Ornament Elaboration Research Articles

Which plumage patches provide information about condition and success in a female fairy-wren?

Abstract Recent evidence suggests that female ornaments can commonly act as signals. However, how signaling functions might be affected by the tendency for reduced ornament elaboration in relation to males is less well-understood. We address this in mutually ornamented purple-crowned fairy-wrens. We investigated putatively ornamental (tail, ear coverts, crown) and non-ornamental (throat, back) plumage patches in females and compared our findings to previous studies in males. Both sexes have brown backs, buff-white throats, and turquoise-blue tails (bluer in males), while ear coverts are rufous in females and black in males. Both sexes also have a seasonal crown (slate-gray in females, black-and-purple in males). Dominant (breeder) females expressed more complete and grayer (more ornamented) crowns, although variation in coloration should not be discriminable by individuals. Unexpectedly, subordinates showed more colorful (saturated) rufous ear coverts, which should be discriminable. Condition-dependence was only evident for crown completeness (% slate-gray cover). Females with more reddish-brown backs were more reproductively successful. Variation in plumage characteristics did not explain differential allocation by mates or chances of gaining dominance. Our outcomes were not entirely consistent with findings in males. The most notable disparity was for the crown, a signal used in male-male competition that in females seems to be expressed as an incomplete version of the male crown that is not associated with fitness benefits. Our study shows that in a species, multiple traits can vary in their information content and that female ornaments can sometimes be less informative than in males, even those that are produced seasonally.

The stabilising impact of natural selection on the allometry of sexual ornaments: Fish that escape locomotor constraints exhibit extravagant ornamentation

Abstract Positive allometry has been considered a hallmark of sexual selection whereby larger males of superior condition develop disproportionately larger ornaments for their body size compared to smaller males of poorer condition. Yet many structures known to be sexually selected often exhibit other allometric patterns. This has led to controversy over the utility of allometry in adequately capturing the signature of sexual selection, particularly if static (within population) and evolutionary (across species) allometries are functionally constrained by stabilising natural selection. To investigate this, we evaluated the allometries of ornamental head crests and dorsal fins across multiple species of blenny fish. In particular, we compared species that occupied an aquatic environment—where swimming performance was expected to have constrained ornament size—with species that have transitioned onto land where such biomechanical constraints on ornament size have been removed. Static allometries of both head crest and dorsal fin ornaments were found to be positive in males, but less so in females, across all species examined. This was consistent with the allometric theory of sexual selection that predicts positive allometry specifically in male ornamentation. Nevertheless, male allometric exponents were constrained in aquatic species whereas males of terrestrial species were free to exaggerate the size of their ornaments. Natural selection therefore appears to limit the evolution of ornament size in aquatics, and probably because of the biomechanical constraints associated with swimming. These differences in within‐population static allometry between aquatic and terrestrial species in turn manifested in a greater across‐species evolutionary allometric elevation, but not exponent, for terrestrial species relative to aquatic species. These findings indicate that the study of ornament allometries can provide useful insights into the role of sexual selection on ornament elaboration and also help reveal the presence of opposing natural selection that might result in alternative allometric patterns. The relationship between static and evolutionary allometries remains complex, and our results caution against the interpretation of evolutionary allometry in the absence of a clear understanding of the underlying static allometries associated with it. A free Plain Language Summary can be found within the Supporting Information of this article.

Male rock lizards may compensate reproductive costs of an immune challenge affecting sexual signals

