A novel method for quantifying the glossiness of animals

The glossy sheen of healthy hair is an ideal of human beauty; however, glossiness has never been quantified in the context of non-human animal signaling. Glossiness, the specular reflectance characteristic of polished surfaces, has the potential to act as a signal of quality because it depends upon material integrity and cleanliness. Here, we undertook two studies of glossiness in avian plumage to determine (a) the repeatability of a recently developed measure of glossiness, (b) the relationship between glossiness and conventional measures of coloration, and (c) how glossiness is associated with quality signaling. Using museum specimens of three North American bird species with glossy plumage (red-winged blackbird, Agelaius phoeniceus; great-tailed grackle, Quiscalus mexicanus; Chihuahuan raven, Corvus cryptoleucus), we found that the glossiness measure was highly repeatable for all species and was significantly correlated with plumage coloration (e.g., chroma, brightness) in male great-tailed grackles. We then used wild-caught grackles to examine sexual dimorphism in plumage glossiness and its correlation to a potentially sexually selected trait in this species, male tail length. We found that males were significantly glossier than females and that male, but not female, glossiness correlated positively with tail length. This study provides a repeatable method to measure glossiness and highlights its potential as a signal of individual quality in animals.     

Absence of status signalling by structurally based ultraviolet plumage in wintering blue tits (<Emphasis Type="Italic">Cyanistes caeruleus</Emphasis>)

Structurally based ultraviolet (UV)-reflective plumage parts can be important cues in mate choice. However, it remains largely unknown if UV plumage variation can also function as a signal of social status during competitive interactions. In blue tits (Cyanistes caeruleus), the UV-reflective crown plumage functions as a female mate choice cue that probably indicates male quality, as males with higher UV reflectance have been shown to have better chances of over-winter survival. Possibly, the UV crown plumage acts as a status signal in the competition over scarce food sources during winter. To test this idea, we related dominance of individuals at an artificial food source during adverse winter conditions to spectrophotometric measurements of their crown plumage. However, while controlling for the confounding effects of sex, age, and distance from territory, we found no significant effect of crown UV reflectance on dominance. Consistent with this result, we also found no relation between crown UV reflectance and over-winter survival. We conclude that the structurally based UV reflectance of the blue tit crown feathers plays little role in competition between individuals during winter despite its importance as a cue in mate choice.     

Sexual selection in a socially monogamous bird: male color predicts paternity success in the mountain bluebird, <Emphasis Type="Italic">Sialia currucoides</Emphasis>

Ornamental traits are thought to evolve because they give individuals an advantage in securing multiple mates. Thus, the presence of ornamentation among males in many monogamous bird species presents something of a conundrum. Under certain conditions, extra-pair paternity can increase the variance in reproductive success among males, thus increasing the potential for sexual selection to act. We addressed this possibility in the mountain bluebird (Sialia currucoides), a socially monogamous songbird in which males possess brilliant ultraviolet (UV)-blue plumage. Specifically, we asked whether a male’s success at siring offspring within his own nest and within the nests of other males was related to his coloration. In pairwise comparisons, males that sired extra-pair offspring were not more colorful than the males that they cuckolded. However, males that sired at least one extra-pair offspring were, on average, brighter and more UV-blue than males that did not sire extra-pair offspring. Brighter, more UV-blue males sired more offspring both with their own mate and tended to sire more offspring with extra-pair mates and thus sired more offspring overall. Our results support the hypothesis that the brilliant UV-blue ornamental plumage of male mountain bluebirds evolved at least in part because it provides males with an advantage in fertilizing the eggs of multiple females.     

