Male Ornaments Research Articles

Testing the role of same-sex sexual behaviour in the evolution of alternative male reproductive phenotypes

Male same-sex sexual behaviour (SSB), where males court or attempt to mate with other males, is common among animal taxa. Recent studies have examined its fitness costs and benefits in attempts to understand its evolutionary maintenance, but the evolutionary consequences of SSB are less commonly considered. One potential impact of SSB might be to facilitate the evolution of traits associated with less sexually dimorphic males, such as alternative reproductive tactics, by diverting costly aggression from other males. To test this, we capitalized on the recent rapid spread of a silent male morph of the field cricket, Teleogryllus oceanicus, which is unable to produce characteristic male acoustic signals, benefits from satellite mating behaviour and has feminized appearance and cuticular hydrocarbon profiles. We tested the prediction that interactions involving these nonsignalling, less sexually dimorphic male morphs would show heightened rates of SSB, which could reduce the strength of male–male competition and permit greater access to females. We found no evidence that SSB was more common in trials involving silent males. Instead, SSB was predicted by courtship of females presented during a pretrial treatment. Our results provide evidence supporting the view that SSB represents a spillover of sexually selected courtship behaviour in a nonadaptive context, but do not support a strong role for SSB in the evolution of less ornamented males in this system.

Extra-pair paternity patterns in European barn swallows Hirundo rustica are best explained by male and female age rather than male ornamentation

Adaptive explanations for the evolution of extra-pair paternity (EPP) in birds often assume cuckolding males to be better-ornamented than cuckolded males. Several studies have confirmed that either male sexual ornamentation is associated with EPP or that phenotypes of cuckolded and cuckolding males differ. Expression of male ornamentation may change with age; however, a recent meta-analysis has identified age itself as an important factor that differed in cuckolding and cuckolded males. The age of social female partner may also affect EPP, though this has received little attention. Here, by using detailed data on age of individual barn swallows (Hirundo rustica rustica), we identified age as the major predictor of male and female promiscuity. Our results revealed that, whereas a male’s ability to obtain an extra-pair mate increased linearly with age, the only predictor of the probability of a male being cuckolded was the age of his social partner, with older females engaging more frequently in EPP. In contrast, male ornamentation was not significantly related to EPP pattern. Tarsus length was the sole significant phenotypic trait in comparison between cuckolding and cuckolded males. Our data provide little support for the hypothesis that extra-pair mate choice in our barn swallow population was ornament driven. This may indicate either a non-adaptive scenario for EPP, for example with older males better able to coerce females into copulation, or EPP mating based on other than absolute mate-choice criteria associated with the expression of male ornamentation. SIGNIFICANCE STATEMENT: We analysed patterns of extra-pair paternity (EPP) in barn swallows. Derived from observation of 160 nests, our results appear to differ from the findings of some previous studies that identified ornamental traits as being associated with extra-pair and within-pair paternity (WPP) in this iconic model taxon of sexual selection. In particular, tail streamer length had no detectable association with WPP or a male’s ability to obtain an extra-pair partner. Pairwise comparisons of cuckolded and cuckolding males, involving 76 mixed paternity nests, also supported the hypothesis that male ornamentation does not play a role in determining EPP patterns in the focal barn swallow population. When statistically controlled for clutch initiation date, the probability of obtaining an extra-pair partner only increased with increasing age of males, while WPP was only associated with the age of their female social partners. Our data provide little support for the hypothesis that extra-pair mate choice in our barn swallow population is ornament driven and indicated that age, rather than ornaments, would be a better predictor of paternity.

Limits to environmental masking of genetic quality in sexual signals.

There is considerable debate over the value of male sexual ornaments as signals of genetic quality. Studies alternately report that environmental variation enhances or diminishes the genetic signal, or leads to crossover where genotypes perform well in one environment but poorly in another. A unified understanding is lacking. We conduct a novel experimental test examining the dual effects of distinct categories of genetic (inbred vs. crossed parental lines) and environmental quality (low, through high to extreme larval food stress) on a condition-dependent male ornament. We find that differences in genetic quality signalled by the ornament (male eyespan in Diasemopsis meigenii stalk-eyed flies) become visible and are amplified under high stress but are overwhelmed in extreme-stress environments. Variance among independent genetic lines increases with environmental stress in both genetic quality classes, but at a slower rate in highquality outcrossed flies. Individual genetic lines generally maintain their ranks across environments, except among highquality lines under low environmental stress, where low genetic variance among lines precludes differentiation between ranks. Our results provide a conceptual advance, demonstrating a unified pattern for how genetic and environmental quality interact. They show when environmental conditions lead to the amplification of differences in signals of genetic quality and thereby enhance the potential indirect genetic benefits gained by female mate choice.

