Dual functions of a melanin-based ornament in the common yellowthroat

Melanin-based ornaments often function as signals in male-male competition, whereas carotenoid-based ornaments appear to be important in female mate choice. This difference in function is thought to occur because carotenoid pigments are more costly to produce than melanins and are thus more reliable indicators of male quality. We examined the role of melanin- and carotenoid-based ornaments in male-male competition and female choice in the common yellowthroat Geothlypis trichas, a sexually dichromatic passerine. Males display a black facial mask produced by melanin pigmentation and a bright yellow bib (throat, breast and belly) produced by carotenoid pigmentation. In controlled aviary experiments, mask size was the best predictor of both male-male competition and female mate choice, and, therefore, mask size may be regarded as an ornament of dual function. These dual functions may help to maintain the reliability of mask size as an indicator of male quality, despite the potentially low cost of production. The size of the bib was unrelated to male-male competition or female choice, but there was a tendency for females to prefer males with more colourful bibs. We propose that the black mask is important in competition for territories with other males and for attracting females. Our results highlight the need for more studies of the mechanisms of sexual selection in species with ornaments composed of different pigment types.     

I'm sexy and I glow it: female ornamentation in a nocturnal capital breeder

In many species, males rely on sexual ornaments to attract females. Females, by contrast, rarely produce ornaments. The glow-worm (Lampyris noctiluca) is an exception where wingless females glow to attract males that fly in search of females. However, little is known about the factors that promote the evolution of female ornaments in a sexual selection context. Here, we investigated if the female ornament of the glow-worm is a signal of fecundity used in male mate choice. In support of this, we found brightness to correlate with female fecundity, and males to prefer brighter dummy females. Thus, the glow emitted by females is a reliable sexual signal of female fecundity. It is likely that male preference for the fecundity-indicating ornament has evolved because of large variation among females in fecundity, and because nocturnal males cannot directly assess female size and fecundity. These results indicate that female ornamentation may evolve in capital breeders (i.e. those in which stored resources are invested in reproduction) when females vary significantly in fecundity and this variation cannot be assessed directly by males.     

Geographic variation in the function of ornaments in the common yellowthroat Geothlypis trichas

We used controlled aviary experiments to study the role of male ornaments in male-male competition and female choice in the common yellowthroat Geothlypis trichas , a sexually dichromatic warbler. Previous aviary studies in Wisconsin, USA, indicated that males with larger black facial masks were dominant over males with smaller masks and preferred by females in mate choice experiments. In this study, we replicated those experiments in a population in New York, USA, where male mating success was related more consistently to the size of the yellow bib (throat, breast, and belly) than to the size of the mask. Similar to the study in Wisconsin, we found that males with larger masks were more likely to be dominant in New York, however, we found that males with larger bibs were preferred by females in New York, and there was no significant preference for males with larger masks. These results are consistent with the hypothesis that carotenoid-based ornaments are selected by female choice and melanin-based ornaments are selected by male-male competition. However, the pattern of female choice appears to vary between New York and Wisconsin. This geographic difference could be related to a variety of environmental factors (habitat, carotenoid and parasite abundance) that affect the costs and benefits of choosing males with particular ornaments in each location.     

Carotenoid-Based Plumage Coloration Predicts Leukocyte Parameters during the Breeding Season in Northern Cardinals (Cardinalis cardinalis)

Dichromatism in songbirds is often associated with polygyny and dimorphic parental investment, and is thought to arise via sexual selection. Northern cardinals (Cardinalis cardinalis) are not only dichromatic but also monogamous and biparental, suggesting that plumage coloration in this species may serve different functions than in more typical dichromatic species. In order to explore the role of sexual selection in the evolution of plumage coloration in cardinals, we used reflectance spectrophotometry to investigate whether two carotenoid‐based ornaments, the male’s red breast and the female’s underwing coverts, contain information that potential mates or competitors could use to assess condition. We found that whereas coloration was not related to body condition (measured as the residual body mass from a regression of body mass on wing chord), more saturated carotenoid coloration was associated with higher heterophil to lymphocyte ratios in males, and with higher white blood cell counts in females. Thus, in both sexes, carotenoid coloration was positively linked to immune measures normally associated with higher levels of stress and infection. These results do not indicate that carotenoid‐based coloration functions as a signal of low levels of stress or disease in this species. We propose instead that because plumage coloration may be related to competitiveness, the more saturated individuals increase their risk of injury, stress, and infection by engaging in more competitive behavior or by secreting more testosterone, or both. Our finding that carotenoid pigmentation is positively associated in males with the size of the cloacal protuberance, an androgen‐sensitive sex character, supports this hypothesis.     