Achromatic color variation in black-capped chickadees,Poecile atricapilla: black and white signals of sex and rank

Sexual dichromatism and phenotypic variation in elaborate male traits are common products of sexual selection. The spectral properties of carotenoid and structurally-based plumage colors and the patch sizes of melanin-based plumage colors have received considerable attention as sexual signals in birds. However, the importance of variation in achromatic plumage colors (white, gray and black) remains virtually unexplored, despite their widespread occurrence. We investigated a potential signal function of the achromatic black and white plumage of black-capped chickadees (Poecile atricapilla). We captured and color-banded 178 free-living chickadees and assessed winter flock dominance hierarchies by tabulating pairwise interactions at feeders. We recaptured 73 of these birds and measured plumage coloration for six body regions using a reflectance spectrometer and the area of melanin-based plumage patches from standardized photographs. We found extensive individual variation in chickadee plumage traits and considerable sexual dichromatism. Male black-capped chickadees have significantly brighter white plumage than females, larger black patches, and greater plumage contrast between adjacent white and black plumage regions. We also found rank differences in the plumage reflectance of males; high-ranking males, who are preferred by females as both social and extra-pair partners, exhibit significantly darker black plumage and grow their feathers more rapidly than low-ranking males. This variation among individuals reveals a potential signal function for achromatic plumage coloration in birds.     

Informative content of multiple plumage-coloured traits in female and male European Rollers

Animals may assess the quality of other individuals by using information that different ornaments may provide. The European Roller (Coracias garrulus) is a socially monogamous species in which males and females display highly conspicuous plumage colouration. According to the mutual selection hypothesis, we predicted that, in this species, plumage coloration could signal individual quality in both sexes because both female and male rollers invest a considerable amount of time caring for their offspring. We used spectrophotometric measurements to investigate the information content of multiple plumage colour traits. We found that the roller is actually a sexually dimorphic and dichromatic species. Different plumage colours from different origins were correlated within individual. Head and back brightness correlated with body condition in both sexes, and in males, head brightness correlated with the number of fledglings in successful nests, while head green-yellow saturation correlated with parental provisioning. Meanwhile, in females, back brightness was related to the number of fledglings in successful nests and to parental provisioning rate. In addition, there was a positive assortative mating in relation to weight, body condition, head green-yellow saturation and back brightness. Finally, we found a positive correlation between parent and offspring coloration. Altogether, these results suggest that multiple colour traits may act as quality indicators in the roller and that they may be used by the two sexes to assess potential mate quality.     

Interacting effects of ambient light and plumage color patterns in displaying Wire-tailed Manakins (Aves,Pipridae)

Sending color signals to conspecifics may attract predators, leading to opposing selection pressures on the evolution of signal expression and display behavior in animals. The costs of signaling can be reduced, however, because conspicuousness is the combined result of the reflectance spectra of the displayer's color pattern and the spectra of ambient light illuminating the animal. Changes in ambient light can alter conspicuousness, even when chemical and structural color-generating mechanisms remain constant, potentially allowing animals to display their colors most fully in light environments where the benefits are greatest relative to the costs. Using spectroradiometric methods, we determined how light habitat use affects conspicuousness in adult males of the Wire-tailed Manakin Pipra filicauda, a lekking bird species with vivid plumage colors. We studied three aspects of visibility, including properties of the entire color pattern, visual contrast within an individual's plumage and a bird's contrast relative to its visual background. Wire-tailed Manakins usually displayed in forest shade environments, which reduced their conspicuousness at larger viewing distances, while maximizing visual contrast within the plumage color pattern at close viewing distances. Compared to sunspots, ambient light in forest shade reduces the contrast of individual bird colors with the background at close viewing distance. However, background contrast of individual bird colors in the shade was still relatively higher during sunny than during cloudy weather which may explain why males were more active when the sun was not blocked by clouds. Assuming that the visual perceptions of predators and other manakins do not differ from the reflectance patterns we measured, Wire-tailed manakins tend to display in light environments that reduce the conflicts between avoiding long distance detection by predators and displaying conspicuous color signals to visiting females.     