Plasticity and divergence in ultraviolet reflecting structures on Dogface butterfly wings

The vast diversity of animal coloration is generated through a combination of pigment and structural colors. These colors can greatly influence the fitness and life history of an organism. Butterflies and their wing colors are an excellent model to study how these colors can impact the development and success of an organism. In this study, we explore species differences in structurally-based ultraviolet coloration in the Zerene butterfly. We show clear species differences in ultraviolet (UV) pattern and reflectance spectra. By varying larval diet, we show evidence for developmental plasticity in the structure and organization of UV reflecting scales in Zerene cesonia. We further show that feeding the larval host plant of Zerene eurydice to Z. cesonia does not result in greater similarity in scale structure or UV coloration to the sister species. These results not only demonstrate a connection between plasticity in a male ornamentation, UV wing pattern, and larval resource acquisition, but also identify candidate structural and organizational changes in wing scales responsible for the trait variation.

The role of male coloration and ornamentation in potential alternative mating strategies of the dimorphic jumping spider, Maevia inclemens

Polymorphism can arise across taxa due to various selection pressures and potentially lead to alternative mating or antipredator strategies. For male jumping spiders, sexual selection and predation risk are often intertwined when courting cannibalistic females and may be a driving factor in the polymorphism of the jumping spider, Maevia inclemens. The dimorphic males of M. inclemens differ dramatically in their complex courtship behavior and display traits that may function as alternative mating strategies to reduce female aggression and maximize mating success. We hypothesized that males of the “tufted” morph honestly communicate condition or body size to females with three conspicuous tufts of setae on their head and males of the “striped” morph reduce female aggression with coloration commonly found in aposematic animals (here, yellow-orange pedipalps and striped legs). We examined correlations between tuft length and symmetry and metrics of body size and condition in field-collected spiders and conducted prey color choice tests (with live color-manipulated prey) to determine if yellow-orange and striped prey are avoided. Tuft length was variable and correlated with male size (but not condition). All prey color types were attacked at equal rates, but spiders oriented to striped prey more often, suggesting that male stripes may attract female attention without increasing predation. This study provides insight into the potential functions of the different courtship and visual displays of M. inclemens males. Using jumping spiders to study polymorphism can provide new insight into how multiple morphs can evolve, as males use mating strategies not only to impress females but also avoid getting eaten by their potential mates.Significance statementUnderstanding the selection pressures that shape and maintain polymorphisms in natural populations is an active area of inquiry, yet most of what we know comes from a small handful of well-studied vertebrate taxa. The unique biology of jumping spiders and the fact that males have to avoid cannibalism during courtship, makes them well-suited to provide novel insights into the functions and maintenance of dimorphic male traits. The unusual male dimorphism of M. inclemens captured the attention of naturalists as early as the 1800s, yet we still do not understand why the two male morphs differ so dramatically in morphology and courtship behavior. Here, we propose and test a novel hypothesis that provides insight into the long-studied but still largely unexplained phenomenon of male dimorphism in this species.

Sexually selected sexual selection: Can evolutionary retribution explain female ornamental colour?

By preferring mates with increasingly costly ornaments or courtship displays, females cause an escalation of male reproductive costs. Such increased costs should promote male selectivity based on fecundity-linked female attributes, leading to female ornamentation in species with traditional sex roles. Consequently, female ornamentation should evolve more frequently in taxa where male reproduction is costly than in comparable taxa where it is cheaper. We assessed the prevalence of female ornamental colouration in two clades of viviparous cyprinodontid fish: the Goodeinae, where stringent female choice imposes male mating costs, and the Poeciliinae, whose males can circumvent female mate choice. We found that although in the Poeciliinae female ornamental colour is a correlated, but paler version of male coloration, females of the Goodeinae often display vivid ornamental colours that are distinct from those of males. Thus, male and female ornaments are not (phylo)genetically correlated in the Goodeinae. Furthermore, phylogenetic signal on male and female colour is clearly detectable in the Poeciliinae, but absent in the Goodeinae, suggesting that ornamental colour of males and females in the latter may be the consequence of selection. Given that enforceable female choice has promoted male ornaments, we propose that evolutionary retribution has promoted distinct female ornaments in the Goodeinae.

