Sexual dichroism and pigment localization in the wing scales of Pieris rapae butterflies

The beads in the wing scales of pierid butterflies play a crucially important role in wing coloration as shown by spectrophotometry and scanning electron microscopy (SEM). The beads contain pterin pigments, which in Pieris rapae absorb predominantly in the ultraviolet (UV). SEM demonstrates that in the European subspecies Pieris rapae rapae, both males and females have dorsal wing scales with a high concentration of beads. In the Japanese subspecies Pieris rapae crucivora, however, only the males have dorsal wing scales studded with beads, and the dorsal scales of females lack beads. Microspectrophotometry of single scales without beads yields reflectance spectra that increase slightly and monotonically with wavelength. With beads, the reflectance is strongly reduced in the UV and enhanced at the longer wavelengths. By stacking several layers of beaded scales, pierid butterflies achieve strong colour contrasts, which are not realized in the dorsal wings of female P. r. crucivora. Consequently, P. r. crucivora exhibits a strong sexual dichroism that is absent in P. r. rapae.     

Aggressive Behavior and Courtship Rejection in Brightly and Plainly Colored Female Keeled Earless Lizards (Holbrookia propinqua)

Relationships between bright secondary sexual coloration and behavior were studied in female Holbrookia propinqua, which develop striking orange and yellow colors during the breeding season. In tethered introduction studies, brightly colored females performed aggressive courtship rejection behaviors toward conspecific males; plainly colored females were not aggressive toward males, but attempted to avoid them. Responses of females of the two color patterns toward conspecific females of both color phases were not detectably different. Experimental introductions of lizards with coloration modified by paint showed that females of both color patterns recognize any other lizard bearing the bright female colors as female, regardless of actual sex. Both the orange and yellow components were shown to contribute to sex recognition. The yellow component alone allowed accurate sex identification, but only half the females responded to males painted with only the orange female component as if they were females. Because females did not behave differently toward other females on the basis of coloration, the hypothesis that bright coloration evolved as an adaptive signal between females is rejected. The dark ventrolateral stripes of male and plainly colored females did not appear to affect intraspecific social responses by females.     

Female mating biases for bright ultraviolet iridescence in the butterfly Eurema hecabe (Pieridae)

Exaggerated male-limited coloration is widespread among butterflies,yet convincing demonstrations of intraspecific mating preferencesfor signal brightness and/or chromaticity are relatively rarein this group. Here, I couple behavioral experiments involvingmanipulations of ambient light environments and male reflectancepatterns with observation of wild mating patterns to investigatevisual mating biases in the large grass yellow (Eurema hecabe).Males in this species possess exaggerated, limited-view ultraviolet(UV) iridescence across most of their dorsal wing surface thathas putative sexual signaling function. In the first experiment,conducted in small (0.7–m³) cages, individuals were significantlyless likely to copulate when the UV portion of natural ambientillumination (i.e., 300–400 nm) was strongly reduced.In 2 subsequent experiments, conducted under full-spectrum sunlightin small and large (5 x 6 x 4 m) cages, males with their UVsignal artificially dulled by 25% consistently copulated withfewer, and smaller, females than sham-control individuals. Importantly,the manipulated levels of UV brightness in these experimentsfall well within the naturally occurring bounds of variationin male UV reflectance. These findings therefore unanimouslysupport the presence of a UV signal–based female bias.In apparent contrast, comparison of 161 in-copula and 188 free-flyingmales from a high-density field assemblage revealed that copulatingmales were significantly older and henceforth actually possessed(subtly) less UV bright wings. Copulating male UV brightnesswas, however, positively related to the size of their mate,which echoes the experimental findings and may represent a signatureof mutual mate choice. I discuss these results in light of thefull complexities of the butterfly mating system and the potentialsignaling value of iridescent coloration in butterflies andanimals generally.     

