi do not differ significantly in plasma carotenoid concentration, male hihi have a greater proportion of carotenoid-rich foods in their diet than do females. This is a consequence of a greater fruit and lower invertebrate intake than females and an avoidance of low-carotenoid content fruit. By combining behavioral observations with quantification of circulating carotenoids, we present evidence that colorful birds forage to maximize carotenoid intake, a conclusion we would not have drawn had we examined plasma carotenoids alone.

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.

On the methodological limitations of detecting oxidative stress: effects of paraquat on measures of oxidative status in greenfinches

Oxidative stress (OS) is widely believed to be responsible for generation of trade-offs in evolutionary ecology by means of constraining investment into a number of components of fitness. Yet the progress in understanding the true role of OS in ecology and evolution has remained elusive. Interpretation of current findings is particularly hampered by the scarcity of experiments demonstrating which of the many available parameters of oxidative status respond most sensitively to and are relevant for measuring OS. We addressed these questions in wild-caught captive greenfinches (Carduelis chloris) by experimental induction of OS by administration of the pro-oxidant compound paraquat with drinking water. Treatment induced 50% of mortality and a significant drop in body mass and an increase in oxidative DNA damage and glutathione levels in erythrocytes among the survivors of the high paraquat (0.2 g/L during 7 days) group. Three days after the end of the treatment, paraquat had no effect on peroxidation of lipids (plasma malondialdehyde), carbonylation of proteins (in erythrocytes), parameters of plasma antioxidant protection (TAC and OXY), uric acid or carotenoids. Our findings of an increase in one marker of damage and one marker of protection from the multitude of measured variables indicate that detection of OS is difficult even under most stringent experimental induction of oxidative insult. We hope that this study highlights the need for reconsideration of over-simplistic models of OS and draws attention to the limitations of detection of OS due to time-lagged and hormetic up-regulation of protective mechanisms. This study also underpins the diagnostic value of measurement of oxidative damage to DNA bases and assessment of erythrocyte glutathione levels.

Differential effects of specific carotenoids on oxidative damage and immune response of gull chicks

Micronutrients are essential for normal metabolic processes during early development. Concretely, it has been suggested that diet-derived carotenoids can play a key role in physiological functions due to their antioxidant and immunostimulant properties. However, their role as antioxidants remains controversial. Additionally, it is also unclear whether oxidative stress mediates their immunostimulatory effects. In this field study, we separately supplemented yellow-legged gull (Larus michahellis, Naumann 1840) chicks with two carotenoids (lutein and β-carotene) with different molecular structure and different transformation pathways into other oxidative forms of carotenoids. We quantified their effect on the oxidative status and the immune response of chicks before and after an oxidative challenge with Paraquat, a pro-oxidant molecule. Prior to oxidative challenge, no carotenoid treatment affected the oxidative status of chicks, but they enhanced the inflammatory response to an antigen compared to controls. The oxidative challenge enhanced plasma vitamin E levels (but not in β-carotene supplemented chicks) and the antioxidant capacity in the short term. Interestingly, lutein-supplemented chicks showed lower oxidative damage to proteins than non-lutein supplemented chicks. After the oxidative challenge, the positive effect of carotenoid supplementation on the immune response disappeared. Thus, these results suggest differential effects of two carotenoids with different molecular structure on the oxidative status. Lutein but not β-carotene helps to combat oxidative damage after a free-radical exposure. Additionally, the results indicate that the immunostimulatory effects of carotenoids are linked to oxidative status during early life.

Attractive skin coloration: harnessing sexual selection to improve diet and health

In this paper we review the mechanisms through which carotenoid coloration could provide a sexually selected cue to condition in species with elaborate color vision. Skin carotenoid pigmentation induced by fruit and vegetable consumption may provide a similar cue to health in humans (particularly light-skinned Asians and Caucasians). Evidence demonstrates that carotenoid-based skin coloration enhances apparent health, and that dietary change can perceptibly impact skin color within weeks. We find that the skin coloration associated with increased fruit and vegetable consumption benefits apparent health to a greater extent than melanin pigmentation. We argue that the benefits to appearance may motivate individuals to improve their diet and that this line of appearance research reveals a potentially powerful strategy for motivating a healthy lifestyle.

Attractive Skin Coloration: Harnessing Sexual Selection to Improve Diet and Health

In this paper we review the mechanisms through which carotenoid coloration could provide a sexually selected cue to condition in species with elaborate color vision. Skin carotenoid pigmentation induced by fruit and vegetable consumption may provide a similar cue to health in humans (particularly light-skinned Asians and Caucasians). Evidence demonstrates that carotenoid-based skin coloration enhances apparent health, and that dietary change can perceptibly impact skin color within weeks. We find that the skin coloration associated with increased fruit and vegetable consumption benefits apparent health to a greater extent than melanin pigmentation. We argue that the benefits to appearance may motivate individuals to improve their diet and that this line of appearance research reveals a potentially powerful strategy for motivating a healthy lifestyle.