Color signals in the black grouse (Tetrao tetrix): signal properties and their condition dependency

Structural colours reflect individual quality: a meta-analysis

Colourful ornaments often communicate salient information to mates, and theory predicts covariance between signal expression and individual quality. This has borne out among pigment-based signals, but the potential for 'honesty' in structural coloration is unresolved. Here, I synthesized the available evidence to test this prediction via meta-analysis and found that, overall, the expression of structurally coloured sexual signals is positively associated with individual quality. The effects varied by the measure of quality, however, with body condition and immune function reliably encoded across taxa, but not age nor parasite resistance. The relationship was apparent for both the colour and brightness of signals and was slightly stronger for iridescent ornaments. These results suggest diverse pathways to the encoding and exchange of information among structural colours while highlighting outstanding questions as to the development, visual ecology and evolution of this striking adornment.

Full spectra coloration and condition-dependent signaling in a skin-based carotenoid sexual ornament

Carotenoid-based traits commonly act as condition-dependent signals of quality to both males and females. Such colors are typically quantified using summary metrics (e.g., redness) derived by partitioning measured reflectance spectra into blocks. However, perceived coloration is a product of the whole spectrum. Recently, new methods have quantified a range of environmental factors and their impact on reflection data at narrow wavebands across the whole spectrum. Using this approach, we modeled the reflectance of red integumentary eye combs displayed by male black grouse (Lyrurus tetrix) as a function of ornament size and variables related to male quality. We investigated the strength and direction of effect sizes of variables at each waveband. The strongest effect on the spectra came from eye comb size, with a negative effect in the red part of the spectrum and a positive effect in ultraviolet reflectance. Plasma carotenoid concentration and body mass were also related to reflectance variance in differing directions across the entire spectra. Comparisons of yearlings and adults showed that the effects were similar but stronger on adult reflectance spectra. These findings suggest that reflectance in different parts of the spectrum is indicative of differing components of quality. This method also allows a more accurate understanding of how biologically relevant variables may interact to produce perceived coloration and multicomponent signals and where the strongest biological effects are found.

Divergent color signals from homologous unfeathered ornaments in two congeneric grouse

Color-based visual signals are important aspects of communication throughout the animal kingdom. Individuals evaluate color to obtain information about age and condition and to behave accordingly. Birds display a variety of striking, conspicuous colors and make ideal subjects for the study of color signaling. While most studies of avian color focus on plumage, bare unfeathered body parts also display a wide range of color signals. Mate choice and intrasexual competitive interactions are easily observed in lekking grouse, which also signal with prominent unfeathered color patches. Most male grouse have one pair of colorful bare part ornaments (combs), and males of several species also have inflatable air sacs in their throat. Previous studies have mostly focused on comb color and size, but little is known about the signaling role of air sac color. We measured comb size and the color properties of combs and air sacs in the Lesser and Greater Prairie-Chickens (Tympanuchus pallidicinctus and T. cupido, respectively), and investigated whether these properties varied with age and mass. We found that mass predicted color properties of air sacs and that age predicted comb size in the Greater Prairie-Chicken, suggesting that these ornaments indicate condition dependence. No conclusive relationships between color and age or size were detected in the Lesser Prairie-Chicken. Color properties of both ornaments differed between the two species. Further research is needed to determine mechanisms that link condition to color and whether the information advertised by color signals from these ornaments is intended for males, females, or both.

The resistance of Eimeria spp. to toltrazuril in black grouse (Lyrurus tetrix) kept in an aviary