Plumage color as a composite trait: developmental and functional integration of sexual ornamentation

Most studies of condition-dependent sexual ornaments have treated such ornaments as single traits. However, sexual ornaments are often composites of several components, each produced by partially independent developmental pathways. Depending on environmental and individual condition, components of these ornaments may reflect different behavioral or physiological properties of an individual. One of the best-known, condition-dependent ornaments is carotenoid-based plumage coloration, which has at least four distinct components: pigment elaboration, patch area, pigment symmetry, and patch area symmetry. Here we examined fitness consequences of variation in individual components of carotenoid ornamentation in male house finches (Carpodacus mexicanus). Over 5 yr and several selection episodes, we studied variation in the plumage components in a large sample (n = 498) of males from a Montana population. The ornament components were partially independent of each other and had distinct fitness consequences. Selection for higher fecundity favored an increase in redness of coloration and a decrease in pigment asymmetry and patch area asymmetry but did not act on patch area itself. In contrast, viability selection favored larger and more symmetrical ornamental patches but did not act on pigment elaboration. Developmental and functional interrelationships among individual components of ornamentation strongly differed between house finch populations. Distinct patterns of selection on individual components of condition-dependent ornaments, combined with partially independent development of components, should favor the evolution of composite sexual traits whose components reliably reflect condition across a wide array of environments.     

Sex of opponent influences response to a potential status signal in house sparrows

We evaluated the possible use of a sexually dimorphic plumage trait as a status signal in inter- and intrasexual social interactions in free-living house sparrows, Passer domesticus. Males with larger black throat patches (bibs) tended to be dominant regardless of the opponent's sex. This relationship was maintained after controlling for body size and age in multivariate analyses. However, the nature of social interactions varied according to the sex of the opponent and a male's bib size. Males with larger bibs tended to be less aggressive to male opponents, and received significantly less aggression from those birds, a key predicted benefit of a status signal. However, when the opponent was a female, males with large bibs displayed significantly more aggression than males with small bibs, and this relationship persisted even after controlling for the dominance status of the participants. Females were significantly more aggressive towards male opponents than towards female opponents, and females interacting with two males of known bib size were more aggressive to the one with the larger bib. These results suggest that the bib of male house sparrows may be a signal of status, but that in interactions with females, the bib may also have other functions. Frequent testing by females of the underlying quality being signalled by the bib may be linked to later mate choice, and might be adaptive if the information value of the bib varies among years. Such testing may also contribute to mechanisms for maintaining the bib as a reliable signal of status. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

Independent Mating Preferences for Male Body Size and Coloration in Female Trinidadian Guppies

Although females in numerous species generally prefer males with larger, brighter and more elaborate sexual traits, there is nonetheless considerable intra‐ and interpopulation variation in mating preferences amongst females that requires explanation. Such variation exists in the Trinidadian guppy, Poecilia reticulata, an important model organism for the study of sexual selection and mate choice. While female guppies tend to prefer more ornamented males as mates, particularly those with greater amounts of orange coloration, there remains variation both in male traits and female mating preferences within and between populations. Male body size is another trait that is sexually selected through female mate choice in some species, but has not been examined as extensively as body coloration in the guppy despite known intra‐ and interpopulation variation in this trait among adult males and its importance for survivorship in this species. In this study, we used a dichotomous‐choice test to quantify the mating preferences of female guppies, originating from a low‐predation population in Trinidad, for two male traits, body length and area of the body covered with orange and black pigmentation, independently of each other. We expected strong female mating preferences for both male body length and coloration in this population, given relaxation from predation and presumably relatively low cost of choice. Females indeed exhibited a strong preference for larger males as expected, but surprisingly a weaker (but nonetheless significant) preference for orange and black coloration. Interestingly, larger females demonstrated stronger preferences for larger males than did smaller females, which could potentially lead to size‐assortative mating in nature.     