An annual cycle of gene regulation in the red-legged salamander mental gland: from hypertrophy to expression of rapidly evolving pheromones

BackgroundCell differentiation is mediated by synchronized waves of coordinated expression for hundreds to thousands of genes, and must be regulated to produce complex tissues and phenotypes. For many animal species, sexual selection has driven the development of elaborate male ornaments, requiring sex-specific differentiation pathways. One such male ornament is the pheromone-producing mental gland of the red-legged salamander (Plethodon shermani). Mental gland development follows an annual cycle of extreme hypertrophy, production of pheromones for the ~ 2 month mating season, and then complete resorption before repeating the process in the following year. At the peak of the mating season, the transcriptional and translational machinery of the mental gland are almost exclusively redirected to the synthesis of rapidly evolving pheromones. Of these pheromones, Plethodontid Modulating Factor (PMF) has experienced an unusual history: following gene duplication, the protein coding sequence diversified from positive sexual selection while the untranslated regions have been conserved by purifying selection. The molecular underpinnings that bridge the processes of gland hypertrophy, pheromone synthesis, and conservation of the untranslated regions remain to be determined.ResultsUsing Illumina sequencing, we prepared a de novo transcriptome of the mental gland at six stages of development. Differential expression analysis and immunohistochemistry revealed that the mental gland initially adopts a highly proliferative, almost tumor-like phenotype, followed by a rapid increase in pheromone mRNA and protein. One likely player in this transition is Cold Inducible RNA Binding Protein (CIRBP), which selectively and cooperatively binds the highly conserved PMF 3′ UTR. CIRBP, along with other proteins associated with stress response, have seemingly been co-opted to aid in mental gland development by helping to regulate pheromone synthesis.ConclusionsThe P. shermani mental gland utilizes a complex system of transcriptional and post-transcriptional gene regulation to facilitate its hypertrophication and pheromone synthesis. The data support the evolutionary interplay of coding and noncoding segments in rapid gene evolution, and necessitate the study of co-evolution between pheromone gene products and their transcriptional/translational regulators. Additionally, the mental gland could be a powerful emerging model of regulated tissue proliferation and subsequent resorption within the dermis and share molecular links to skin cancer biology.

Release from intralocus sexual conflict? Evolved loss of a male sexual trait demasculinizes female gene expression.

The loss of sexual ornaments is observed across taxa, and pleiotropic effects of such losses provide an opportunity to gain insight into underlying dynamics of sex-biased gene expression and intralocus sexual conflict (IASC). We investigated this in a Hawaiian field cricket, Teleogryllus oceanicus, in which an X-linked genotype ( flatwing) feminizes males' wings and eliminates their ability to produce sexually selected songs. We profiled adult gene expression across somatic and reproductive tissues of both sexes. Despite the feminizing effect of flatwing on male wings, we found no evidence of feminized gene expression in males. Instead, female transcriptomes were more strongly affected by flatwing than males', and exhibited demasculinized gene expression. These findings are consistent with a relaxation of IASC constraining female gene expression through loss of a male sexual ornament. In a follow-up experiment, we found reduced testes mass in flatwing males, whereas female carriers showed no reduction in egg production. By contrast, female carriers exhibited greater measures of body condition. Our results suggest sex-limited phenotypic expression offers only partial resolution to IASC, owing to pleiotropic effects of the loci involved. Benefits conferred by release from intralocus conflict could help explain widespread loss of sexual ornaments across taxa.

Sexual ornaments but not weapons trade off against testes size in primates.

Males must partition their limited reproductive investments between traits that promote access to females (sexual ornaments and weapons) and traits that enhance fertilization success, such as testes and ejaculates. Recent studies show that if the most weaponized males can monopolize access to females through contest competition, thereby reducing the risk of sperm competition, they tend to invest less in sperm production. However, how males invest in sexual ornaments relative to sperm production remains less clear. If male ornaments serve as badges of status, with high-ranking males attaining near-exclusive access to females, similar to monopolizing females through combat, their expression should also covary negatively with investment in post-mating traits. In a comparative study across primates, which exhibit considerable diversification in sexual ornamentation, male weaponry and testes size, we found relative testes size to decrease with sexual ornaments but increase with canine size. These contrasting evolutionary trajectories might be driven by differential selection, functional constraints or temporal patterns of metabolic investment between the different types of sexual traits. Importantly, however, our results indicate that the theory of relative investments between weapons and testes in the context of monopolizing females can extend to male ornaments.