AbstractSexual signals can be evolutionarily stable if they are condition dependent or costly to the signaler. One of these costs may be the trade-off between maintaining the immune system and the elaboration of ornaments. Experimental immune challenges in captivity show a reduction in the expression of sexual signals, but it is not clear whether these detrimental effects are important in nature and, more importantly, whether they have reproductive consequences. We designed a field experiment to challenge the immune system of wild male Carpetan rock lizards, Iberolacerta cyreni, with a bacterial antigen (lipopolysaccharide). The immune challenge decreased relative reflectance of ultraviolet structural and melanin-dependent sexual coloration in the throat and the lateral ocelli, whereas the carotenoid-dependent dorsal green coloration was not affected. Immune activation also decreased proportions of ergosterol and cholesta-5,7-dien-3-ol in femoral secretions. These results support a trade-off between the immune system and both visual and chemical sexual ornaments. Moreover, the reproductive success of males, estimated with DNA microsatellites, depended on the expression of some color and chemical traits. However, the immune challenge did not cause overall differences in reproductive success, although it increased with body size/age in control but not in challenged males. This suggests the use of alternative reproductive strategies (e.g., forced matings) in challenged males, particularly in smaller ones. These males might consider that their survival probabilities are low and increase reproductive effort as a form of terminal investment in spite of their “low-quality” sexual signals and potential survival costs.

Badge Size Reflects Sperm Oxidative Status within Social Groups in the House Sparrow Passer domesticus

The phenotype-linked fertility hypothesis proposes that male ornaments reflect male fertility. Male ornaments could honestly signal sperm quality due to the high susceptibility of sperm to free radicals on the one hand and the negative impact of oxidative stress on ornament elaboration on the other hand. Thus, only males with superior antioxidant defences could bear the cost of more elaborated sexual ornaments without suffering adverse fitness costs. Yet, in species where males experience differential access to fertile females, a trade-off emerges between investing into traits favouring mating opportunities (e.g. secondary sexual ornaments, social dominance, mate-guarding behaviours, etc.) or into traits favouring sperm competitive ability (e.g. sperm numbers and quality). When male sexual ornaments promote greater access to fertile females, a negative relationship can then be predicted between ornamentation and sperm quality. We tested the latter hypothesis and the phenotype-linked fertility hypothesis in wild House Sparrows Passer domesticus by exploring the relationships between sperm quality, melanin-based ornamentation, and redox status in blood and sperm. We found no correlation between badge size and sperm swimming performance. However, we found that within a social group, large-badged males better protect their ejaculates from oxidative stress, and thus produce less oxidized ejaculates. Additionally, we found that badge size did not reflect social dominance, and thus the protection of the ejaculate is independent of males’ ability to monopolize resources. Our results suggest that badge size might reflect male investment into the antioxidant protection of their sperm relative to a given social environment, and thus females may accrue both direct and indirect benefits by mating with large-badged males producing less oxidized ejaculates.

Honest sexual signaling in turtles: experimental evidence of a trade-off between immune response and coloration in red-eared sliders Trachemys scripta elegans.

Sexual signals can be evolutionarily stable if they are honest and condition dependent or costly to the signaler. One possible cost is the existence of a trade-off between maintaining the immune system and the elaboration of ornaments. This hypothesis has been experimentally tested in some groups of animals but not in others such as turtles. We experimentally challenged the immune system of female red-eared sliders Trachemys scripta elegans, with a bacterial antigen (lipopolysaccharide (LPS)) without pathogenic effects to explore whether the immune activation affected visual colorful ornaments of the head. The LPS injection altered the reflectance patterns of color ornaments. In comparison to the control animals, the yellow chin stripes of injected animals exhibited (1) reduced brightness, (2) lower long wavelength (>470nm) reflectance, and (3) lower values for carotenoid chroma. The postorbital patches of injected individuals also showed reduced very long wavelength (>570nm) reflectance but did not change in carotenoid chroma. Thus, experimental turtles showed darker and less "yellowish" chin stripes and less "reddish" postorbital patches at the end of the experiment, whereas control turtles did not change their coloration. This is the first experimental evidence supporting the existence of a trade-off between the immune system and the expression of visual ornaments in turtles. We suggest that this trade-off may allow turtles to honestly signal individual quality via characteristics of coloration, which may have an important role in intersexual selection processes.

The evolution of honest communication: integrating social and physiological costs of ornamentation.

