Dynamic sexual dichromatism in an explosively breeding Neotropical toad

Sexual selection often promotes the evolution of elaborate colour signals in males, but the importance of sexually selected colour signals remains poorly studied in amphibians. We used reflectance spectrometry to document pronounced sexual dichromatism and dramatic colour change in Bufo luetkenii, a toad that breeds in large aggregations at the onset of the rainy season in Costa Rica. Our observations suggest that males fade rapidly from a vibrant lemon yellow to a dull brown once they have paired with a female. We demonstrate this by showing that males are much brighter than females and that unpaired males are more colourful than males in amplexus. We also show that coloration fades rapidly when males are briefly held captive. This is, to our knowledge, the first study to document such dynamic change in male coloration and sexual dichromatism in anurans.     

The role of sexual and natural selection in shaping patterns of sexual dichromatism in the largest family of songbirds (Aves: Thraupidae)

Males and females can be under different evolutionary pressures if sexual and natural selection is differentially operating in each sex. As a result, many species have evolved sexual dichromatism, or differences in coloration between sexes. Although sexual dichromatism is often used as an index of the magnitude of sexual selection, sexual dichromatism is a composite trait. Here, we examine the evolution of sexual dichromatism in one of the largest and most ecologically diverse families of birds, the tanagers, using the avian visual perspective and a species‐level phylogeny. Our results demonstrate that the evolutionary decreases of sexual dichromatism are more often associated with larger and more frequent changes in male plumage coloration, and evolutionary increases are not more often associated with larger changes in either sex. Furthermore, we show that the crown and ventral plumage regions are correlated with sexual dichromatism in males, and that only male plumage complexity is positively correlated with sexual dichromatism. Finally, we demonstrate that light environment is important in shaping both plumage brilliance and complexity. By conducting a multilevel analysis of plumage evolution in males and females, we show that sexual dichromatism evolves via a mosaic of sexual and natural selection in both sexes.     

Size dimorphism and avian‐perceived sexual dichromatism in a New Zealand endemic bird,the whitehead Mohoua albicilla

Sex differences in behavior, morphology, and physiology are common in animals. In many bird species, differences in the feather colors of the sexes are apparent when judged by human observers and using physical measures of plumage reflectance, cryptic (to human) plumage dichromatism has also been detected in several additional avian lineages. However, it remains to be confirmed in almost all species whether sexual dichromatism is perceivable by individuals of the studied species. This latter step is essential because it allows the evaluation of alternative hypotheses regarding the signaling and communication functions of plumage variation. We applied perceptual modeling of the avian visual system for the first time to an endemic New Zealand bird to provide evidence of subtle but consistent sexual dichromatism in the whitehead, Mohoua albicilla. Molecular sexing techniques were also used in this species to confirm the extent of the sexual size dimorphism in plumage and body mass. Despite the small sample sizes, we now validate previous reports based on human perception that in male whiteheads head and chest feathers are physically brighter than in females. We further suggest that the extent of sexual plumage dichromatism is pronounced and can be perceived by these birds. In contrast, although sexual dimorphism was also detectable in the mass among the DNA‐sexed individuals, it was found to be less extensive than previously thought. Sexual size dimorphism and intraspecifically perceivable plumage dichromatism represent reliable traits that differ between female and male whiteheads. These traits, in turn, may contribute to honest communication displays within the complex social recognition systems of communally breeding whitehead and other group‐breeding taxa. J. Morphol., 2010. © 2010 Wiley‐Liss, Inc.     

Evolution of sexual dichromatism: contribution of carotenoid- versus melanin-based coloration

In birds, carotenoid-based plumage coloration is more dependent on physical condition and foraging abilities and less constrained developmentally than is melanin-based coloration. Thus, female mate choice for honest signals should result in more intense sexual selection on carotenoid- than on melanin-based plumage coloration. Using variation in sexual dimorphism as an indirect measure of the intensity of sexual selection, we tested the prediction mat variation in sexual dimorphism is driven more by change in carotenoid-based coloration between males and females dian by change in melanin-based coloration. Examination of historical changes in carotenoid- versus melanin-based pigmentation in 126 extant species of Cardueline finches supported this prediction. We found that carotenoid-derived coloration changed more frequendy among congeners dian melanin-based coloration. In both sexes, increase in carotenoid-based coloration score, but not in melanin-based coloration score, was strongly associated with increase in sexual dichromatism. In addition, sexual dimorphism in carotenoid-based coloration contributed more to overall dichromatism than dimorphism in melanin-based plumage. Our results supported die hypothesis that melanin-based and carotenoid-based coloration have fundamentally different signal content and suggest that combining melanin-based and carotenoid-based coloration in comparative analyses is not appropriate.     