Dangerous love? Predation risk does not affect female mate choice in blue‐black grassquits

AbstractPredation risk may be an important factor affecting female mate choice. Hypothetically, females could choose extravagantly ornamented males that survive in high predation risk environments. However, this decision could be different if choosing a conspicuous male under high predation risk is costly for females or results in reduced offspring survival. In such contexts, females could become indifferent to male quality or prefer inconspicuous males. We tested this idea using captive blue‐black grassquits (Volatinia jacarina, Linnaeus, 1766), a species in which males perform conspicuous leap displays coupled with songs during the breeding season, which presumably subjects females and offspring to higher predation risk. Females were placed in an arena with speakers on opposite sides emitting male courtship songs. One speaker emitted songs at a high rate (proxy for a conspicuous male) while the other speaker broadcast songs at a low rate (proxy for a less conspicuous male). While the female evaluated the two male songs, a third speaker emitted vocalizations characterizing three levels of risk: adult predator, nest predator, and no‐risk control. Females showed no preference for either male stimuli across the predation risk treatments. This lack of preference relative to frequency of male vocal displays suggests that leap‐song frequency is not used by females during mate choice. We suggest that in addition to its role in courtship, male grassquit displays also signal status to other males when competing for territories. Thus, we propose that predation risk does not directly influence blue‐black grassquit intersexual selection and that females in this species may exercise indirect mate choice, choosing social mates based on male ability to establish and defend a territory, and relying secondarily upon other aspects of male display attributes, such as its visual components.

Post-natal corticosterone exposure affects ornaments in adult male house sparrows (Passer domesticus)

In vertebrates, the ontogeny of several crucial organismal systems is known to occur early in life. Developmental conditions can ultimately have important consequences on adult fitness by affecting individual phenotype. These developmental effects are thought to be primarily mediated by endocrine systems, and especially by glucocorticoids. In this study, we tested how post-natal exposure to corticosterone (the primary avian glucocorticoid) may subsequently affect the expression of ornaments in adult male house sparrows (Passer domesticus). Specifically, we investigated the long-term consequences of this manipulation on the size and color of several visual signals: badge, wing bar, tarsus and beak. Post-natal corticosterone exposure had a strong negative impact on the size, but not the color, of some male ornaments (badge and wing bar surface area). Because wing bar and badge surface area are used as sexual and/or hierarchical signals in house sparrow, we showed that early life stress can affect some aspect of attractiveness and social status in this species with potentially important fitness consequences (e.g. sexual selection and reproductive performance).Future studies need now to explore the costs and benefits of this developmental plasticity for individuals (i.e. fitness).

Female choice for sick males over healthy males: Consequences for offspring

AbstractSexual selection theory indicates that ornament expression in males is in close relation to their condition. This “honesty” relationship serves as the basis for female choice: Females would mate with healthy males over sick males after assessing male ornament signal expression and derive benefits for their progeny. Here, we investigated female mate choice for infected and non‐infected males, male survival after infection (to corroborate the negative effect of infection), and fitness consequences of female preferences using Tenebrio molitor beetles. Male infection was produced having two types of challenges as follows: males infected with entomopathogenic fungi and males infected with nylon implants. Similar to previous studies, we corroborated that females preferred fungus‐infected males over positive control, negative control, and nylon‐challenged males. Survival was the lowest for fungus‐treated males followed by nylon‐treated and control males. Females mated with fungus‐treated males laid fewer and smaller eggs, and the laid eggs had less lipid content with a reduced eclosion success compared to females mated with non‐challenged males. Our interpretation is that fungus‐treated males invested their energetic resources to increase their attractiveness at the risk of survival, in a terminal investment fashion. Females, however, would have corrected their choice by investing less in their offspring.

Facial masculinity does not appear to be a condition-dependent male ornament and does not reflect MHC heterozygosity in humans

Recent studies have called into question the idea that facial masculinity is a condition-dependent male ornament that indicates immunocompetence in humans. We add to this growing body of research by calculating an objective measure of facial masculinity/femininity using 3D images in a large sample (n = 1,233) of people of European ancestry. We show that facial masculinity is positively correlated with adult height in both males and females. However, facial masculinity scales with growth similarly in males and females, suggesting that facial masculinity is not exclusively a male ornament, as male ornaments are typically more sensitive to growth in males compared with females. Additionally, we measured immunocompetence via heterozygosity at the major histocompatibility complex (MHC), a widely-used genetic marker of immunity. We show that, while height is positively correlated with MHC heterozygosity, facial masculinity is not. Thus, facial masculinity does not reflect immunocompetence measured by MHC heterozygosity in humans. Overall, we find no support for the idea that facial masculinity is a condition-dependent male ornament that has evolved to indicate immunocompetence.

