UV plumage color is an honest signal of quality in male budgerigars

Trait Variation and Spatiotemporal Dynamics across Avian Secondary Contact Zones

A secondary contact zone (SCZ) is an area where incipient species or divergent populations may meet, mate, and hybridize. Due to the diverse patterns of interspecific hybridization, SCZs function as field labs for illuminating the on-going evolutionary processes of speciation and the establishment of reproductive isolation. Interspecific hybridization is widely present in avian populations, making them an ideal system for SCZ studies. This review exhaustively summarizes the variations in unique traits within avian SCZs (vocalization, plumage, beak, and migratory traits) and the various movement patterns of SCZs observed in previous publications. It also highlights several potential future research directions in the genomic era, such as the relationship between phenotypic and genomic differentiation in SCZs, the genomic basis of trait differentiation, SCZs shared by multiple species, and accurate predictive models for forecasting future movements under climate change and human disturbances. This review aims to provide a more comprehensive understanding of speciation processes and offers a theoretical foundation for species conservation.

Canalised and plastic components of melanin-based colouration: a diet-manipulation experiment in house sparrows

Whether melanin-based plumage colouration accurately reflects a bird's quality is still controversial. To better understand potential mechanisms behind the observed variation in plumage colouration, we shifted our attention from a high-level expression of colour to low-level physiological phenomena by targeting the microstructure and pigment content of the feather. In a well-studied model system, the house sparrow (Passer domesticus), we combined an experimental manipulation of birds' physiological condition and availability of resources that are key to the production of the studied colouration (phenylalanine and tyrosine (PT). We found that feathers from sparrows fed with the control diet had noticeably lower values of brightness, suggesting a higher quality of the ornamental "blackness" in comparison to those sampled from birds fed with a PT-reduced diet. Electron paramagnetic resonance (EPR) spectroscopy detected higher melanin concentrations in samples from the control than the PT-reduced group. Our multi-level analysis excluded mechanisms such as barbule density and melanosomes' distribution, clearly pointing to the finest-level proxy of colour: the concentration of melanin in melanosomes themselves. Despite melanins being manufactured by birds endogenously, the efficiency of melanogenesis can be noticeably limited by diet. As a result, the birds' plumage colouration is affected, which may entail consequences in social signalling.

Intersexual differences in the exploratory behaviour of blue tits (Cyanistes caeruleus)

Breeding performance and survival are mechanisms through which animal personality affects fitness. Exploration behaviour (EB) is an easily obtainable personality trait that correlates to multiple life-history traits, thus it has been widely used in animal personality studies. Through in situ Novel Environment exploration tests, we assessed the exploration behaviour of blue tits (Cyanistes caeruleus) during two breeding seasons. A principal component analysis revealed two behavioural axes regarding the exploration behaviour: slow-fast and timid-daring. We found that male blue tits are significantly slower explorers than female blue tits, suggesting that evolutionary processes act unequally across sexes in our study area. To investigate these processes, we assessed the relationship between exploration behaviour and two fitness aspects: reproduction and survival. Slower-exploring males bred in nests with significantly shorter incubation periods, and timid males had higher local return ratios. Interactions between male and female EB revealed that disassortative pairs were more successful breeders than assortative ones. Further research should explore potential evolutionary implications of intersexual EB differences.

Heightened condition dependent expression of structural coloration in the faces, but not wings, of male and female flies

Structurally colored sexual signals are a conspicuous and widespread class of ornament used in mate choice, though the extent to which they encode information on the quality of their bearers is not fully resolved. Theory predicts that signaling traits under strong sexual selection as honest indicators should evolve to be more developmentally integrated and exaggerated than nonsexual traits, thereby leading to heightened condition dependence. Here, we test this prediction through examination of the sexually dimorphic faces and wings of the cursorial fly Lispe cana. Males and females possess structural UV-white and golden faces, respectively, and males present their faces and wings to females during close-range, ground-based courtship displays, thereby creating the opportunity for mutual inspection. Across a field-collected sample of individuals, we found that the appearance of the faces of both sexes scaled positively with individual condition, though along separate axes. Males in better condition expressed brighter faces as modeled according to conspecific flies, whereas condition scaled with facial saturation in females. We found no such relationships for their wing interference pattern nor abdomens, with the latter included as a nonsexual control. Our results suggest that the structurally colored faces, but not the iridescent wings, of male and female L. cana are reliable guides to individual quality and support the broader potential for structural colors as honest signals. They also highlight the potential for mutual mate choice in this system, while arguing for 1 of several alternate signaling roles for wing interferences patterns among the myriad taxa which bear them.