Ecological constraint and the evolution of sexual dichromatism in darters

It is not known how environmental pressures and sexual selection interact to influence the evolution of extravagant male traits. Sexual and natural selection are often viewed as antagonistic forces shaping the evolution of visual signals, where conspicuousness is favored by sexual selection and crypsis is favored by natural selection. Although typically investigated independently, the interaction between natural and sexual selection remains poorly understood. Here, we investigate whether sexual dichromatism evolves stochastically, independent from, or in concert with habitat use in darters, a species‐rich lineage of North American freshwater fish. We find the evolution of sexual dichromatism is coupled to habitat use in darter species. Comparative analyses reveal that mid‐water darter lineages exhibit a narrow distribution of dichromatism trait space surrounding a low optimum, suggesting a constraint imposed on the evolution of dichromatism, potentially through predator‐mediated selection. Alternatively, the transition to benthic habitats coincides with greater variability in the levels of dichromatism that surround a higher optimum, likely due to relaxation of the predator‐mediated selection and heterogeneous microhabitat dependent selection regimes. These results suggest a complex interaction of sexual selection with potentially two mechanisms of natural selection, predation and sensory drive, that influence the evolution of diverse male nuptial coloration in darters.     

The contribution of structural-, psittacofulvin- and melanin-based colouration to sexual dichromatism in Australasian parrots

Colour ornamentation in animals is exceptionally diverse, but some colours may provide better signals of individual quality or more efficient visual stimuli and, thus, be more often used as sexual signals. This may depend on physiological costs, which depend on the mechanism of colour production (e.g. exogenously acquired colouration in passerine birds appears to be most sexually dichromatic). We studied sexual dichromatism in a sample of 27 Australasian parrot species with pigment- (melanin and psittacofulvin) and structural-based colouration, to test whether some of these types of colouration are more prominent in sexual ornamentation. Unlike passerines, in which long wavelength colouration (yellow to red) usually involves exogenous and costly carotenoid pigments, yellow to red colouration in parrots is based on endogenously synthesized psittacofulvin pigments. This allows us to assess whether costly exogenous pigments are necessary for these plumage colours to have a prominent role in sexual signalling. Structural blue colouration showed the largest and most consistent sexual dichromatism, both in area and perceptually relevant chromatic differences, indicating that it is often ornamental in parrots. By contrast, we found little evidence for consistent sexual dichromatism in melanin-based colouration. Unlike passerines, yellow to red colouration was not strongly sexually dichromatic: although the area of colouration was generally larger in males, colour differences between the sexes were on average imperceptible to parrots. This is consistent with the idea that the prominent yellow to red sexual dichromatism in passerines is related to the use of carotenoid pigments, rather than resulting from sensory bias for these colours.     

Sexual dimorphism and dichromatism in Steere's Liocichla (Liocichla steerii)

ABSTRACT.   Although sexual differences in birds can be extreme, differences between males and females in body size and plumage color are more subtle in many species. We used a genetic-based approach to determine the sex of male and female Steere's Liocichla ( Liocichla steerii ) and examine the degree of size dimorphism and plumage dichromatism in this apparently monomorphic species. We found that males were significantly larger than females. In addition, Steere's Liocichla have a prominent yellow plumage patch on the lores that was significantly larger in males than females for both live birds and museum specimens. We also used reflectance spectrometry to quantify the color of the yellow-green breast feathers of Steere's Liocichla and found no significant differences between males and females in brightness, intensity, saturation, or hue. However, females tended to have brighter breast plumage, particularly at long wavelengths. Collectively, these color variables were useful in discriminating birds according to sex when used in a discriminant function analysis. Our study suggests that sexual selection may be more widespread than once assumed, even among birds considered monomorphic, and emphasizes the need for additional data from tropical and subtropical species.     