No fecundity cost of female secondary sexual trait expression in the horned beetle Onthophagus sagittarius

Typically males bear the products of sexual selection in the form of ornaments and/or weapons used to compete for and attract females. Secondary sexual traits in females have been thought of as the product of correlated responses to sexual selection on males. However, there is increasing phylogenetic evidence that female secondary sexual traits can arise independently of selection on males, and may be subject to sexual selection. Theoretical models of the evolution of female ornamentation via male mate choice have assumed that females suffer a cost of ornament expression via reduced fecundity, and hence female ornaments are less likely to evolve than male ornaments. In the dung beetle Onthophagus sagittarius, there has been an independent evolutionary origin of horns in females that are qualitatively different from the horns produced by males. We use this system as a model to examine the costs of horn expression for females within a life-history context. We identified a longevity cost of reproduction for females that was independent of horn expression. Large females lived longer, and after controlling for lifespan, had a higher lifetime fecundity, and invested more heavily in maternal provisioning than did small females. We found no evidence of a cost to females of investment in horns. Rather, the rate of increase in fecundity and horn expression with body size were equal, so that absolute horn size provides an accurate indicator of body size and maternal quality. The effects we observe were independent of female contest competition and/or male mate choice, which were excluded in our experimental protocol. However, we speculate on the potential functional contributions female horns might make to female fitness.     

Evolution of female carotenoid coloration by sexual constraint in <Emphasis Type="Italic">Carduelis</Emphasis> finches

Background  

Females often express the same ornaments as males to a similar or lesser degree. Female ornaments can be adaptive, but little is known regarding their origins and mode of evolution. Current utility does not imply evolutionary causation, and therefore it is possible that female ornamentation evolved due to selection on females, as a correlated response to selection on males (sexual constraint), or a combination of both. We tested these ideas simulating simple models for the evolution of male and female correlated traits, and compared their predictions against the coloration of finches in the genus Carduelis.     

Reproductive effort of both male and female Bar‐throated Apalis Apalis thoracica is predicted by ornamentation of self and mate

Melanin‐based plumage ornaments have been shown to play an important role in male–male competition, but also influence inter‐sexual communication. Consequently, ornaments may be associated with reproductive effort of both males and females. Females mated to males with larger melanin ornaments may acquire access to better territories or benefit from increased paternal care. Here we investigated whether the melanin‐based breast‐band of male and female Bar‐throated Apalis Apalis thoracica is a signal of information about its bearer and is associated with male and female reproductive effort. Breast‐band size was a highly variable morphometric trait in both sexes, but only in males was it associated with body mass. We then assessed whether male and female breast‐band size predicted maternal and paternal investment. Egg mass increased with male breast‐band size, but decreased with female breast‐band size. Whether females adjust maternal hormone allocation in response to their partner's ornamentation remains a contentious issue. We found that yolk testosterone and androstenedione concentrations were not predicted by male ornamentation or body mass. Finally, males with larger breast‐bands provided their mates with more food, allowing those females to spend more time incubating. Reproductive effort of both parents is therefore predicted by their own and their mate's ornamentation in Bar‐throated Apalis, and thus breast‐band size potentially acts as a signal of reproductive performance in both sexes. These results highlight the need for more comprehensive analyses of a relationship between melanin‐based ornaments and fitness, incorporating multiple behavioural variables associated with reproductive effort.     