An integrative measure of cognitive performance, but not individual task performance, is linked to male reproductive output in budgerigars

Cognitive abilities such as learning and memory are key for survival and reproduction. Individuals with high cognitive abilities may be more successful at attracting mates and producing offspring. However, empirical tests of and evidence supporting this hypothesis remain scarce. We measured cognitive performance of male budgerigars in four tasks: problem solving, detour reaching, seed discrimination, and spatial memory. We then tested female choice for male cognition at three stages of the mating choice process: social pairing, extra-pair mating, and continued reproductive investment with a social mate. We also measured female reproductive output. We used an integrative measure of male cognitive performance that encapsulates performance across all tasks, the 'composite cognitive score' by summing performance on the four tasks. In the first stage, females did not choose their social mates based on any of the measures of male cognitive performance. In the second stage, however, males with higher composite cognitive scores sired and raised more offspring. In the third stage, females increased their reproductive investment after the first breeding attempt when paired with males with higher detour-reaching scores. These results suggest that female reproductive decisions may shape overall male cognitive performance.

The colour of success: does female mate choice rely on male colour change in the chameleon Furcifer pardalis?

Colour change is involved in various functions ranging from thermo- and hydroregulation to camouflage and communication. The role of colour change in communication has received increased attention over the past few decades, yet has been studied predominantly in the context of intrasexual competition. Here, we investigate the role of colour change in mate choice in an animal that can change its colour, the panther chameleon (Furcifer pardalis). We conducted behavioural experiments and colour analysis to investigate whether colour changes, including in the UV range, are involved in mate choice. This study presents evidence of female mate choice for specific aspects of colour change in courting males, both in the visible (i.e. human visible range: 400-700 nm) and the UV range. Females chose males exhibiting more saturation changes regardless of the body region and spectral range. In addition, females chose males showing fewer brightness changes at the level of the lateral line and males showing lower hue changes at the level of the bands and the interbands, in the visible range. At UV wavelengths, selected males showed more brightness changes and higher maximum brightness. These results suggest that male colour change is important in female mate choice in the panther chameleon.

Male ultraviolet reflectance and female mating history influence female mate choice and male mating success in a polyandrous lizard

Pre-copulatory female mate choice based on male ultraviolet (UV) coloration has been demonstrated in several vertebrate species; however, post-copulatory mechanisms have been largely overlooked. Here, we investigated female mate preference based on male UV coloration in the common lizard Zootoca vivipara, in which males display conspicuous UV coloration on their throat. During two successive years, we staged sequential mating trials between females and four different males with UV-reduced or control belly and throat coloration. We recorded pre-copulatory female behaviour, copulation behaviour and assigned paternity to all offspring. Females were more aggressive towards UV-reduced males and, during the second year, UV-reduced males had a lower probability of siring at least one egg (fertilization success) during the last mating trials. However, in the second year, copulation was shorter with control males. Altogether, our results suggest that females exert subtle pre-copulatory mate preference based on male UV ornaments and, conditional on the study year and female mating history, some degree of post-copulatory preference for UV-control males leading to differential male fertilization success. This study suggests that UV-based female mate choice may be more widespread than previously thought in vertebrates, and emphasizes the importance of using a study design well adapted to the species reproductive behaviour.

Structural colours reflect individual quality: a meta-analysis

Colourful ornaments often communicate salient information to mates, and theory predicts covariance between signal expression and individual quality. This has borne out among pigment-based signals, but the potential for 'honesty' in structural coloration is unresolved. Here, I synthesized the available evidence to test this prediction via meta-analysis and found that, overall, the expression of structurally coloured sexual signals is positively associated with individual quality. The effects varied by the measure of quality, however, with body condition and immune function reliably encoded across taxa, but not age nor parasite resistance. The relationship was apparent for both the colour and brightness of signals and was slightly stronger for iridescent ornaments. These results suggest diverse pathways to the encoding and exchange of information among structural colours while highlighting outstanding questions as to the development, visual ecology and evolution of this striking adornment.