Much research on animal communication has addressed how costs such as social costs or physiological costs favor the accuracy of signals. Previous work has largely considered these costs separately, but we may be missing essential connections by studying costs in isolation. After all, social interactions produce rapid changes in hormone titers which can then affect individual behavior and physiology. As a result, social costs are likely to have widespread physiological consequences. Here, I present a new perspective on the factors that maintain honest signals by describing how the interplay between social costs and physiological costs may maintain an accurate link between an animal's abilities and ornament elaboration. I outline three specific mechanisms by which the interaction between social behavior and hormones could favor honest signals and present specific predictions for each of the three models. Then, I review how ornaments alter agonistic behavior, agonistic behavior influences hormones, and how these hormonal effects influence fitness. I also describe the few previous studies that have directly tested how ornaments influence hormones. Finally, opportunities for future work are discussed. Considering the interaction between social behavior and physiology may address some challenges associated with both social and physiological models of costs. Understanding the dynamic feedbacks between physiology and social costs has potential to transform our understanding of the stability of animals' communication systems.

Head coloration reflects health state in the red-eared slider Trachemys scripta elegans

Sexual signals can be evolutionarily stable if they are honest and condition-dependent or costly to the signaler. One possible cost is the existence of a trade-off between maintaining physiological health and elaboration of ornaments, such that only healthier individuals may afford to produce more elaborate sexual displays. We analyzed the relationship between head coloration and health state of Trachemys scripta elegans turtles. Results showed that turtles with a higher immune response and with a higher body condition had postorbital red patches with brighter coloration with higher values of long-wavelength reflectance (i.e., more reddish). Similarly, turtles with a higher immune response and with a lower heterophil/lymphocyte (H/L) ratio had chin yellow stripes with darker coloration with higher values of medium wavelengths (i.e., more yellowish). These relationships suggest that the health state of T. scripta elegans turtles is reflected by the colorful skin patches and stripes of the head. Characteristics of coloration did not differ between sexes, suggesting that this visual signal may be used by both sexes in intrasexual and intersexual communication. Because many other turtle species have similar colorful patches, it is likely that coloration may have a still unexplored significant role in sexual selection in many turtles.

Copper Ornaments in the Iberian Chalcolithic

A general feature of the first metallurgy in the Old World is the manufacture of personal ornaments. In the Iberian Peninsula, however, the earliest copper items during the Chalcolithic are not ornaments, but tools and tool-weapons. This situation changes radically in the Bronze Age, when copper ornaments constitute the largest group of objects. Technological divergences, which are also evident, speak of an autonomous and distinct development of metallurgy in Iberia, one that could have limited the elaboration of ornaments. The question we raise is: why was copper not used in Iberia for the manufacture of personal ornaments during the Chalcolithic? We try to answer this by analysing the technological and socioeconomic aspects of the social demand for these products in the Chalcolithic.

Copper Ornaments in the Iberian Chalcolithic: Technology versus Social Demand

A general feature of the first metallurgy in the Old World is the manufacture of personal ornaments. In the Iberian Peninsula, however, the earliest copper items during the Chalcolithic are not ornaments, but tools and tool-weapons. This situation changes radically in the Bronze Age, when copper ornaments constitute the largest group of objects. Technological divergences, which are also evident, speak of an autonomous and distinct development of metallurgy in Iberia, one that could have limited the elaboration of ornaments. The question we raise is: why was copper not used in Iberia for the manufacture of personal ornaments during the Chalcolithic? We try to answer this by analysing the technological and socioeconomic aspects of the social demand for these products in the Chalcolithic.

Structure of Social Networks in a Passerine Bird: Consequences for Sexual Selection and the Evolution of Mating Strategies

The social environment is a critical determinant of fitness and, in many taxa, is shaped by an individual's behavioral discrimination among social contexts, suggesting that animals can actively influence the selection they experience. In competition to attract females, males may modify sexual selection by choosing social environments in which they are more attractive relative to rivals. Across the population, such behaviors should influence sexual selection patterns by altering the relationship between male mating success and sexual ornament elaboration. Here we use network analysis to examine patterns of male social behavior in relation to plumage ornamentation and mating success in a free-living population of house finches. During the nonbreeding season, less elaborate males changed associations with distinct social groups more frequently, compared to more elaborate males that showed greater fidelity to a single social group. By the onset of pair formation, socially labile males effectively increased their attractiveness relative to other males in the same flocks. Consequently, males that frequently moved between social groups had greater pairing success than less social individuals with equivalent sexual ornamentation. We discuss these results in relation to conditional mating tactics and the role of social behavior in evolutionary change by sexual selection.