The black grouse is a critically endangered species in Europe. Aviary breeding and re-introduction programs have contributed to the reinstatement of black grouse populations. Parasitic diseases can influence the species' behavior, choice of habitats, and survival rates. Protozoa of the genus Eimeria are the most prevalent parasites in black grouse. The aim of this study was to evaluate the effectiveness of toltrazuril (TOL) against Eimeria spp. in black grouse kept in an aviary. Eimeria spp. oocysts (mean OPG for all birds = 7,047.84; SD = 5,625.05) were detected in feces samples in a routine parasitological examination. Parasitic infections were treated with toltrazuril Baycox 2.5% (Bayer, Leverkusen, Germany) at 1 ml/1 l H2O administered per os with drinking water twice a day for 2 d, 12 h/24 h. High oocysts counts persisted upon the second parasitic examination. Therefore, the birds were divided into 3 groups in 3 existing aviaries. Toltrazuril was administered according to the same protocol, and its uptake with drinking water was monitored. A parasitological examination conducted 5 d later revealed the presence of parasites, and the TOL dose was increased to 3 ml/1 l H2O. When oocysts were detected in a successive exam, the TOL dose was further increased to 5 ml/1 l H2O. The increase in TOL dose did not eliminate parasites, therefore, the treatment was discontinued. The fecal egg count reduction test (FECRT) revealed that 1 ml of TOL reduced fecal oocysts counts (OPG) by 0.02%, the second application of the same TOL dose reduced OPG by 0%, 3 ml of TOL reduced OPG by 0%, and 5 ml of TOL reduced OPG by 65.7%. The applied doses of TOL did not induce significant differences in the fecal oocysts counts of any of the identified species of coccidia. The results indicate that the coccidia species infecting black grouse have natural resistance to TOL.

Age- and quality-dependent DNA methylation correlate with melanin-based coloration in a wild bird

Secondary sexual trait expression can be influenced by fixed individual factors (such as genetic quality) as well as by dynamic factors (such as age and environmentally induced gene expression) that may be associated with variation in condition or quality. In particular, melanin-based traits are known to relate to condition and there is a well-characterized genetic pathway underpinning their expression. However, the mechanisms linking variable trait expression to genetic quality remain unclear. One plausible mechanism is that genetic quality could influence trait expression via differential methylation and differential gene expression. We therefore conducted a pilot study examining DNA methylation at a candidate gene (agouti-related neuropeptide: AgRP) in the black grouse Lyrurus tetrix. We specifically tested whether CpG methylation covaries with age and multilocus heterozygosity (a proxy of genetic quality) and from there whether the expression of a melanin-based ornament (ultraviolet-blue chroma) correlates with DNA methylation. Consistent with expectations, we found clear evidence for age- and heterozygosity-specific patterns of DNA methylation, with two CpG sites showing the greatest DNA methylation in highly heterozygous males at their peak age of reproduction. Furthermore, DNA methylation at three CpG sites was significantly positively correlated with ultraviolet-blue chroma. Ours is the first study to our knowledge to document age- and quality-dependent variation in DNA methylation and to show that dynamic sexual trait expression across the lifespan of an organism is associated with patterns of DNA methylation. Although we cannot demonstrate causality, our work provides empirical support for a mechanism that could potentially link key individual factors to variation in sexual trait expression in a wild vertebrate.

What maintains signal honesty in animal colour displays used in mate choice?

Many of the colour displays of animals are proposed to have evolved in response to female mate choice for honest signals of quality, but such honest signalling requires mechanisms to prevent cheating. The most widely accepted and cited mechanisms for ensuring signal honesty are based on the costly signalling hypothesis, which posits that costs associated with ornamentation prevent low-quality males from being highly ornamented. Alternatively, by the index hypothesis, honesty can be achieved via cost-free mechanisms if ornament production is causally linked to core physiological pathways. In this essay, we review how a costly signalling framework has shaped empirical research in mate choice for colourful male ornaments and emphasize that alternative interpretations are plausible under an index signalling framework. We discuss the challenges in both empirically testing and distinguishing between the two hypotheses, noting that they need not be mutually exclusive. Finally, we advocate for a comprehensive approach to studies of colour signals that includes the explicit consideration of cost-free mechanisms for honesty.This article is part of the themed issue 'Animal coloration: production, perception, function and application'.