Melanin‐based sexual dichromatism in the Western Palearctic avifauna implies darker males and lighter females

Melanins are the most common pigments providing coloration in the plumage and bare skin of birds and other vertebrates. Numerous species are dichromatic in the adult or definitive plumage, but the direction of this type of sexual dichromatism (i.e. whether one sex tends to be darker than the other) has not been thoroughly investigated. Using color plates, we analysed the presence of melanin‐based color patches in 666 species belonging to 69 families regularly breeding in the Western Palearctic. Sexual dichromatism based on melanins in at least one integumentary part involved 205 (30.7%) species. The body parts contributing more frequently to dichromatism were the dorsal areas, head and breast, whereas the less dichromatic body parts were the belly and the exposed integumentary parts (i.e. bill and legs). Regarding the phylogenetic spread of dichromatisms, 37 (53.6%) families contained at least one species with melanin‐based sexual dimorphism in the definitive adult plumage. As for the direction of the color difference, males are darker than females in a majority of species, meaning that males tend to produce more eumelanin and females tend to synthesize more pheomelanin. This survey has revealed the high prevalence of melanins in the emergence of sexual dichromatism in birds, at least in the Western Palearctic. Whether the described pattern is due to sexual selection promoting more conspicuous males or to natural selection for more cryptic females remains to be determined. Given that pheomelanin synthesis concurrently consumes the antioxidant glutathione but may also reduces toxic cysteine, sex‐biased physiological factors should also be given consideration in the evolution of bird plumages.     

“Reverse” differential staining of sister chromatids

A technique for the differential staining of sister chromatids with basic fuchsin is described. The resulting staining pattern is the reverse of that obtained with a similar technique using Giemsa dye.     

Background matching,disruptive coloration,and differential use of microhabitats in two neotropical grasshoppers with sexual dichromatism

Cryptic coloration is an adaptative defensive mechanism against predators. Color patterns can become cryptic through background coloration‐matching and disruptive coloration. Disruptive coloration may evolve in visually heterogeneous microhabitats, whereas background matching could be favored in chromatically homogeneous microhabitats. In this work, we used digital photography to explore the potential use of disruptive coloration and background matching in males and females of two grasshopper species of the Sphenarium genus in different habitats. We found chromatic differences in the two grasshopper species that may be explained by local adaptation. We also found that the females and males of both species are dichromatic and seem to follow different color cryptic strategies, males are more disruptive than females, whereas females have a high background matching with less disruptive elements. The selective pressures of the predators in different microhabitats and the differences in mobility between sexes may explain the color pattern divergence between females and males. Nevertheless, more field experiments are needed in order to understand the relative importance of disruptive and background matching coloration in the evolution of sexual dichromatism in these grasshoppers.     

Flight initiation distances in relation to sexual dichromatism and body size in birds from three continents

Predators exert strong selection pressures on their prey. Prey would therefore benefit by adjusting their behaviour to the risk of predation, while predators conversely would benefit from adjusting their behaviour to that of their prey. Extravagant ornamentation has evolved to attract mates and/or successfully compete with conspecifics of the same sex to secure high mating success, even if that occurs at a cost of increased risk of predation. Thus, sexually dichromatic species may be more susceptible to predation than sexually monochromatic species, and the presence of compensation is indicative of such species being more vulnerable. If extravagant ornamentation is costly in terms of predation risk, then we should expect sexually dichromatic species to have longer flight initiation distances (FID) than sexually monochromatic species. If ornamentation is acquired as a handicap with only individuals in prime condition being able to display with the smallest viability cost, we should expect sexually dichromatic individuals to have shorter FID than sexually monochromatic individuals. Such differences among individuals should, on an evolutionary time scale, translate into differences in FID being related to differences in sexual dichromatism among species. We investigated the relationship between FID and sexual dichromatism in phylogenetic analyses, while accounting for effects of continent (Australia, North America, and Europe), body mass, the interaction between sexual dichromatism and body mass and the interaction between sexual dichromatism and continent. In an analysis of 447 species we found shorter FID in sexually dichromatic than in sexually monochromatic species (consistent with the handicap hypothesis because sexually dichromatic species took greater risks), especially so at large body size. FID differed among continents and the relative difference in FID between sexually monochromatic and sexually dichromatic species was larger in Europe than in Australia and North America. These differences among continents may be attributed to latitudinal effects of predation. These findings are important for current ideas about the evolution of secondary sexual characters because they imply covarying continental differences in predation, especially for large bodied sexually dichromatic species.     