A novel method of comparing mating success and survival reveals similar sexual and viability selection for mobility traits in female tree crickets

The relationship between sexual and viability selection in females is necessarily different than that in males, as investment in sexual traits potentially comes at the expense of both fecundity and survival. Accordingly, females do not usually invest in sexually selected traits. However, direct benefits obtained from mating, such as nuptial gifts, may encourage competition among females and subsidize investment into sexually selected traits. We compared sexual and viability selection on female tree crickets Oecanthus nigricornis, a species where females mate frequently to obtain nuptial gifts and sexual selection on females is likely. If male choice determines female mating success in this species, we expect sexual selection for fecundity traits, as males of many species prefer more fecund females. Alternatively, intrasexual scramble or combat competition on females may select for larger jumping legs or wider heads (respectively). We estimated mating success in wild caught crickets using microsatellite analysis of stored sperm and estimated relative viability by comparing surviving female O. nigricornis to those captured by a common wasp predator. In support of the scramble competition hypothesis, we found sexual selection for females with larger hind legs and narrower heads. We also found stabilizing viability selection for intermediate head width and hind leg size. As predicted, traits under viability and sexual selection were very similar, and the direction of that selection was not opposing. However, because the shape of sexual and viability selection differs, these episodes of selection may favour slightly different trait sizes.     

Eggshell coloration and its importance in postmating sexual selection

Avian eggshell color seems to fulfill multiple functions, some of them being structural and others signaling. In this study, we tested whether or not eggshell coloration may play a role in sexual selection of Tree Sparrows (Passer montanus). According to the “Sexually selected eggshell coloration” hypothesis, eggshell coloration signals female, egg or chick quality and males adjust parental investment according to this signal. Eggs of this species are covered with brown spots and patches, and variation between clutches is high. We found that eggshell coloration correlates with both protoporphyrin and biliverdin, but protoporphyrin concentrations are ten times higher. Eggshell coloration reflects egg and offspring quality, but not female quality. Thus, eggshell coloration may signal female postmating investment in offspring rather than female quality. Furthermore, differential allocation in terms of maternal investment is supported by the fact that females lay more pigmented clutches when mated to males with bigger melanin‐based ornaments relative to their own. Moreover, males invested proportionally more to chicks that hatched from more pigmented clutches. Our correlative results thus seem to support a role of sexual selection in the evolution of eggshell coloration in birds laying brown eggs, pigmented mainly by protoporphyrin.     

Does Breeding Ornamentation Signal Genetic Quality in Arctic charr, <Emphasis Type="Italic">Salvelinus alpinus</Emphasis>?

Genetic theories of sexual selection predict that most ornamental secondary sexual traits provide reliable indication of the genetic quality of their bearers. Accordingly, also the offspring of mates with elaborate mating display should perform better than those of less conspicuous counterparts. In this study, we used Arctic charr (Salvelinus alpinus) as a model species to investigate whether the variation in a carotenoid-based red breeding coloration (a sexually dichromatic trait) in different sexes would reflect differences in individual genetic variability, one measure of individual quality, and/or indirectly, be manifested in variation in the offspring’s early viability and growth. We created maternal half-sibling families by artificially fertilizing the eggs with milt from bright- and pale-coloured males and then held the resulting progenies under identical hatchery conditions. The expression of red coloration among parental fish was not associated with their genetic diversity estimates in either sex nor did offspring sired by bright males consistently differ in terms of embryo survival or endogenous growth efficiency from offspring sired by pale males. By contrast, maternal effects were notably strong and, additionally, the degree of female coloration was negatively linked to their reproductive potential. The more intensely coloured females had a smaller relative fecundity and they also produced offspring of lower viability, implying a significant trade-off in resource allocation between ornamentation and offspring. Our results indicate that the red breeding ornamentation of Arctic charr is likely to be informative rather among females than males when the reproductive quality is predicted on grounds of the number of offspring produced. Nevertheless, this study does not support the direct selection hypothesis in explaining the evolution of female ornamentation, but rather suggests that the less intense coloration of female charr compared to males may reflect inter-sexual differences in the trade-off between natural and sexual selection.     