The role of colour in signalling and male choice in the agamid lizard Ctenophorus ornatus

Bright coloration and complex visual displays are frequent and well described in many lizard families. Reflectance spectrometry which extends into the ultraviolet (UV) allows measurement of such coloration independent of our visual system. We examined the role of colour in signalling and mate choice in the agamid lizard Ctenophorus ornatus. We found that throat reflectance strongly contrasted against the granite background of the lizards' habitat. The throat may act as a signal via the head-bobbing and push-up displays of C. ornatus. Dorsal coloration provided camouflage against the granite background, particularly in females. C. ornatus was sexually dichromatic for all traits examined including throat UV reflectance which is beyond human visual perception. Female throats were highly variable in spectral reflectance and males preferred females with higher throat chroma between 370 and 400 nm. However, female throat UV chroma is strongly correlated to both throat brightness and chest UV chroma and males may choose females on a combination of these colour variables. There was no evidence that female throat or chest coloration was an indicator of female quality. However, female brightness significantly predicted a female's laying date and, thus, may signal receptivity. One function of visual display in this species appears to be intersexual signalling, resulting in male choice of females.     

Reflectance spectra and mating patterns support intraspecific mimicry in the colour polymorphic damselfly <Emphasis Type="Italic">Ischnura elegans</Emphasis>

Coexistence of female colour morphs in animal populations is often considered the result of sexual conflict, where polymorphic females benefit from reduced male sexual harassment. Mate-searching males easily detect suitable partners when only one type of female is present, but become challenged when multiple female morphs coexist, which may result in frequency-dependent mate preferences. Intriguingly, in damselflies, one female morph often closely resembles the conspecific male in body coloration, which has lead to hypotheses regarding intra-specific male-mimicry. However, few studies have quantitatively evaluated the correspondence between colour reflectance spectra from males and male-like females, relying instead on qualitative visual assessments of coloration. Using colour analyses of reflectance spectra, we compared characteristics of the body coloration of ontogenetic male and female colour morphs of the damselfly Ischnura elegans. In addition, we evaluated whether males appear to (1) discriminate between immature and mature female colour morphs, and (2) whether male-like females experience reduced male mating attention and low mating frequencies as predicted from male-mimicry. Spectral reflectance data show that immature female morphs differ substantially in coloration from mature individuals. Mating frequencies were much lower for immature than mature female morphs. For the male-like female morph, measures of colour were statistically indistinguishable from that of both immature and mature conspecific males. Mating frequencies of male-like females were lower than those of other mature female morphs under field and experimental conditions. Together, our results indicate that males may use the observed spectral differences in mate choice decisions. Furthermore, male-like females may be regarded as functional mimics that have reduced attractiveness and lowered rates of sexual harassment by mate-searching males.     

The Role of Wing Pigmentation, UV and Fluorescence as Signals in a Neotropical Damselfly

Pigmentation patterns, ultraviolet reflection and fluorescent emission are often involved in mate recognition and mate quality functions in many animal taxa. We investigated the role of wing ultra-violet reflection, fluorescence emission, and pigmentation on age and sexual signals in the damselfly Mnesarete pudica. In this species, wings are sexually dimorphic in colour and exhibit age dependency: males and females show a smoky black colouration when young, turning red in mature males while it turns brown in females. First, we investigated wing UV patterns through reflectance and emission spectra. Second, behavioural experiments were undertaken to show male and female responses to manipulated wing pigmentation and experimentally reduced UV (UV-). Reflectance spectra of the wings of juvenile and mature males and females were used to show the differences between controls and individuals with manipulated colouration used in the behavioural experiment. UV-reduced, females with wings painted red, and control males and females were tethered and presented to conspecific males and females, and their behavioral responses were recorded. The male red wing pigmentation and females with red wings elicited an aggressive response in territorial males and a sexual response in females. Both males and females showed neutral responses towards individuals with reduced UV. Wing signals of juvenile individuals also provoked neutral responses. These results suggest that UV, together with pigmentation, plays a role during mate recognition in males and females. Other than butterflies and spiders, it seems that fluorescence signals and UV reflectance can also be part of communication in odonates.     