Sneaker “jack” males outcompete dominant “hooknose” males under sperm competition in Chinook salmon (Oncorhynchus tshawytscha)

In a variety of taxa, males deploy alternative reproductive tactics to secure fertilizations. In many species, small “sneaker” males attempt to steal fertilizations while avoiding encounters with larger, more aggressive, dominant males. Sneaker males usually face a number of disadvantages, including reduced access to females and the higher likelihood that upon ejaculation, their sperm face competition from other males. Nevertheless, sneaker males represent an evolutionarily stable strategy under a wide range of conditions. Game theory suggests that sneaker males compensate for these disadvantages by investing disproportionately in spermatogenesis, by producing more sperm per unit body mass (the “fair raffle”) and/or by producing higher quality sperm (the “loaded raffle”). Here, we test these models by competing sperm from sneaker “jack” males against sperm from dominant “hooknose” males in Chinook salmon. Using two complementary approaches, we reject the fair raffle in favor of the loaded raffle and estimate that jack males were ~1.35 times as likely as hooknose males to fertilize eggs under controlled competitive conditions. Interestingly, the direction and magnitude of this skew in paternity shifted according to individual female egg donors, suggesting cryptic female choice could moderate the outcomes of sperm competition in this externally fertilizing species.     

Sexually selected dichromatism in the hihi Notiomystis cincta: multiple colours for multiple receivers

Why do some bird species show dramatic sexual dichromatism in their plumage? Sexual selection is the most common answer to this question. However, other competing explanations mean it is unwise to assume that all sexual dichromatism has evolved by this mechanism. Even if sexual selection is involved, further work is necessary to determine whether dichromatism results from competition amongst rival males, or by female choice for attractive traits, or both. Here, we test whether sexually dichromatic hihi (Notiomystis cincta) plumage is currently under sexual selection, with detailed behavioural and genetic analyses of a free‐living island population. Bateman gradients measured for males and females reveal the potential for sexual selection, whilst selection gradients, relating reproductive success to specific colourful traits, show that there is stabilizing selection on white ear tuft length in males. By correlating colourful male plumage with different components of reproductive success, we show that properties of yellow plumage are most likely a product of male–male competition, whilst properties of the black and white plumage are an outcome of both male–male competition and female choice. Male plumage therefore potentially signals to multiple receivers (rival males and potential mates), and this may explain the multicoloured appearance of one of the most strikingly dichromatic species in New Zealand.     

Ontogenetic colour change signals sexual maturity in a non-territorial damselfly

Conspicuous colouration increases male reproductive success through female preferences and/or male–male competition. Despite the advantages of conspicuous colouration, inconspicuous male morphs can exist simultaneously in a population due to genetic diversity, condition dependence or developmental constraints. We are interested in explaining the male dichromatism in Xanthagrion erythroneurum damselflies. We reared these damselflies in outdoor insectaries under natural conditions and showed that this species undergoes ontogenetic colour changes. The younger males are yellow and change colour to red 6–7 days after their emergence. We took red and yellow male reflectance spectra and found that red males are brighter than yellow males. Next, we aimed to determine whether ontogenetic colour change signals sexual maturity with field observations and laboratory experiments. Our field observational data showed that red males are in higher abundance in the breeding territory, and they have a higher mating frequency than yellow males. We confirmed these field observations by enclosing a red and a yellow male with two females and found that yellow males do not mate in presence of red males. To determine whether colour change signals sexual maturity, we measured mating success of males before and after colour changes by enclosing a single male at different age (day 3-day 7) and colour (yellow, intermediate and red) with a single female in a mating cage. Males did not mate when yellow but the same male mated after it changed colour to red, suggesting the ontogenetic colour change signals sexual maturity in this species. Our study shows that male dichromatism can be age-dependent and ontogenetic colour change can signal age and sexual readiness in non-territorial insects.     

Behaviors associated with mating success in the “purring” wolf spider Gladicosa gulosa

Studies of sexual selection in animal systems frequently use courtship and receptivity behaviors as metrics for mating success, and this is generally built upon data associating such behaviors with mating. Spider systems are becoming increasingly popular in sexual selection research, and the wolf spider Gladicosa gulosa has the potential to help answer unique questions in animal communication. However, this species is understudied, and no quantitative data exist that demonstrate which male and female behaviors may be indicators of mating success. We paired male and female G. gulosa and recorded the behaviors involved in their courtship and mating. We found a number of significant behaviors from both males and females, and we suggest which behaviors/variables are perhaps most predictive of mating success. Beyond building a data‐based foundation for future work, we also suggest that the relationship of certain male and female variables may shed light on some of the more unique features of this species.     