Parental effects on carotenoid-based plumage coloration in nestling great tits, Parus major

Carotenoid pigments have attracted much interest in behavioural and evolutionary ecology because of their dual function in immune physiology and as color signals. In vertebrates, carotenoids must ultimately be obtained from the diet, and the mechanisms and magnitude of this environmental dependence are central for understanding carotenoid signal functions and evolution. In the present cross-fostering experiment with great tits Parus major, we investigate pre- and postnatal parental effects (egg yolk carotenoids, parental coloration) on nestling size and carotenoid coloration, using HPLC analysis of egg yolk carotenoids, and a reflectance-based measure of ‘chroma’ that reflects the plumage pigment concentration. Both rearing environment and origin influenced offspring size and plumage chroma. Maternal allocation of carotenoids to eggs had a weak positive effect on nestling plumage chroma, whereas we found no prenatal maternal effects (egg size or yolk carotenoid concentration) on size. Nestling plumage chroma was also significantly predicted by the chroma of the rearing father, but not by the color of the rearing mother or either of the original (genetical) parents. Thus, both prenatal maternal effects and postnatal paternal effects influence the carotenoid-based plumage coloration of nestling great tits. Future studies will reveal if parental effects have long-term consequences for plumage development and associated fitness components.     

Variation in the temporal and spatial use of signals and its implications for multimodal communication

The use of signals across multiple sensory modalities in communication is common in animals and plants. Determining the information that each signal component conveys has provided unique insights into why multimodal signals evolve. However, how these complex signals are assessed by receivers will also influence their evolution, a hypothesis that has received less attention. Here, we explore multimodal signal assessment in a closely related complex of island flycatchers that have diverged in visual and acoustic signals. Using field experiments that manipulated song and plumage colour, we tested if song, a possible long-range signal, is assessed before plumage colour in conspecific recognition. We find that divergent song and colour are assessed in sequence, and this pattern of sequential assessment is likely mediated by habitat structure and the extent of differences in signal characteristics. A broad survey of the literature suggests that many organisms from a wide range of taxa sequentially assess multimodal signals, with long-range signals attracting conspecifics for further assessment of close-range signals. Our results highlight the need to consider how signals are assessed when understanding multimodal signal evolution. Finally, given the results of our field experiments indicating sequential assessment of divergent song and colour in the recognition of conspecifics, we discuss the consequences of multimodal signal divergence for the origin of species, as changes in signals across different sensory modalities may influence the evolution of premating reproductive isolation.     

Aggression and fighting ability are correlated in the swordtail fish Xiphophorus cortezi: the advantage of being barless

Males of the swordtail fish Xiphophorus cortezi are polymorphic for the pigment pattern vertical bars. Previous studies indicate that barred males exhibit higher levels of aggression towards males with bars than those without, while barless males fail to exhibit differential levels of aggression to either morph. In this study I matched barred and barless males for size and paired them in dyadic contests in order to determine if either morph was more dominant and if so, if dominance was the result of higher aggression levels. I found that barless males had higher bite frequencies and were able to win a majority of the contests while barred males consistently escalated to biting first, even though in most cases they were ultimately the losers. In order to determine whether the observed aggression levels and fighting abilities were inherent to being barless or a consequence of responses to the bars themselves, the dyads were re-paired once after barless males were given temporary bars and once after barred males had their bars removed. Thus, each morph encountered his opponent in both a barred and barless state. Regardless of bar state, naturally barless males continued to be more aggressive and more dominant than their barred counterparts. In addition, naturally barred males only won contests in which they bit more. These results indicate that for this species, aggression is an important component of winning contests when opponents are roughly the same size. As a result, naturally barless males as a whole appear to have higher resource holding potential (RHP) than naturally barred males of the same size because of their greater aggression levels.     

Female pied flycatchers Ficedula hypoleuca choose male characteristics in homogeneous habitats

Summary The paper reports the results of a 2-year study of pairing success of male pied flycatchers in a homogeneous habitat. A handicapping experiment was carried out in which certain wing and tail feathers were removed from randomly selected males. Handicapped males had reduced pairing success, they lost weight, and they sang less frequently than control males. Male pairing success was positively correlated with the darkness of the plumage, body-size, and previous breeding experience. Earlier studies on the same species have failed to detect any relationships between pairing success and male characteristics, possibly because of habitat heterogeneity and variation in nest site quality. The evolutionary basis for female choice of male characteristics is discussed. There are reports that males with attractive traits (e.g. black plumage) provide a high quality of parental care. However, the fact that male pairing success was related to male conspicuousness makes it difficult to discriminate between active and passive female choice.     