Female butterflies prefer males bearing bright iridescent ornamentation

Butterflies are among nature's most colorful animals, and provide a living showcase for how extremely bright, chromatic and iridescent coloration can be generated by complex optical mechanisms. The gross characteristics of male butterfly colour patterns are understood to function for species and/or sex recognition, but it is not known whether female mate choice promotes visual exaggeration of this coloration. Here I show that females of the sexually dichromatic species Hypolimnas bolina prefer conspecific males that possess bright iridescent blue/ultraviolet dorsal ornamentation. In separate field and enclosure experiments, using both dramatic and graded wing colour manipulations, I demonstrate that a moderate qualitative reduction in signal brightness and chromaticity has the same consequences as removing the signal entirely. These findings validate a long-held hypothesis, and argue for the importance of intra- versus interspecific selection as the driving force behind the exaggeration of bright, iridescent butterfly colour patterns.     

Resource-mediated condition dependence in sexually dichromatic butterfly wing coloration

Theory predicts that traits subject to strong sexual selection should evolve to be more exaggerated and developmentally integrated than nonsexual traits, thus leading to heightened condition dependence. Until recently, however, efforts to evaluate this prediction have suffered from either a purely correlational (nonmanipulative) approach, or from using manipulations of doubtful ecological relevance. Here I address these issues by integrating observation and manipulation to study condition- and sex-related color variation in a butterfly. The focal species, Eurema hecabe (Pieridae), possesses three sexually homologous and morphogenetically discrete dorsal wing color elements-coherently scattered ultraviolet (UV), pteridine yellow, and melaninic black. The UV is most strongly sexually selected, and is also the only color element with restricted distribution across female wings. Condition dependence and sexual dichromatism were pervasive, characterizing all color traits except the melanic black, and acting such that low condition males resembled high condition females. Although female coloration tended to exhibit greater phenotypic variation, size-scaled UV was more variable and condition dependent in males. Importantly, manipulation of larval resources was sufficient to closely reconstruct the extent and patterns of field-observed phenotypic variation in condition, and color trait expression, which implicates larval resource acquisition as a primary driver of condition dependence. These results support theories regarding phenotypic variation in sexually selected traits.     

Wing colors of Chrysozephyrus butterflies (Lepidoptera; Lycaenidae): ultraviolet reflection by males

Wing colors of the four species of Chrysozephyrus butterflies were analyzed by a spectrophotometer. As the dorsal wing surface of males showed a strong reflectance when the specimen was tilted, measurements were made by the tilting method. The dorsal wing surface of males which appears green to the human eye reflected UV (315-350 nm) as well as green light (530-550 nm). The reflectance rate of UV to visible green light varied among species with a higher rate for C. hisamatsusanus and C. ataxus, and a lower rate for C. smaragdinus and C. brillantinus. The peak wavelength and the peak height did not shift when the specimen was exposed to direct sunlight at least for 16 hr. Artificial removal of scales by scratching the wing surface decreased reflectance. Blue marks on the forewings of C. brillantinus, C. hisamatsusanus and C. ataxus females reflected UV to visible light of short wavelength, and orange marks on the dorsal surface of the forewing and the ventral surface of the hindwing of C. samaragdinus females showed a higher reflectance at longer wavelengths.     

Blue structural coloration of male eastern bluebirds Sialia sialis predicts incubation provisioning to females

Plumage coloration has been suggested to serve as an honest signal of benefits that males provide to females. One benefit proposed for females that choose to mate with elaborately colored males is that such males might provide more food resources to the females. To test this hypothesis, we investigated the relationship between the rates at which males provisioned incubating females and the structural ultraviolet (UV)/blue coloration and melanin-based chestnut coloration of male eastern bluebirds Sialia sialis , a sexually dichromatic songbird in which coloration has been shown to be sexually selected in males. We found that males with brighter UV/blue coloration provisioned incubating females more often than did drabber males. Melanin coloration, however, was not correlated with provisioning rates. Moreover, provisioning rates were correlated with the length of time that females spent off the nest, indicating an important benefit of increased male provisioning. These data support the hypothesis that female bluebirds receive direct resource benefits by pairing with males with bright structural coloration.     