Sex‐specific selection on plant architecture through “budget” and “direct” effects in experimental populations of the wind‐pollinated herb,Mercurialis annua

Sexual selection may contribute to the evolution of plant sexual dimorphism by favoring architectural traits in males that improve pollen dispersal to mates. In both sexes, larger individuals may be favored by allowing the allocation of more resources to gamete production (a “budget” effect of size). In wind‐pollinated plants, large size may also benefit males by allowing the liberation of pollen from a greater height, fostering its dispersal (a “direct” effect of size). To assess these effects and their implications for trait selection, we measured selection on plant morphology in both males and females of the wind‐pollinated dioecious herb Mercurialis annua in two separate experimental common gardens at contrasting density. In both gardens, selection strongly favored males that disperse their pollen further. Selection for pollen production was observed in the high‐density garden only, and was weak. In addition, male morphologies associated with increased mean pollen dispersal differed between the two gardens, as elongated branches were favored in the high‐density garden, whereas shorter plants with longer inflorescence stalks were favored in the low‐density garden. Larger females were selected in both gardens. Our results point to the importance of both a direct effect of selection on male traits that affect pollen dispersal, and, to a lesser extent, a budget effect of selection on pollen production.     

Extreme sequential polyandry insures against nest failure in a frog

Sequential polyandry may evolve as an insurance mechanism to reduce the risk of choosing a mate that is infertile, closely related, genetically inferior or incompatible, but polyandry also might insure against nest failure in unpredictable environments. Most animals are oviparous, and in species where males provide nest sites whose quality varies substantially and unpredictably, polyandrous females might insure offspring success by distributing their eggs across multiple nests. Here, we test this hypothesis in a wild population of an Australian terrestrial toadlet, a polyandrous species, where males construct nests and remain with broods. We found that females partitioned their eggs across the nests of two to eight males and that more polyandrous females gained a significant increase in mean offspring survivorship. Our results provide evidence for the most extreme case of sequential polyandry yet discovered in a vertebrate and also suggest that insurance against nest failure might favour the evolution of polyandry. We propose that insurance against nest failure might be widespread among oviparous taxa and provide an important explanation for the prevalence of sequential polyandry in nature.     

“Active” and “passive” ecological restoration strategies in meta‐analysis

One of the means of creating a more robust methodology for ecological restoration involves reducing the gap between ecological theory and restoration practices. A common strategy to do so is using meta‐analysis to understand key drivers of restoration outcomes. “Active” and “passive” is a dichotomy often used to separate restoration strategies in such meta‐analyses. We investigate previously raised concerns about selection bias and subjective categorization of strategies. We promote a paired experimental design in future empirical research and propose the use of three categories of restoration strategy in lieu of “passive” and “active” to alleviate inconsistency in definitions and categorization.     

Migration and the evolution of sexual dichromatism: evolutionary loss of female coloration with migration among wood-warblers

The mechanisms underlying evolutionary changes in sexual dimorphism have long been of interest to biologists. A striking gradient in sexual dichromatism exists among songbirds in North America, including the wood-warblers (Parulidae): males are generally more colourful than females at northern latitudes, while the sexes are similarly ornamented at lower latitudes. We use phylogenetically controlled comparative analysis to test three non-mutually exclusive hypotheses for the evolution of sexual dichromatism among wood-warblers. The first two hypotheses focus on the loss of female coloration with the evolution of migration, either owing to the costs imposed by visual predators during migration, or owing to the relaxation of selection for female social signalling at higher latitudes. The third hypothesis focuses on whether sexual dichromatism evolved owing to changes in male ornamentation as the strength of sexual selection increases with breeding latitude. To test these hypotheses, we compared sexual dichromatism to three variables: the presence of migration, migration distance, and breeding latitude. We found that the presence of migration and migration distance were both positively correlated with sexual dichromatism, but models including breeding latitude alone were not strongly supported. Ancestral state reconstruction supports the hypothesis that the ancestral wood-warblers were monochromatic, with both colourful males and females. Combined, these results are consistent with the hypotheses that the evolution of migration is associated with the relaxation of selection for social signalling among females and that there are increased predatory costs along longer migratory routes for colourful females. These results suggest that loss of female ornamentation can be a driver of sexual dichromatism and that social or natural selection may be a stronger contributor to variation in dichromatism than sexual selection.