Influence of haemosporidian infection status on structural and carotenoid‐based colouration in the blue titCyanistes caeruleus

Hypotheses postulating parasite‐mediated mate choice intrinsically assume that parasitic infections deteriorate the quality of male ornamentation. Although this assumption has often been studied in the context of carotenoid‐based colouration, only few studies investigated this with reference to structural feather colouration, which in many species plays a vital role in sexual selection. Here, using a three‐years dataset from a wild blue titCyanistes caeruleuspopulation, we examined the relationship between the haemosporidian infection status and the structural as well as the carotenoid‐based colouration of adult birds. Furthermore, we investigated potential differences in the impact on feather colouration between two examined parasite genera:PlasmodiumandHaemoproteus. For analysis of the feathers spectral reflectance we used both the tristimulus model and the avian tetrahedral colour space model, incorporating visual phenotype of the blue tit. Contrary to expectations we found that infected birds showed higher brightness, in both the structural and the carotenoid‐based colours. We also found no differences in the feather colouration between birds infected withPlasmodiumandHaemoproteus. Observed pattern might be best explained by the parasite‐mediated selection hypothesis, as only individuals of superior quality should be able to survive the acute stage of infection and therefore they could produce more elaborate ornamental colouration.

Carotenoid-based skin ornaments reflect foraging propensity in a seabird,Sula leucogaster

Carotenoid-based ornaments are common signalling features in animals. It has long been proposed that such ornaments communicate information about foraging abilities to potential mates. However, evidence linking foraging with ornamentation is largely missing from unmanipulated, free-ranging populations. To investigate this relationship, we studied a coastal population of brown booby (Sula leucogaster brewsteri), a seabird with a carotenoid-based gular skin ornament. δ 13C values from both feathers and blood plasma were negatively correlated with male gular colour, indicating birds that consumed more pelagic prey in offshore locations had more ornamented skin than those that fed on nearshore, benthic prey. This relationship was supported by our GPS tracking results, which revealed longer, more offshore foraging trips among highly ornamented males. Our data show that brown booby ornaments are honest indicators of foraging propensity; a link consistent with the rarity hypothesis and potentially driven by the concentration of carotenoids found in phytoplankton versus benthic algae. Carotenoid-based ornaments may reflect foraging tendencies in animals such as coastal predators that use food webs with distinct carotenoid profiles.

Performance, behavior and offspring morphology may offset reproductive costs of male‐typical ornamentation for female lizards

Females of sexually dimorphic species that bear male-typical secondary sexual traits, such as male ornaments, may incur costs. However, in some species, ornamented females can be found in high numbers, suggesting that they may experience benefits that offset these costs. Some female fence lizards (Sceloporus undulatus) possess blue badges that are similar to, but less dramatic than, those displayed by males during courtship and dominance contests. Females bearing these badges have reduced reproductive investment and are less desirable as mates, costs that are difficult to reconcile with the high prevalence of this trait (up to 95% of females) in some populations. To test the hypothesis that there are potential benefits associated with male-typical ornamentation in females, we compared fitness-relevant performance, behavior and morphology of ornamented and unornamented female fence lizards and their offspring. Ornamented females achieved faster sprinting speeds and had offspring that were more likely to flee from predatory attacks and had longer bodies; these traits are associated with higher survival in this species. These results suggest the presence of two female morphotypes that may be differentially favored under contrasting environmental conditions. Unornamented females appear to have a reproductive advantage that should be favored under low predation environments. By contrast, ornamented females have traits that would confer a fitness advantage in high-predation environments. This work suggests a potential mechanism maintaining male-typical female ornamentation in nature.