Advertised quality, caste and food availability influence the survival cost of juvenile hormone in paper wasps

Life-history trade-offs are often hormonally mediated. Here, we provide a comparative perspective on the endocrine basis of life-history trade-offs by examining the invertebrate hormone juvenile hormone (JH). JH is often associated with benefits, including increased dominance and reproductive success. We tested whether JH reduced survival of Polistes dominulus wasps and whether this survival cost was influenced by factors such as advertised quality, food availability, caste and body size. Overall, JH reduced individual survival. Among fed and unfed queens, JH reduced survival in a dose-dependent manner. Among workers, JH had a stronger effect on survival of fed workers than unfed workers. Unfed workers died quickly and body size was the best predictor of survival. Surprisingly, queens and workers treated with JH survived longer when they had signals advertising high quality than when they had signals advertising low quality. The relationship between advertised quality and ability to withstand high levels of JH suggests that there are differential physiological costs associated with ornament elaboration that could play a role in maintaining signal accuracy over evolutionary time. Overall, the convergence of endocrine-mediated costs across diverse systems suggests that endocrine-mediated trade-offs may be an adaptive way to optimize resource allocation rather than a non-adaptive constraint specific to a particular hormone.

Immune challenge affects sexual coloration of male Iberian wall lizards

Sexual signals can be evolutionarily stable if they are honest and condition dependent or costly to the signaler. One possible cost is the existence of a trade-off between maintaining the immune system and the elaboration of ornaments. We experimentally challenged the immune system of male Iberian wall lizards, Podarcis hispanica, with a bacterial antigen (lipopolysaccharide) without pathogenic effects to explore whether the immune activation affected sexually dimorphic visual ornaments. Ventrolateral coloration changed in all males, but immune activation affected some characteristics of coloration of experimental males (i.e., challenged males failed to increase brightness and medium wavelengths over time as control males did, and the proportion of yellow pigments decreased after the immune activation) but not others (i.e., proportion of blue, green and red pigments changed equally in all males). Results suggested the existence of a trade-off between physiological regulation of the immune system and the allocation of essential compounds (probably carotenoids) to sexual ornaments. We suggest that this trade-off may allow one to honestly signal individual male quality via characteristics of coloration in lizards, which may have an important role in both intra- and intersexual selection processes.

Immune activation affects chemical sexual ornaments of male Iberian wall lizards

Many animals use chemical signals in sexual selection, but it is not clear how these sexual traits might have evolved to signal honestly male condition. It is possible that there is a trade-off between maintaining the immune system and the elaboration of ornaments. We experimentally challenged the immune system of male Iberian wall lizards, Podarcis hispanica, with a bacterial antigen (lipopolysaccharide), without pathogenic effects, to explore whether the immune activation affected chemical ornaments. Immune activation resulted in decreased proportions of a major chemical in femoral secretions (cholesta-5,7-dien-3-ol = provitamin D(3)) known to be selected in scent of males by females and which active form (vitamin D) has a variety of important effects on immune system function. This result suggests the existence of a potential trade-off between physiological regulation of the immune system and the allocation of essential nutrients (vitamins) to sexual chemical ornaments in male lizards.

Habitat as a source of intrapopulational variation of ornament size in Galápagos lava lizards (Microlophus albemarlensis complex)

AbstractGradients in habitat structure are expected to influence the outcome of selection on traits that contribute to communicative display. Galápagos lava lizards (Microlophus albemarlensis complex) on Isla Plaza Sur in the Galápagos Islands occur across a gradient of vegetative cover. Previous work in this population has shown that traits associated with predator avoidance are magnified in habitats with low vegetative cover. This pattern suggests that predation pressure differs by habitat and thus, may act to select against the elaboration of ornamentation. We measured the size of the chin patch, an ornament known to be used in intraspecific signaling, to test this hypothesis. The area of the chin patch was dependent on both snout-vent length and residual body mass. In contrast to expectation, males had larger chin patches in the sparsely vegetated habitat suggested to have high predation risk. This result raises questions about the presumed survival cost of ornament elaboration.