Insulin-like growth factor 1 is related to the expression of plumage traits in a passerine species

Abstract

Avian plumage colors and ornaments are excellent models to study the endocrine mechanisms linking sexually selected traits and individual parameters of quality and condition. Insulin-like growth factor 1 (IGF-1) is an evolutionarily highly conserved peptide hormone. Its regulatory role in cell proliferation and differentiation and its high sensitivity to the nutritional state of individuals suggest it as an interesting candidate, possibly providing a link between body condition and individual capacity to grow elaborated ornamental features. We investigated whether IGF-1 levels during molting correlate with the expression of multiple ornaments in a sexually dichromatic passerine species, the bearded reedling (Panurus biarmicus). We collected blood samples of males and females shortly before the molting completed and measured the size and colors of ornamental traits. Our results indicate that in males, structural plumage colors, the size of the melanin-based ornament (beard), and tail length are independent traits. IGF-1 levels are associated with the length of the tail and the expression of male structural plumage components (UV coloration), but not the melanin-based ornament. In females, plumage color and tail length were independent traits, which were not related to IGF-1 levels. To the best of our knowledge, this study provides the first evidence that IGF-1 could play a role in the development of secondary sexual characters in a bird species.

Significance statement

IGF-1 is an evolutionarily highly conserved peptide hormone, which recently entered the center stage of research enquiry in evolutionary biology. It is considered as one of the key factors shaping individual life histories, but little is known about its effects on sexually selected traits. We investigated whether IGF-1 levels during molting predict the elaboration of multiple ornamental plumage traits in male and female bearded reedlings (Panurus biarmicus). Our results indicate that higher IGF-1 levels had positive effects on male structural plumage colors and tail feather length. This is the first study, bringing indication for a potential role of IGF-1 in the expression of plumage ornaments in a bird species. Our findings suggest that IGF-1 might serve as an ideal candidate to study the mechanisms linking condition and the capacity to develop sexually selected ornaments.

Feeling the heat: Extreme temperatures compromise constitutive innate humoral immunity and skin color in a desert dwelling lizard

Environmental temperature, particularly in habitats with extreme temperature fluctuations, may shape selection pressures on life history traits. Especially in ectotherms, temperature affects performance, physiology, and in some species, skin color. Skin color can be a sexual ornament signaling the bearer's ability to resist infections, when only high-quality individuals are able to invest both in high immune defense and elaborate ornament expression. However, how the information content of these sexual traits may vary with environmental conditions has been less studied. Dickerson's collared lizard (Crotaphytus dickersonae) males are blue and have a black and white collar. This conspicuous coloration signals performance and immune response, and is related to body temperature. Here, by maintaining males at higher, lower, and mean environmental temperatures we evaluated whether temperature variation influences color and constitutive innate humoral immunity (agglutination and lysis titers, estimated through hemolysis-hemagglutination assays), and whether extreme temperatures impose trade-offs between color and humoral immunity. We found that at low and high temperature treatments males had lower agglutination and lysis titers, and at low temperature, blue chroma from the dorsum declined and males became greener. Interestingly, at low and control temperature treatments, agglutination titer and blue coloration were positively correlated, whereas high temperatures revealed a trade-off between increasing agglutination titers and displaying bluer skin color. Our results suggest that in the Dickerson collared lizard even short-term variation of environmental temperature affects performance of constitutive innate humoral immunity and the brilliant blue skin color. Particularly, high temperatures may compromise some components of male's immunity and sexual signaling.

Effects of energy and thermoregulation time on physiological state and sexual signal in a lizard

Theory of sexual selection states that males often develop showy signals, which reduce their survival but increase their reproductive success. During mate choice, these conspicuous signals can be honest indicators of individual quality conveying information about the signaler's physiological state. Sexually selected signals are influenced by many environmental factors; however, whether signals and physiological state are affected together is rarely studied. The ultraviolet-blue throat color of male Lacerta viridis is an intra- and intersexually selected signal connected to blood parasite infection and influenced by environmental factors. The aim of this study was to experimentally investigate how ecologically relevant environmental factors affect color signal intensity and key physiological traits parallel. During the mating season, we exposed 40 adult male lizards infected with blood parasites to food and basking time treatments in a full factorial design. We measured color, amount of reactive oxygen metabolites (ROMs), hematocrit, immunocompetence, and blood parasite intensity before and after treatments. High basking time resulted in elevated immunocompetence coupled with increased ROMs. The high food treatment increased nuptial color brightness, but also increased ROMs and decreased immunocompetence. In summary, our study provides experimental evidence about environmentally induced parallel changes in an honest sexual signal and several quality-indicator physiological traits. We showed that available energy and time for high metabolism have independent and sometimes opposite effects on individual state.