Greater flamingos <Emphasis Type="Italic">Phoenicopterus roseus</Emphasis> use uropygial secretions as make-up

It was long thought that the colour of bird feathers does not change after plumage moult. However, there is increasing evidence that the colour of feathers may change due to abrasion, photochemical change and staining, either accidental or deliberate. The coloration of plumage due to deliberate staining, i.e. with cosmetic purposes, may help individuals to communicate their quality to conspecifics. The presence of carotenoids in preen oils has been previously only suggested, and here we confirm for the first time its presence in such oils. Moreover, the carotenoids in the uropygial secretions were the same specific pigments found in feathers. We show not only that the colour of feathers of greater flamingos Phoenicopterus roseus became more colourful due to the application of carotenoids from uropygial secretions over the plumage but also that the feathers became more colourful with the quantity of pigments applied over them, thus providing evidence of cosmetic coloration. Flamingos used uropygial secretions as cosmetic much more frequently during periods when they were displaying in groups than during the rest of the year, suggesting that the primary function of cosmetic coloration is mate choice. Individuals with more colourful plumage initiated nesting earlier. There was a correlation between plumage coloration before and after removal of uropygial secretions from feathers’ surfaces, suggesting that the use of these pigmented secretions may function as a signal amplifier by increasing the perceptibility of plumage colour, and hence of individual quality. As the cosmetic coloration strengthens signal intensity by reinforcing base-plumage colour, its use may help to the understanding of selection for signal efficacy by making interindividual differences more apparent.     

The importance of white on black: unmelanized plumage proportion predicts display complexity in birds

Animal ritualized displays have been classically viewed as behavioral characters that decrease signal ambiguity or that facilitate the evaluation of costly exhibitions. It has been shown that their prevalence and level of complexity across species can reflect phylogenetic relationships between them, but the adaptive function of these behavioral traits is poorly known. Here, I hypothesize that, given that the efficacy of visual displays basically depends on conspicuousness and level of performance, species with low levels of conspicuousness may be forced to perform more complex varieties of a given display to get the same signal efficiency than other more conspicuous species. Thus, the evolution of display complexity, considered as the level of exaggeration of ritualized movements, may be explained as an adaptive trait and not only by phylogenetic inertia. I illustrate and test this hypothesis with the case of black-and-white plumage patches of pelecaniform birds. As predicted, there was a negative correlation between level of complexity and species conspicuousness (proportion of unmelanized plumage) for two different social displays. This indicates that classical ideas on the adaptiveness of ritualized displays should be considered to understand the present variation in signal form across species, which sheds light on the evolution of multiple signals.     

Testosterone and melanin-based black plumage coloration: a comparative study

Despite the functional significance of melanin-based plumage coloration in social and sexual signaling, the mechanisms controlling its information content are poorly understood. The T-regulation hypothesis proposes that melanin ornaments signal competitive abilities via the effects of testosterone (T) mediating both melanization and sexual/aggressive behaviors. Using the phylogenetic comparative approach, we tested whether frontal black melanization is associated with elevated T around the time of breeding plumage development across all bird species with available T-data. We found a context-dependent relationship between melanization and T, varying with the type of ornamentation (patchy or full-black) and with the presumed taxonomic distribution of the hormonal control of plumage dichromatism. Within two taxa in which male plumage development is assumed androgen-dependent (Charadriiformes, Corvida), evolutionary increases in male melanization, and melanin dichromatism correlated with increases in T in most analyses but not within the basal lineage (ratites, Galloanseriformes) with androgen-independent male plumage. Among Passeroidea with presumably genetically or luteinizing-hormone-based male plumage, melanization and its dichromatism correlated with T only in species with <100% frontal melanization. These results were robust as we controlled for several confounding variables such as mating and parental behaviors. This study is the first to test and support the T-regulation hypothesis interspecifically, suggesting that among-species differences in melanization may have evolved in response to differences in circulating T in certain avian taxa. Our results imply that the extent of black ornamentation may serve as an honest indicator of male competitiveness in those species that evolved an appropriate hormonal basis (T dependence) for color production. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Mating systems,parasites and plumage dimorphism in waterfowl