Offspring sex ratios in relation to mutual ornamentation and extra‐pair paternity in the Black Swan Cygnus atratus

In sexually dichromatic birds, females may adaptively adjust the sex ratio of their offspring prior to hatching in relation to male ornamentation, for example, by producing more sons when paired to a highly attractive partner. However, to our knowledge no studies have investigated offspring sex ratio modification in species in which both sexes are ornamented, and it is unknown whether such a process would be adaptive. Here we examine variation in offspring sex ratio in the mutually ornamented Black Swan Cygnus atratus. Brood sex ratio was not related to the degree of ornament elaboration in either parent, or to extra‐pair paternity. We suggest that parental attractiveness may not be inherited in a sex‐linked manner, or may be largely non‐heritable. Thus, females may not benefit from biasing the sex ratio of their offspring in relation to parental attractiveness.

Adaptive genetic complementarity in mate choice coexists with selection for elaborate sexual traits

Choice of genetically unrelated mates is widely documented, yet it is not known how self-referential mate choice can co-occur with commonly observed directional selection on sexual displays. Across 10 breeding seasons in a wild bird population, we found strong fitness benefits of matings between genetically unrelated partners and show that self-referential choice of genetically unrelated mates alternates with sexual selection on elaborate plumage. Seasonal cycles of diminishing variation in ornamentation, caused by early pairing of the most elaborated males, and influx of increasingly genetically unrelated available mates caused by female-biased dispersal, lead to temporal fluctuations in the target of mate choice and enabled coexistence of directional selection for ornament elaboration with adaptive pairing of genetically unrelated partners.

Complexity and integration in sexual ornamentation: an example with carotenoid and melanin plumage pigmentation

Sexual ornaments often consist of several components produced by distinct developmental processes. The complexity of sexual ornaments might be favoured by mate choice of individual components in different environments which ultimately results in weak interrelationships (integration) among the developmental processes that produce these components. At the same time, sexual selection for greater exaggeration of individual components favours their stronger co-dependence on organismal resources. This should ultimately produce stronger condition-mediated integration among ornaments' components in individuals with the most exaggerated ornamentation. Here we distinguish between these two sources of integration by examining the relationship between integration and elaboration of sexual ornamentation in three bird species: two with carotenoid-based sexual ornamentation (the house finch, Carpodacus mexicanus and common redpoll, Carduelis flammea) and a species with melanin-based sexual ornamentation (house sparrow, Passer domesticus). We found that integration of components varied with elaboration of carotenoid-based ornamentation but not of melanin ornamentation. In the house finches, integration was the highest in individuals with small ornaments and decreased with ornament elaboration whereas the pattern was the opposite in common redpolls. These results suggest that in these species integration and complexity of carotenoid-based ornamental components are due to shared condition-dependence of distinct developmental pathways, whereas integration and complexity of the melanin ornamentation is due to organismal integration of developmental pathways and is largely condition- and environment-invariant. Thus, functionally, ornamentation of the house sparrows can be considered a single trait, whereas complexity of the house finch and redpoll ornamentation varies with ornament elaboration and individual condition.

Context-dependent sexual advertisement: plasticity in development of sexual ornamentation throughout the lifetime of a passerine bird.

Male investment into sexual ornamentation is a reproductive decision that depends on the context of breeding and life history state. In turn, selection for state- and context-specific expression of sexual ornamentation should favour the evolution of developmental pathways that enable the flexible allocation of resources into sexual ornamentation. We studied lifelong variation in the expression and condition-dependence of a sexual ornament in relation to age and the context of breeding in male house finches (Carpodacus mexicanus)--a species that develops a new sexual ornament once a year after breeding. Throughout males' lifetime, the elaboration of ornamentation and the allocation of resources to the development of sexual ornamentation depended strongly on pairing status in the preceding breeding season--males that were single invested more resources into sexual ornamentation and changed ornamentation more than males that were paired. During the initial (post-juvenile) moult, the expression of ornamentation was closely dependent on individual condition, however the condition-dependence of ornamentation sharply decreased throughout a male's lifetime and in older males expression of sexual ornamentation was largely independent of condition during moult. Selection for early breeding favoured greater ornamentation in males that were single in the preceding seasons and the strength of this selection increased with age. On the contrary, the strength of selection on sexual ornamentation decreased with age in males that were paired in the preceding breeding season. Our results reveal strong context-dependency in investment into sexual ornamentation as well as a high flexibility in the development of sexual ornamentation throughout a male's life.