Flexible timing of reproductive effort as an alternative mating tactic in black grouse (Lyrurus tetrix) males

Alternative reproductive tactics often take the form of dichotomous behavioural phenotypes. Focusing attention on such obvious dichotomy means that flexible patterns of behaviour within tactics is largely ignored. Using a long-term dataset of black grouse (Lyrurus tetrix) lek behaviours, we tested whether there were fine-scale differences in reproductive effort (lek attendance, fighting rates) and whether these were related to age and phenotype. Yearling males increased their lek attendance and fighting rate to a peak when adult male effort was declining. Adults and yearlings allocated reproductive effort according to their body mass but this was unrelated to differences in timing of effort. In adult males, different patterns of lek attendance were associated with different costs of reproduction, measured by mass loss or gain. Overall, our work demonstrates that individuals can use flexible patterns of reproductive effort both in terms of their own condition, their age and the likely costs of behaviours.

Limited indirect fitness benefits of male group membership in a lekking species

In group living species, individuals may gain the indirect fitness benefits characterizing kin selection when groups contain close relatives. However, tests of kin selection have primarily focused on cooperatively breeding and eusocial species, whereas its importance in other forms of group living remains to be fully understood. Lekking is a form of grouping where males display on small aggregated territories, which females then visit to mate. As females prefer larger aggregations, territorial males might gain indirect fitness benefits if their presence increases the fitness of close relatives. Previous studies have tested specific predictions of kin selection models using measures such as group-level relatedness. However, a full understanding of the contribution of kin selection in the evolution of group living requires estimating individuals' indirect fitness benefits across multiple sites and years. Using behavioural and genetic data from the black grouse (Tetrao tetrix), we show that the indirect fitness benefits of group membership were very small because newcomers joined leks containing few close relatives who had limited mating success. Males' indirect fitness benefits were higher in yearlings during increasing population density but marginally changed the variation in male mating success. Kin selection acting through increasing group size is therefore unlikely to contribute substantially to the evolution and maintenance of lekking in this black grouse population.

Heterospecific female mimicry in Ficedula flycatchers

Mimicry is a widespread phenomenon. Vertebrate visual mimicry often operates in an intraspecific sexual context, with some males resembling conspecific females. Pied flycatcher (Ficedula hypoleuca) dorsal plumage varies from the ancestral black to female-like brown. Experimental studies have shown that conspecific and heterospecific (collared flycatcher, F. albicollis) individuals of both sexes respond, at least initially, to brown individuals as if they were female. We quantified the perceptual and biochemical differences between brown feathers and found that brown pied flycatcher males are indistinguishable from heterospecific, but not from conspecific, females in both aspects. To our knowledge, this is the first evidence of a visual mimetic signalling system in a sexual context where the model is heterospecific to the mimic. By only mimicking heterospecific females, brown pied flycatcher males can establish territories next to the more dominant collared flycatcher in sympatry, suffer less aggression by darker conspecifics in allopatry and preserve within-species sexual recognition throughout the breeding range. A closer look at the evolutionary history and ecology of these two species illustrates how such a mimetic system can evolve. Although likely rare, this phenomenon might not be unique to Ficedula flycatchers.

Bare-part color in female budgerigars changes from brown to structural blue following testosterone treatment but is not strongly masculinized

Whereas several studies have shown that experimentally increased levels of the androgenic steroid testosterone can affect female behavior, fewer studies have focused on the activational effects of exogenous testosterone on female morphology. With respect to colorful displays in birds, almost exclusively the effects of testosterone manipulation on female carotenoid-based colorations have been studied. Other color types such as structural colors (i.e. UV, blue and violet colors that result from differential light reflection in the nanostructures of the tissue) remain largely unstudied. Here, we investigated the short- and long-term effects of exogenous testosterone on the expression of structural bare-part coloration in female budgerigars, Melopsittacus undulatus. In this parrot species, bare-part coloration is expressed in the cere, a structure over the beak which is brown in females and structural blue in males. We experimentally increased plasma testosterone levels in testosterone-treated females (T-females) compared to controls (C-females) and we performed weekly spectrophotometric measurements of the cere for five weeks after implantation and one measurement after ten weeks. We also estimated the extent to which testosterone masculinized female cere color by comparing the experimental females with untreated males. We found significant effects of testosterone on cere color from week four after implantation onwards. T-females expressed significantly bluer ceres than C-females with higher values for brightness and UV reflectance. T-female cere color, however, remained significantly less blue than in males, while values for brightness and UV reflectance were significantly higher in T-females than in males. Our quantitative results show that exogenous testosterone induces the expression of structural blue color in females but does not strongly masculinize female cere coloration. We provide several potential pathways for the action of testosterone on structural color.