Parental care and UV coloration in blue tits: opposite correlations in males and females between provisioning rate and mate’s coloration

Parental investment and sexually‐selected signals can be intimately related, either because the signals indicate the amount of investment that an individual is prepared to make, and hence its value as a mate (the ‘good parent process’), or because individuals are selected to vary their own investment in relation to their mate’s signals (‘differential allocation’ or ‘reproductive compensation’). Correlations between parental investment and the sexually selected signals of both an individual and its mate are therefore of central interest in sexual selection. Blue tits Cyanistes caeruleus are an ideal study species to investigate such correlations because they provide substantial amounts of biparental care and possess sexually‐selected structural UV coloration that seems to signal attractiveness in both sexes. We investigated whether feeding rates of male and female blue tits were correlated with either their own or their mate’s UV coloration, and whether any such correlation was affected by the sex ratio of the brood. We also investigated whether any such correlations were reflected in offspring phenotype. Feeding rates were not correlated with either sex of parent’s own UV coloration. However, they were correlated with the mate’s UV coloration, but in opposite directions in males and females: females had higher feeding rates when mated to bright UV males, implying differential allocation, while males had lower feeding rates when mated to bright UV females, implying reproductive compensation. These relationships were unaffected by the sex ratio of the brood. In addition, fledgling tarsus length, but not mass, was related to male UV coloration, and to female UV coloration in interaction with male age. These results suggest that both male and female attractiveness influence parental investment of the mate, and that this in turn affects offspring phenotype. We found no evidence for differential sex allocation.     

No Evidence that Male Choice Contributes to the Maintenance of a Shared, Sex-Limited Trait in Mimetic and Non-mimetic Female Tiger Swallowtail Butterflies, Papilio glaucus

In animal species that have morphological polymorphisms maintained by unique or divergent selection pressures, understanding the preservation of shared traits is important for identifying the factors that are influencing overall evolutionary processes. In the Eastern tiger swallowtail butterfly, Papilio glaucus, females are dimorphic. One morph (‘dark-morph’) is mostly black and mimics the toxic pipevine swallowtail, Battus philenor. These females have large amounts of blue coloration on the dorsal hind wings that enhances their mimetic resemblance. Conversely, the alternate female type (‘yellow-morph’) is similar to males in coloration with the exception of extensive dorsal blue coloration, comparable to dark-morph females. Such coloration is almost completely absent in males. We examined dorsal blue coloration in P. glaucus to determine if mimetic resemblance in dark morphs is predominantly responsible for the maintenance of dorsal blue color in both female types, or whether mate recognition and/or sexual selection by males has a stronger influence on this trait. We measured the relative amount and variance of dorsal and ventral blue coloration in females of both color morphs, as well as males. We also compared these measurements to similar ones taken in the sister species, P. canadensis (which does not exhibit female dimorphism). Lastly, we investigated mate recognition and preferences of wild males. Our results suggest that mimetic resemblance may be more important than sexual selection for sustaining dorsal blue coloration in dark-morph females and that yellow-morphs could have elevated levels of blue due to currently unknown genetic associations. Although trait correlation between sexes is common, intrasexual trait correlation in a sex-limited, polymorphic species has not been frequently observed.     

Bare-Part Color in Female Budgerigars Changes from Brown to Structural Blue following Testosterone Treatment but Is Not Strongly Masculinized

Whereas several studies have shown that experimentally increased levels of the androgenic steroid testosterone can affect female behavior, fewer studies have focused on the activational effects of exogenous testosterone on female morphology. With respect to colorful displays in birds, almost exclusively the effects of testosterone manipulation on female carotenoid-based colorations have been studied. Other color types such as structural colors (i.e. UV, blue and violet colors that result from differential light reflection in the nanostructures of the tissue) remain largely unstudied. Here, we investigated the short- and long-term effects of exogenous testosterone on the expression of structural bare-part coloration in female budgerigars, Melopsittacus undulatus. In this parrot species, bare-part coloration is expressed in the cere, a structure over the beak which is brown in females and structural blue in males. We experimentally increased plasma testosterone levels in testosterone-treated females (T-females) compared to controls (C-females) and we performed weekly spectrophotometric measurements of the cere for five weeks after implantation and one measurement after ten weeks. We also estimated the extent to which testosterone masculinized female cere color by comparing the experimental females with untreated males. We found significant effects of testosterone on cere color from week four after implantation onwards. T-females expressed significantly bluer ceres than C-females with higher values for brightness and UV reflectance. T-female cere color, however, remained significantly less blue than in males, while values for brightness and UV reflectance were significantly higher in T-females than in males. Our quantitative results show that exogenous testosterone induces the expression of structural blue color in females but does not strongly masculinize female cere coloration. We provide several potential pathways for the action of testosterone on structural color.     