Avian Sexual Dichromatism in Relation to Phylogeny and Ecology

▪ Abstract The extent and diversity of sexual dichromatism in birds is thought to be due to the intensity of current sexual selection on the plumage ornamentation of males and females. This view leads to an expectation of concordance between ecological conditions and sexual dichromatism. Yet, because expression of dichromatism is the result of not only current selection, but also historical patterns of development, function, and selection, the concordance between ecology and current sexual dichromatism is not straightforward. Recent studies have revealed a number of trends in the evolution of avian sexual ornamentation that seem contrary to what is expected if current sexual selection is the primary force shaping dichromatism. For example, change in sexual dichromatism is often the result of evolutionary changes in female rather than male ornamentation. Moreover, sexual dichromatism is often an ancestral rather than a derived state; current expression of dichromatism is frequently the result of selection for lesser ornamentation in one sex and not for ornament elaboration. Loss and gain of sexual ornamentation sometimes precedes changes in preference for sexual ornamentation, and sexual ornaments can have high evolutionary lability despite their developmental and functional complexity. These findings emphasize that phylogenetic reconstructions must play a central role in attempts to understand the function and evolution of sexual dichromatism. With a historical perspective, one can test the relative importance of direct selection, indirect selection, and drift in relation to changes of sexual dichromatism. If sexual selection is invoked, the mechanisms of sexual selection can be explored by examining the concordance between the elaboration of ornamentation and the preferences for ornamentation across species and by tracing phylogenetic trajectories of sexual ornaments. Finally, placing physiological, genetic, and developmental mechanisms of sexual ornamentation into such a phylogenetic framework will enable greater inference about the past evolution and current function of sexual dichromatism in birds.

Paternal care as a conditional strategy: distinct reproductive tactics associated with elaboration of plumage ornamentation in the house finch

When individuals in a population differ in physiological condition and residual reproductive value, selection should favor phenotypic plasticity in reproductive investment such that individuals are able to adopt the reproductive tactic that results in the highest fitness under given conditions. Here we examined reproductive tactics in relation to the elaboration of conditiondependent sexual ornamentation (carotenoid breast coloration) in a Montana population of the house finches (Carpodacus mexicanus). Males used distinct reproductive tactics depending on elaboration of their sexual ornamentation. Males with red pigmentation (maximum ornament elaboration) paired with females that nested earlier, but these males did little provisioning of incubating females and nestlings. In contrast, males with yellow coloration paired with females that nested later, but these males fed female and nestlings more. Consequently, for red males offspring recruitment was primarily affected by earlier nest initiation, whereas in yellow males it was affected most by male provisioning. In males with intermediate plumage coloration, all measured components, nest initiation, provisioning of incubating female, and nestling feeding, strongly contributed to offspring recruitment. The fitness consequences of alternative reproductive tactics of males were influenced by breeding experience and fidelity of their mates. Among first-time breeders, red males achieved the highest fecundity because of the advantage gained through early nesting and pairing with more experienced females and because of compensation by their mates for low male provisioning of nestlings. Among experienced breeders, males with intermediate plumage coloration achieved the highest fecundity because of the combined benefits of relatively early pairing and high parental care. High variation in sexual ornamentation in a Montana population of house finches may favor distinct associations of sexual displays with a particular set of reproductive behaviors. Key words: Carpodacus mexicanus, conditional strategies, house finches, parental care, reproductive investment, secondary sexual traits. [Behav Ecol 13:591–597 (2002)]