Sexual traits are sensitive to genetic stress and predict extinction risk in the stalk-eyed fly, Diasemopsis meigenii

The handicap principle predicts that sexual traits are more susceptible to inbreeding depression than nonsexual traits. However, this hypothesis has received little testing and results are inconsistent. We used 11 generations of full-sibling mating to test the effect of inbreeding on sexual and nonsexual traits in the stalk-eyed fly Diasemopsis meigenii. Consistent with the theoretical predictions, the male sexual trait (eyespan) decreased more than nonsexual traits (female eyespan and male wing length), even after controlling for body size variation. In addition, male eyespan was a reliable predictor of line extinction, unlike other nonsexual traits. After 11 generations, inbred lines were crossed to generate inbred and outbred families. All morphological traits were larger in outbred individuals than inbred individuals. This heterosis was greater in male eyespan than in male wing length, but not female eyespan. The elevated response in male eyespan to genetic stress mirrored the result found using environmental stress during larval development and suggests that common mechanisms underlie the patterns observed. Overall, these results support the hypothesis that male sexual traits suffer more from inbreeding depression than nonsexual traits and are in line with predictions based on the handicap principle.

Environmental and genetic effects on pigment-based vs. structural component of yellow feather colouration

Background

Carotenoid plumage is of widespread use in bird communication. Carotenoid-based feather colouration has recently been shown to be dependent on both pigment concentration and feather structure. If these two components are determined differently, one plumage patch may potentially convey different aspects of individual quality.

Methodology/Principal Findings

We evaluated the effects of genetic and environmental factors on carotenoid-based yellow breast colouration of Great Tit (Parus major) nestlings. By partial cross-fostering, we separated the genetic and pre-natal vs. post-natal parental effects on both the structural and the pigment-based component of carotenoid-based plumage colouration. We also simultaneously manipulated the post-hatching environment by brood size manipulation. The structural component of nestling colouration reflected features of female colouration. On the other hand, the pigment-based component was more affected by rearing conditions presumably representing food quality. While the structural component was related to both origin- and environment-related factors, the pigment-based component seemed to be environment-dependent only. These results support the notion that pigment-based and structural components of feather colouration are determined differently.

Conclusions/Significance

Chromatic and achromatic components of carotenoid-based feather colouration reflected different aspects of individual quality and history, and thus may potentially form a multicomponent signal.

Age-dependent inbreeding risk and offspring fitness costs in female black grouse

Dispersal is an important mechanism used to avoid inbreeding. However, dispersal may only be effective for part of an individual's lifespan since, post-dispersal individuals that breed over multiple reproductive events may risk mating with kin of the philopatric sex as they age. We tested this hypothesis in black grouse Tetrao tetrix, and show that yearling females never mated with close relatives whereas older females did. However, matings were not with direct kin suggesting that short-distance dispersal to sites containing kin and subsequent overlap of reproductive lifespans between males and females were causing this pattern. Chick mass was lower when kinship was high, suggesting important fitness costs associated with inbred matings. This study shows that increased inbreeding risk might be a widespread yet rarely considered cost of ageing.

Female plumage coloration is sensitive to the cost of reproduction. An experiment in blue tits