Viability selection on female fly finery in the wild

Female ornaments have evolved in a few taxa in which females compete for access to important resources provided by their mates. However, the effects of these sexually selected traits on survival have not been studied. Elaborate leg‐scale and/or abdominal ornaments are displayed by females of some Rhamphomyia dance flies (Diptera: Empididae) to flying males carrying prey gifts (females do not hunt). Previous analyses have shown significant sexual selection on these female traits. We studied viability selection on the traits by sampling the webs of two spider species and comparing prey R. longicauda females to survivors. We also investigated viability selection from one of the spiders over two seasons. We found that the direction of viability selection on R. longicauda from sticky Tetragnatha spider webs was consistent over two seasons. For abdominal ornaments the form of viability selection was positive and primarily directional (linear). Viability selection also favoured shorter tibiae but there was no significant selection on the size of residual tibial scale area. However, with the addition of dance fly kills from the non‐sticky, leaf‐covering webs of a Dictyna spider, abundant in only one of the seasons, the overall direction of viability selection favoured larger tibial ornaments. While noting that this viability selection on tibial scale ornaments may be a statistical artefact of the fewer traits in the two‐predator analysis (abdominal structures were missing from most Dictyna prey), we suggest that simple differences in the natural history of selective agents causing mortality may partly explain the variation in whether sexual traits are under viability selection. Viability selection on ornamental traits may vary greatly between seasons with changes in the abundances of different natural enemies so that net directional selection on these traits over many generations may be weak.     

Sexual pigmentation and parental risk‐taking in yellow warblers Setophaga petechia

Adult‐directed predation risk imposes important behavioral constraints on parents and might thus alter relationships between costly sexual ornaments and parental performance. For instance, under low predation risk, highly ornamented individuals might display better parental performance than others, as predicted by ‘good parent’ models of sexual selection. However, under high risk of predation, highly ornamented individuals might abandon parental effort if conspicuous to predators, or if social partners are more willing to take parental risks when paired with highly ornamented mates. We experimentally elevated perceived adult‐directed predation risk near nests to explore how carotenoid‐ and phaeomelanin‐based pigmentation in both sexes relate to parental risk‐taking for offspring in the yellow warbler Setophaga petechia. Compared to other males, males with more intense carotenoid‐based pigmentation maintained higher levels of paternal effort under predation risk at highly concealed nests, but reduced nestling provisioning rate more at exposed nests. Further, when faced with predation risk, females with more phaeomelanin‐based pigmentation reduced nestling provisioning rate less than other females, regardless of nest concealment. Females displayed higher parental effort across treatments when paired to males with more colorful carotenoid pigmentation. However, birds did not reduce parental effort under risk less when paired to a highly ornamented mate, suggesting that predation risk did not accentuate differential allocation. Males did not take fewer parental risks than females. Results indicate that nest concealment modifies parental risk‐taking by males with colorful carotenoid‐based pigmentation, and suggest that female melanin‐based pigmentation may indicate boldness and greater a propensity to take parental risks.     

SEXUAL SELECTION,MULTIPLE MALE ORNAMENTS,AND AGE‐ AND CONDITION‐DEPENDENT SIGNALING IN THE COMMON YELLOWTHROAT

In many animals, sexual selection has resulted in complex signaling systems in which males advertise aspects of their phenotypic or genetic quality through elaborate ornamentation and display behaviors. Different ornaments might convey different information or be directed at different receivers, but they might also be redundant signals of quality that function reliably at different times (ages) or in different contexts. We explored sexual selection and age‐ and condition‐dependent signaling in the common yellowthroat (Geothlypis trichas), a sexually dichromatic warbler with two prominent plumage ornaments—a melanin‐based, black facial “mask” and carotenoid‐based, UV‐yellow “bib.” In a three‐year study, variance among males in the number of social (M_w) and extra‐pair (M_e) mates generated strong sexual selection on mask and bib attributes. Some traits (mask size, bib yellow brightness) were correlated with male age and did not experience selection beyond age‐related increases in M_w and M_e. Other traits showed age‐specific (bib size) or age‐reversed (ultraviolet brightness) patterns of selection that paralleled changes in the information‐content of each ornament. The components of male fitness generating selection in young versus old males were distinct, reflecting different sources of variation in male fertilization success. Age‐ and context‐dependent changes in the strength, direction, and target of selection may help explain the maintenance of multiple ornaments in this and other species.     