Summary Despite a tendency for males of polygynous bird species to show bright or elaborate plumage, comparative analyses have failed to show any consistent relationship between male brightness or plumage dimorphism and the form of breeding systems. Here we argue that this may be partly because the opportunity for sexual selection varies between species showing serial or seasonal monogamy and life-long monogamy. In waterfowl, both the brightness of male plumage and sexual dimorphism in colouration vary between these categories of monogamy. Other ecological factors related to male brightness or plumage dimorphism include male assistance in protecting young, latitude and an index of parasite sharing. The adaptive significance of these trends is discussed. Offprint requests to: D.K. Scott     

Racketed tail of the male and female turquoise-browed motmot: male but not female tail length correlates with pairing success,performance, and reproductive success

Both males and females of many avian species maintain elaborate plumage traits, and elaborate monomorphic plumage may convey adaptive benefits to one or both sexes as inter- or intraspecific signals. Both sexes of the turquoise-browed motmot (Eumomota superciliosa) are elaborately plumed with long racket-tipped tail. I investigated whether the racketed tail functions as a sexually selected signal in one or both sexes by testing the predictions that males and/or females with the largest tails have: (1) greater pairing success, (2) greater reproductive performance (clutch-initiation date, clutch size, and hatching success), and (3) greater reproductive success. Yearling males with longer denuded rachises (wires) on the central tail feathers had greater pairing success. In addition, adult males with longer wires paired with females who laid larger clutches, had greater hatching success independent of clutch size, and fledged more young. There was no relationship between female tail plumage and pairing success, reproductive performance, or fledgling success. These results are consistent with the hypothesis that male tail plumage functions as a mate choice or status signal, but that the tail of the female does not function in a sexually selected context. I discuss alternative hypotheses for the evolutionary maintenance of the elaborate female tail plumage.     

Plumage characteristics, reproductive investment and assortative mating in tree swallows Tachycineta bicolor

Elaborate ornamental plumage has been associated with various measures of individual quality in many species of birds. Male plumage characteristics, which have been relatively well studied, have been shown to reflect past reproductive investment, as well as the potential for reproductive investment in the current breeding attempt. In contrast, the signalling functions of female traits remain largely unexplored. In this study, we investigated the relationship between plumage attributes of breeding adult tree swallows and past reproductive investment, current reproductive investment and social mate pairing strategy. Both males and older females possess metallic green to metallic blue iridescent plumage on their dorsal surface, making this a suitable species for this type of investigation. We did not find any effects of past reproductive investment and success on the plumage attributes of returning breeders. In contrast, female plumage hue covaried with fledging success, and female plumage brightness was positively associated with mean clutch egg mass. In addition, we found that social pairs mated assortatively with respect to plumage brightness. We argue that since plumage characteristics vary with age in both male and female tree swallows, plumage attributes in this species are indicative of breeding experience and may be honest signals of quality. Positive assortative pairing could be the result of mutual mate choice or intra-sexual competition for nest sites by both males and females.     

Selectivity in Mammalian Extinction Risk and Threat Types: a New Measure of Phylogenetic Signal Strength in Binary Traits

Abstract: The strength of phylogenetic signal in extinction risk can give insight into the mechanisms behind species’ declines. Nevertheless, no existing measure of phylogenetic pattern in a binary trait, such as extinction‐risk status, measures signal strength in a way that can be compared among data sets. We developed a new measure for phylogenetic signal of binary traits, D, which simulations show gives robust results with data sets of more than 50 species, even when the proportion of threatened species is low. We applied D to the red‐list status of British birds and the world's mammals and found that the threat status for both groups exhibited moderately strong phylogenetic clumping. We also tested the hypothesis that the phylogenetic pattern of species threatened by harvesting will be more strongly clumped than for those species threatened by either habitat loss or invasive species because the life‐history traits mediating the effects of harvesting show strong evolutionary pattern. For mammals, our results supported our hypothesis; there was significant but weaker phylogenetic signal in the risk caused by the other two drivers (habitat loss and invasive species). We conclude that D is likely to be a useful measure of the strength of phylogenetic pattern in many binary traits.     