1. A growing number of studies suggest that female ornaments are linked to maternal quality and influence male mate choice. These findings challenge the traditional male-biased view of sexual selection and the hypothesis that female ornaments are the outcome of a genetic correlation with male ornaments. To further test the hypothesis that female traits have a function, it is now essential to investigate their honesty and to determine how signalling and reproduction interact in females. If female traits are honest indicators of quality, then they are likely to have a specific signalling function. 2. We investigated whether carry-over effects of reproduction might ensure the honesty of plumage colour signalling of a bird species with conspicuous UV-blue and yellow coloration, the blue tit Cyanistes caeruleus. Reproductive effort was manipulated by removing clutches, thereby forcing both sexes to reproduce twice and to raise chicks later in the breeding season when food is less abundant. In the year following this manipulation, we investigated the change in plumage in experimental and control males and females. The change was measured in the two putative feather ornaments, the UV-blue cap and the yellow breast, and another feather trait probably less likely to be sexually selected: the wing length. We also tested whether higher-quality females had their coloration less affected by the experiment. 3. We found that control but not manipulated males and females increased their signal towards UV. In addition, in the manipulated group, females that were able to lay more eggs had their UV-blue coloration less affected by the treatment. For yellow coloration, we found that manipulated yearlings but not manipulated adults decreased their yellow chroma in comparison with control. Lastly, our results show that the condition of the manipulated females tended to be positively correlated with yellow chroma. 4. These results show that the trade-offs between reproduction and signalling can ensure the honesty of conspicuous plumage traits in female and male blue tits. In addition, they suggest that female traits have the potential to evolve under sexual selection in this and other bird species.

Male mutation bias and possible long-term effects of human activities

The ability of a population to adapt to changing environments depends critically on the amount and kind of genetic variability it possesses. Mutations are an important source of new genetic variability and may lead to new adaptations, especially if the population size is large. Mutation rates are extremely variable between and within species, and males usually have higher mutation rates as a result of elevated rates of male germ cell division. This male bias affects the overall mutation rate. We examined the factors that influence male mutation bias, and focused on the effects of classical life-history parameters, such as the average age at reproduction and elevated rates of sperm production in response to sexual selection and sperm competition. We argue that human-induced changes in age at reproduction or in sexual selection will affect male mutation biases and hence overall mutation rates. Depending on the effective population size, these changes are likely to influence the long-term persistence of a population.

Carotenoid maintenance handicap and the physiology of carotenoid-based signalisation of health

Despite a reasonable scientific interest in sexual selection, the general principles of health signalisation via ornamental traits remain still unresolved in many aspects. This is also true for the mechanism preserving honesty of carotenoid-based signals. Although it is widely accepted that this type of ornamentation reflects an allocation trade-off between the physiological utilisation of carotenoids (mainly in antioxidative processes) and their deposition in ornaments, some recent evidence suggests more complex interactions. Here, we further develop the models currently proposed to explain the honesty of carotenoid-based signalisation of heath status by adding the handicap principle concept regulated by testosterone. We propose that under certain circumstances carotenoids may be dangerous for the organism because they easily transform into toxic cleavage products. When reserves of other protective antioxidants are insufficient, physiological trade-offs may exist between maintenance of carotenoids for ornament expression and their removal from the body. Furthermore, we suggest that testosterone which enhances ornamentation by increasing carotenoid bioavailability may also promote oxidative stress and hence lower antioxidant reserves. The presence of high levels of carotenoids required for high-quality ornament expression may therefore represent a handicap and only individuals in prime health could afford to produce elaborate colourful ornaments. Although further testing is needed, this 'carotenoid maintenance handicap' hypothesis may offer a new insight into the physiological aspects of the relationship between carotenoid function, immunity and ornamentation.

The oxidation handicap hypothesis and the carotenoid allocation trade-off

The oxidation handicap hypothesis proposes that testosterone mediates the trade-off between the expression of secondary sexual traits and the fight against free radicals. Coloured traits controlled by testosterone can be produced by carotenoid pigments (yellow-orange-red traits), but carotenoids also help to quench free radicals. Recently, it has been shown that testosterone increases the amount of circulating carotenoids in birds. Here, a testosterone-mediated trade-off in the carotenoid allocation between colour expression and the fight against oxidative stress is proposed. Male red-legged partridges were treated with testosterone, anti-androgens or manipulated as controls. Testosterone-treated males maintained the highest circulating carotenoid levels, but showed the palest red traits and no evidence of oxidative damage. Increased levels of a key intracellular antioxidant (i.e. glutathione) indicated that an oxidative challenge was in fact induced but controlled. The trade-off was apparently solved by reducing redness, allowing increased carotenoid availability, which could have contributed to buffer oxidative stress.