Geographic variation in sexual selection among populations of an iguanid lizard,Sauromalus obesus (=ater)

Abstract.— Geographic variation in selection pressures may result in population divergence and speciation, especially if sexual selection varies among populations. Yet spatial variation in targets and intensity of sexual selection is well studied in only a few species. Even more rare are simultaneous studies of multiple populations combining observations from natural settings with controlled behavioral experiments. We investigated how sexual selection varies among populations of the chuckwalla, Sauromalus obesus. Chuckwallas are sexually dimorphic in color, and males vary in coloration among populations. Using field observations and multiple regression techniques, we investigated how sexual selection acts on various male traits in three populations in which males differed in coloration. The influence of sexual selection on male coloration was then investigated in more detail using controlled experiments. Results from field observations indicate that phenotypic selection was acting on territory quality in all three populations. In two populations, selection was also acting either directly or indirectly on male coloration. Male color likely functions as an indicator of food resources to females because male color is based partly on carotenoid pigments. In controlled experiments, significantly more females from these two populations chose males with brighter colors over dull males, a result consistent with studies on carotenoid pigments in other taxa. In a third population, no evidence of sexual selection on male coloration was found in either the field study or controlled experiment. Lack of female preferences for male color in this population, in which chuckwalla densities are low and home ranges are large, may result from searching costs to females.     

Female‐biased size dimorphism in a diapausing caddisfly,Mesophylax aspersus: effect of fecundity and natural and sexual selection

1. The effect of mating success, female fecundity and survival probability associated with intra‐sex variation in body size was studied in Mesophylax aspersus, a caddisfly species with female‐biased sexual size dimorphism, which inhabits temporary streams and aestivates in caves. Adults of this species do not feed and females have to mature eggs during aestivation. 2. Thus, females of larger size should have a fitness advantage because they can harbour more energy reserves that could influence fecundity and probability of survival until reproduction. In contrast, males of smaller size might have competitive advantages over others in mating success. 3. These hypotheses were tested by comparing the sex ratio and body size of individuals captured before and after the aestivation period. The associations between body size and female fecundity, and between mating success and body size of males, were explored under laboratory conditions. 4. During the aestivation period, the sex ratio changed from 1 : 1 to male biased (4 : 1), and a directional selection on body size was detected for females but not for males. Moreover, larger clutches were laid by females of larger size. Finally, differences in mating success between small and large males were not detected. These results suggest that natural selection (i.e. the differential mortality of females associated with body size) together with possible fecundity advantages, are important factors responsible of the sexual size dimorphism of M. aspersus. 5. These results highlight the importance of taking into account mechanisms other than those traditionally used to explain sexual dimorphism. Natural selection acting on sources of variation, such as survival, may be as important as fecundity and sexual selection in driving the evolution of sexual size dimorphism.     

Stronger condition dependence in female size explains altitudinal variation in sexual size dimorphism of a Tibetan frog

The present study investigated altitudinal variation in sexual size dimorphism of a Tibetan frog Nanorana parkeri. Size dimorphism was female‐biased in all populations, although this bias became less at higher altitudes because of a steeper altitudinal decrease in female size than male size. Operational sex ratios, an indicator of the opportunity for sexual selection on larger males, changed independently of altitude. Clutch volume, an indicator of the strength of fecundity selection on larger females, was positively with female size, and tended to decrease approaching high altitudes. Females lived longer and grew more slowly than males, and the mean age in both sexes increased and growth rate decreased altitudinally, although the changes were more rapid in females than males. These results suggest that, relative to males, females (i.e. the sex that typically bears greater reproductive costs and experiences stronger directional selection for larger size to take fecundity advantages) should be more sensitive to environments, attaining a larger size via enhancing growth under favourable lower‐latitude conditions but a smaller size as a result of retarding growth when conditions become harsher at higher altitudes. This supports the condition‐dependence hypothesis with respect to intraspecific variation in sexual size dimorphism. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 107 , 558–565.     

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.