Males are sensitive — sex-dependent effect of rearing conditions on nestling growth

The sex-dependent effect of environmental conditions on nestlings has been extensively studied in size dimorphic birds. Whether males or females are more sensitive to poor conditions is not yet clear; however, the degree of sexual size-dimorphism, brood size and their interactions seem to influence the pattern. Much less is known about sex-dependent environmental sensitivity in size-monomorphic species, even though it may result in biased sex allocation. We altered the rearing conditions by brood size manipulation in the size-monomorphic collared flycatcher and then examined the sex-specific development of the nestlings. In all analyses, we controlled for the effect of paternity, because one may expect extra-pair young to be of better genetic quality and perform better at least under poor conditions. However, this was not the case, because we did not find any difference in growth rate or fledging size between extra- and within-pair young. We found that male nestlings had the potential for faster growth under favourable conditions, but suffered more under poor conditions. We found no sex × environment interaction for fledging size probably because the growth curves level off before fledging, and the disadvantaged nestlings can catch up with their siblings. The larger sensitivity of males does not explain the previously found seasonal shift in brood sex ratios and contradicts previous findings in another size-monomorphic species where females were more sensitive. This suggests that even in size-monomorphic species, no general rule exists, which determines the more sensitive sex.     

Female choice for parasite-free male satin bowerbirds and the evolution of bright male plumage

Summary Hamilton and Zuk proposed that bright male plumage may have evolved in males of polygynous species as a result of female preferences for males that are able to demonstrate their resistance to disease. They predicted an inverse correlation between female mating preferences and the level of parasitic infection of males. We found such a correlation between the level of infection by a common ectoparasite (Myrsidea ptilonorhynchi: Menoponidae) and mating success of male satin bowerbirds (Ptilonorhynchus violaceus). In addition, we tested and were able to confirm three other predictions derived from their model: that (1) older males had fewer parasites than their younger counterparts, (2) levels of individual parasitic infection are highly correlated between years, and (3) that individuals resighted in successive years are less parasitized than those that fail to return. These results support the bright male model, but they are also consistent with two other hypotheses that may explain plumage dimorphism based on the level of parasitic infection. The correlated infection model suggests that females choose males with few ectoparasites because of a correlation between the level of ectoparasitic infection and heritable resistance to internal infections. In the parasite avoidance model, females favor parasitefree males because it lowers their own prospects for parasitic infection. Our data did not show the predicted relationship between parasite numbers with plumage quality that is needed to support the bright male hypothesis, nor did it show the inverse correlation between male condition and parasite numbers that is predicted by both the bright male and correlated infection hypotheses. Our results are most consistent with the parasite avoidance hypothesis.     

Intra-clonal variation in the red seaweed Gracilaria chilensis

The phenotypic plasticity often found in seaweed populations has been explained only from the perspective of inter-population or inter-individual differences. However, many seaweeds grow and propagate by fragmentation of genetically identical units, each with the capacity to function on its own. If significant differences in performance exist among these supposedly identical units, such differences should be expressed upon the release and growth of these units. In this study we document two such types of variation in the red seaweed Gracilaria chilensis. Populations of sporelings, each grown under similar culture conditions and derived from carpospores shed by the same cystocarp exhibit significant differences in growth. In this species, each cystocarp develops from a simple gametic fusion, and cystocarp fusions occur too infrequently to account for the growth differences observed among recruits. In adult thalli, branches (ramets) derived from the same thallus (genet) and grown under similar conditions exhibit significant variation in growth rates and morphology. These findings have several implications. They suggest that carpospore production is not only an example of zygote amplification but that it also could increase variability among mitotically replicated units. Intra-clonal variability followed by fragmentation and re-attachment may increase intra-population variation which, in species of Gracilaria, is often larger than inter-population variation. In addition, the existence of intra-clonal variability suggests that strain selection in commercially important species may require a more continuous screening of highquality strains because of frequent genotypic or phenotypic changes in the various cultivars.