Female Japanese quail visually differentiate testosterone-dependent male attractiveness for mating preferences

Biased mating due to female preferences towards certain traits in males is a major mechanism driving sexual selection, and may constitute an important evolutionary force in organisms with sexual reproduction. In birds, although the role of male ornamentation, plumage coloration, genetic dissimilarity, and body size have on mate selection by females have been examined extensively, few studies have clarified exactly how these characteristics affect female mate preferences. Here, we show that testosterone (T)-dependent male attractiveness enhances female preference for males of a polygamous species, the Japanese quail. A significant positive correlation between female mating preference and circulating T in the male was observed. The cheek feathers of attractive males contained higher levels of melanin and were more brightly colored. The ability of females to distinguish attractive males from other males was negated when the light source was covered with a sharp cut filter (cutoff; < 640 nm). When females were maintained under short-day conditions, the expression of retinal red-sensitive opsin decreased dramatically and they became insensitive to male attractiveness. Our results showed that female preference in quail is strongly stimulated by male feather coloration in a T-dependent manner and that female birds develop a keen sense for this coloration due to upregulation of retinal red-sensitive opsin under breeding conditions.

Sexual Selection Mechanisms for Male Plumage Ornaments in Japanese Barn Swallows

The Barn Swallow Hirundo rustica is a model species for studying sexual selection. After its long tail was shown to be the target of female mate choice, its plumage characteristics, including the red throat patch and the size of white tail spots in addition to tail length, have attracted the attention of many researchers. Although there have been several reviews of sexual selection in this species, these reviews have focused mostly on general patterns of sexual selection within and among subspecies, rather than on the functional differences between plumage characteristics in sexual selection. Here, I review mechanisms of sexual selection for male plumage ornaments in Japanese Barn Swallows H. r. gutturalis. In the last decade, studies of sexual selection in Japanese Barn Swallows have focussed on tail length, the size of the white tail spots, and the size and coloration of red throat patches. Each ornamental trait appears to be involved in different sexual selection mechanisms: the size of the white tail spots is important for several mechanisms of intersexual selection, including the Darwin-Fisher mechanism and differential allocation, whereas throat coloration may be intra-sexually selected, at least in part. The size of the throat patch, in contrast, is the target of an intersexual selection mechanism, differential access, indicating a partially independent evolution of the two components of the red throat patch. Multiple mechanisms of sexual selection would explain the multiple ornaments and their geographical variation, and even the probability of speciation and extinction.

Age, condition and dominance‐related sexual ornament size before and during the breeding season in the black grouse Lyrurus tetrix

Male ornaments function as honest cues of male quality in many species and are subject to intra‐ and intersexual selection. These ornaments are generally studied during peak expression, however their size outside the breeding season may determine ultimate ornament size and costliness, and as such reproductive success. We investigated whether male black grouse Lyrurus tetrix eye comb size was related to age, condition and measures of male dominance before and during the breeding season. Total combined eye comb size began to increase ~70 d before the start of the breeding season. Adult males (aged ≥ 2 yr old) had consistently larger eye combs than younger males (1 yr old) both before and during the breeding season. Heavier and more dominant adult males (attending the lek more frequently and successfully reproducing) had larger eye combs. For younger males, those that were heavier had larger eye combs. Additionally, males that spent more time on the lek showed increased eye comb size as the breeding season approached. Overall we find that ornament size is positively related to dominance and condition before and during the breeding season. Since dominance is accrued through year‐round interactions in many species, the ability to maintain larger signals over prolonged periods, including outside of the breeding season, is likely to be beneficial for adults. For younger males, it is likely that they cannot sustain or are constrained from producing larger eye combs over long periods of time. They therefore prioritise growth of their ornaments later, and according to the amount of time they spend on the lek.

Plumage coloration and social context influence male investment in song.

Animals use multiple signals to attract mates, including elaborate song, brightly coloured ornaments and physical displays. Female birds often prefer both elaborate male song and intense carotenoid-based plumage coloration. This could lead less visually ornamented males to increase song production to maximize their attractiveness to females. We tested this possibility in the highly social and non-territorial house finch (Haemorhous mexicanus), in which females discriminate among males based on both song and on the intensity of red carotenoid-based plumage coloration. We manipulated male plumage coloration through carotenoid supplementation during moult, so that males were either red or yellow. Males were then housed under three social environments: (i) all red birds, (ii) all yellow birds or (iii) a mixture of red/yellow birds. We recorded song after presentation of a female. Red males produced more song than yellow males. But when yellow males were housed with red conspecifics, they produced more song relative to yellow males housed with equally unattractive yellow males. This study provides novel evidence that a male's plumage coloration and the plumage colour of his social competitors influence investment in song.