Experimental stress during molt suggests the evolution of condition-dependent and condition-independent ornaments in the king penguin

Sexual selection and social selection are two important theories proposed for explaining the evolution of colorful ornamental traits in animals. Understanding signal honesty requires studying how environmental and physiological factors during development influence the showy nature of sexual and social ornaments. We experimentally manipulated physiological stress and immunity status during the molt in adult king penguins (Aptenodytes patagonicus), and studied the consequences of our treatments on colourful ornaments (yellow‐orange and UV beak spots and yellow‐orange auricular feather patches) known to be used in sexual and social contexts in this species. Whereas some ornamental features showed strong condition‐dependence (yellow auricular feather chroma, yellow and UV chroma of the beak), others were condition‐independent and remained highly correlated before and after the molt (auricular patch size and beak UV hue). Our study provides a rare examination of the links between ornament determinism and selection processes in the wild. We highlight the coexistence of ornaments costly to produce that may be honest signals used in mate choice, and ornaments for which honesty may be enforced by social mediation or rely on genetic constraints.

Sexual and non-sexual social preferences in male and female white-eyed bulbuls

While the function of ornaments shaped by sexual selection is to attract mates or drive off rivals, these signals may also evolve through social selection, in which the social context affects the fitness of signallers and receivers. Classical ‘mate choice’ experiments often reveal preferences for ornaments, but few studies have considered whether these are strictly sexual or reflect general social preferences. Indeed, an alternative possibility is that ornaments evolve through ‘non-sexual social selection’ (hereafter ‘social selection’). We examined the role of ornamentation (yellow ventral patch) and familiarity (individuals recognize group mates with which they have had previous interactions) on mate choice (opposite-sex stimuli preference) and social choice (same-sex stimuli preference) in both male and female white-eyed bulbuls (Pycnonotus xanthopygos). In the mate choice test, females preferred unfamiliar males with increased yellow. There were no biologically important differences in male preferences based on familiarity or intensity of patch colour. In the social choice test, females preferred to associate with familiar females. Males preferred to associate with familiar males but also preferred to associate with less ornamented males. Our results suggest that ornamentation and familiarity are important features, playing different roles in males and females, in both social and sexual selection processes.

Himalayan black bulbuls (Hypsipetes leucocephalus niggerimus) exhibit sexual dichromatism under ultraviolet light that is invisible to the human eye

Sexual dichromatism is a key proxy for the intensity of sexual selection. Studies of dichromatism in birds may, however, have underestimated the intensity and complexity of sexual selection because they used museum specimens alone without taking colour-fading into account or only measured conspicuous visual traits in live animals. We investigated whether the Himalayan black bulbul (Hypsipetes leucocephalus nigerrimus), which is sexually monomorphic to the human eye, exhibits sexual dichromatism distinguishable by a spectrometer. We measured the reflectance (within both the human visual perceptive and the ultraviolet ranges) of two carotenoid-based parts and eight dull and melanin-based parts for each individual live bird or museum skin sampled. According to an avian model of colour discrimination thresholds, we found that males exhibited perceptibly redder beaks, brighter tarsi and darker plumage than did females. This suggests the existence of multiple cryptic sexually dichromatic traits within this species. Moreover, we also observed detectable colour fading in the museum skin specimens compared with the live birds, indicating that sexual dichromatism could be underestimated if analysed using skin specimens alone.

Immune challenges and visual signalling in tree frogs

In animals, mate-choice is often based on sexual signals that carry information and help the receiver make the best choice to improve the receiver's fitness. Orange visual sexual signals have been hypothesised to carry immune information because they are often due to carotenoid pigments which are also involved in immunity response. Although many studies have focused on the direct relationships between coloration and immunocompetence, few studies have simultaneously studied immunocompetent response and coloration variation after an immune challenge. We tested this hypothesis on starved and ad libitum-fed males of the European tree frog Hyla arborea. Our results show that male coloration is not a reliable indicator of its immune response capacity in this species. However, after an immune challenge induced by a PHA (Phaseolus vulgaris phytohaemagglutinin) injection, starved males presented a significant coloration loss and this alteration was related to the immune response intensity. Taken together, these results suggest that the brighter (lighter) coloration may be used as a cue by female to exclude males with a recent immune challenge, due to diseases or parasites for example.