Female secondary sexual coloration and sex recognition in the keeled earless lizard, Holbrookia propinqua

Relationships between male social behaviour and female reproductive coloration in the iguanid lizard Holbrookia propinqua were examined by introduction of tethered non-resident lizards into the territories of adult males. Introduced lizards were plainly coloured females, females having bright reproductive coloration, adult males, plain females painted to resemble brightly coloured females, bright females painted to mimic plain females, and males painted with the bright yellow and orange secondary sexual coloration of females. Resident males courted all unpainted females, despite being aggressively rejected by the brightly coloured ones. They also courted all but two females in each of the painted groups. All unaltered non-resident males were challenged, attacked or subjected to other aggressive behaviour by residents, but all non-resident males painted to resemble bright females were courted. This differential treatment is highly significant. It clearly demonstrates that the bright female pigmentation functions in sex recognition, identifying a female to males. Other stimuli, especially pheromonal and behavioural cues, may contribute to sex recognition. Because sex recognition alone seems insufficient to account for the evolution of bright female coloration in H. propinqua and several other iguanids, several proposed additional functions are discussed.     

Structural coloration and sexual selection in the barn swallow Hirundo rustica

Structural coloration has been hypothesized to play a role insexual selection, and we tested whether this was the case ina field study of the barn swallow Hirundo rustica. The dorsaliridescent plumage of barn swallows has a strong reflectancein the ultraviolet (UV) region, with adult males on averagereflecting 8-9% more than adult females, as revealed by a 2-yearstudy in southwestern Spain. The correlation between structural coloration (described by the reflectance in the UV part of thespectrum, UV chroma and blue chroma) and three other secondarysexual characters significantly associated with male matingsuccess (tail length, tail asymmetry, and red facial coloration)was weak and generally nonsignificant. Nor was there a significantrelationship between color parameters and body condition. Wetested for an association between structural coloration of the dorsal plumage and sexual selection in a number of independenttests. Arrival date of males was not significantly relatedto color; there was no significant relationship between colorationand probability of survival or age; mated males did not havestronger reflectance than unmated males; and the duration ofthe premating period was not significantly related to color.Reproductive success was not significantly correlated withplumage coloration in males, nor was the feeding rate of offspringby brightly colored males higher than that of males with lessbright plumage. Given that sample sizes were large, and the power of statistical tests high, we conclude that current sexualselection on the coloration of the dorsal plumage in the barnswallow is, at best, weak.     

Ultraviolet and carotenoid‐based coloration in the viviparous lizard Zootoca vivipara (Squamata: Lacertidae) in relation to age,sex, and morphology

Lizards display structural and pigment‐based colorations, and their visual system is sensitive to wavelengths of 300–700 nm. However, few studies in squamate reptiles have quantified interindividual colour variation that includes the structural ultraviolet (UV) component (300–400 nm). In the present study, we investigated variability of a ventral UV/yellow–red ornamentation in the common lizard Zootoca vivipara, including an analysis of spatial distribution, as well as sex and age differences. We also investigated whether the expression of coloration is related to body size and condition. Our analyses revealed two distinct patches: a gular patch with a strong UV reflectance and a belly patch with a dominant yellow–red reflectance. Males displayed a less saturated throat coloration with higher UV chroma and UV hue, and had a redder but duller belly coloration than females. Yearlings had less elaborate ornaments than adults, although they already displayed a yellow–red sexual dichromatism on the belly. UV sexual dichromatism was only apparent in adults as a result of a weaker UV reflectance in females, suggesting potential fitness costs of a bright UV coloration in that sex. Different colour traits were related to body size in both sexes, as well as to body condition in males. We discuss the potential evolutionary scenarios leading to the maintenance of this ornament in common lizards. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 128–141.     

Male-specific Iridescent Coloration in the Pipevine Swallowtail (Battus philenor) is Used in Mate Choice by Females but not Sexual Discrimination by Males

We investigated the potential roles in behavioral interactions of sexually dichromatic iridescent blue coloration found on the dorsal hindwing of male Pipevine Swallowtails (Battus philenor). Behavioral experiments in a large enclosure addressed whether male dorsal hindwing coloration mediated sexual recognition by males, female choice of mates, or both. Models presented to males in the enclosure produced responses that show that males discriminate females from other males using chemical but not visual cues. In contrast, ablation of male dorsal iridescence significantly reduced male mating success with virgin females in the enclosure. The results support the hypothesis that male-specific iridescent coloration in this species is a signal that is important for females in assessing either male species identity or quality in intersexual interactions.     

Conspicuous blue tails,dorsal pattern morphs and escape behaviour in hatchling Iberian wall lizards (Podarcis hispanicus)

Predation has profound effects on the phenotypes of animal prey and, in lizards, the relationship between coloration and antipredatory behaviour has been studied in depth. However, studies that address the relationships between dorsal patterns and tail coloration with escape behaviour in polymorphic lizards are absent in the literature. We describe dorsal morphs and measured tail coloration and escape behaviour in hatchling Iberian wall lizards, Podarcis hispanicus, a species with a previously undescribed female‐restricted dorsal polymorphism (reticulated‐blotched males, and either striped or reticulated‐blotched females) and juvenile tails with conspicuous blue coloration, which is probably used to divert predator attacks towards the autotomizable tail. Overall we provide evidence for the existence of sexual dimorphism in tail ultraviolet reflectance between reticulated females and males, with striped females being intermediate. We identified sex/dorsal morph, body size and tail brightness as predictors of different aspects of escape behaviour and suggest the existence of two alternative escape strategies between striped and reticulated‐blotched females that may be dependent on dorsal morph differences, independently of sex. Reticulated‐blotched females, and also males (all reticulated‐blotched), ran faster and spent less time paused than striped females, which might reflect an escape behaviour strategy based on endurance in striped females. In addition, lowland males displayed tail waving as a ‘last resort’ antipredator strategy that may be related to fatigue. We concluded that hatchling antipredatory behaviour is influenced by both dorsal pattern and tail conspicuousness. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 1094–1106.     

Color patterns and species recognition in four closely related species of Lake Malawi cichlid

We examined interspecific female mating preferences in fourclosely related species of cichlid belonging to the Pseudotropheuszebra species complex of Lake Malawi. These species differin coloration but are similar in other respects, suggestingthat male color patterns may be important to female mate choicein species recognition. To test this hypothesis, we presented females from each species with a choice of four males, one ofher own species and three others that were each of a differentspecies. We also gave each female a choice between the threeheterospecific males only. In all four species, females showeda significant preference for conspecific males in the four-waychoice and chose the male with the most similar color patternto the conspecific male in the three-way choice. These resultsare discussed with reference to the theory of sexual selectionon color patterns as a means of sympatric speciation in cichlids.     

Female Bicyclus anynana butterflies choose males on the basis of their dorsal UV-reflective eyespot pupils

Sexual and natural selection pressures are thought to shape the characteristic wing patterns of butterfly species. Here we test whether sexual selection by female choice plays a role in the maintenance of the male wing pattern in the butterfly Bicyclus anynana. We perform one of the most extensive series of wing pattern manipulations in butterflies, dissecting every component of the 'bulls-eye' eyespot patterns in both ventral and dorsal wing surfaces of males to test the trait's appeal to females. We conclude that females select males on the basis of the size and brightness of the dorsal eyespot's ultraviolet reflecting pupils. Pupil absence is strongly selected against, as are artificially enlarged pupils. Small to intermediate (normal sized) pupils seem to function equally well. This work contradicts earlier experiments that suggest that the size of dorsal eyespots plays a role in female choice and explains why male dorsal eyespots are very variable in size and often have indistinct rings of coloration, as the only feature under selection by females seems to be the central white pupil. We propose that sexual selection by female choice, rather than predator avoidance, may have been an important selective factor in the early stages of eyespot evolution in ancestral Lepidopteran lineages.