Carry-over effects of conditions at the wintering grounds on breeding plumage signals in a migratory bird: roles of phenotypic plasticity and selection

To understand the consequences of ever-changing environment on the dynamics of phenotypic traits, distinguishing between selection processes and individual plasticity is crucial. We examined individual consistency/plasticity in several male secondary sexual traits expressed during the breeding season (white wing and forehead patch size, UV reflectance of white wing patch and dorsal melanin coloration) in a migratory pied flycatcher (Ficedula hypoleuca) population over an 11-year period. Furthermore, we studied carry-over effects of three environmental variables (NAO, a climatic index; NDVI, a vegetation index; and rainfall) at the wintering grounds (during prebreeding moult) on the expression of these breeding plumage traits of pied flycatcher males at individual and population levels. Whereas NAO correlates negatively with moisture in West Africa, NDVI correlates positively with primary production. Forehead patch size and melanin coloration were highly consistent within individuals among years, whereas the consistency of the other two traits was moderate. Wing patch size decreased with higher NAO and increased with higher rainfall and NDVI at the individual level. Interestingly, small-patched males suffered lower survival during high NAO winters than large-patched males, and vice versa during low NAO winters. These counteracting processes meant that the individual-level change was masked at the population level where no relationship was found. Our results provide a good example of how variation in the phenotypic composition of a natural population can be a result of both environment-dependent individual plasticity and short-term microevolution. Moreover, when plasticity and viability selection operate simultaneously, their impacts on population composition may not be evident.

Influence of mate preference and laying order on maternal allocation in a monogamous parrot species with extreme hatching asynchrony

It is well established that in many avian species, prenatal maternal resource allocation varies both between and within clutches and may affect offspring fitness. Differential allocation of maternal resources, in terms of egg weight and yolk composition, may therefore allow the female to adjust brood reduction and to fine-tune reproductive investment in accordance with the expected fitness returns. The adaptive value of such maternal resource allocation is thought to be context-dependent as well as species-specific. We investigated the effects of female preference for her mate on the allocation of prenatal maternal resources in the budgerigar, Melopsittacus undulatus, a monogamous species of parrot that shows an extreme hatching asynchrony. We assessed mate preferences in a two-way preference test and allowed females two breeding rounds: one with the preferred and one with the non-preferred partner. We found no effect of preference on either latency to lay or clutch size, but females mated with the preferred partner laid eggs that contained significantly more yolk. Their eggs also contained significantly more androstenedione but not testosterone. Our results suggest that in this species, female preference may influence maternal resource allocation, and that the functional roles of each androgen in the yolk should be considered separately. In addition, we found a significant effect of laying order on egg and yolk weight as well as on yolk testosterone and androstenedione levels. These measures, however, did not change linearly with the laying order and render it unlikely that female budgerigars compensate for the extreme hatching asynchrony by adjusting within-clutch allocation of prenatal maternal resources.

Relationships among plumage coloration, blood selenium concentrations, and immune responses of adult and nestling tree swallows

In a number of taxa, males and females both display ornaments that may be associated with individual quality and could be reliable signals to potential mates or rivals. We examined the iridescent blue/green back and white breast of adult tree swallows (Tachycineta bicolor) to determine if plumage reflectance was related to adult or offspring immune responses. We simultaneously addressed the influence of blood selenium levels and the interaction between blood selenium and plumage coloration on adult and nestling immunity. Selenium is a well-known antioxidant necessary for mounting a robust immune response but its importance in wild birds remains poorly understood. In females, the brightness of white breast coloration was positively associated with bactericidal capacity, but there was no association with blood selenium. In contrast, male bactericidal capacity was associated with an interactive effect between dorsal plumage coloration and blood selenium concentrations. Males with bluer hues and greater blue chroma showed increased bactericidal capacity as blood selenium concentrations increased, while bactericidal capacity declined in greener males at higher blood selenium concentrations. In nestlings, bactericidal capacity was positively associated with nestling blood selenium concentrations and white brightness of both social parents. These results suggest that white plumage reflectance is indicative of quality in tree swallows and that greater attention should be paid to the reflectance of large white plumage patches. Additionally, the role of micronutrients, such as selenium, in mediating relationships between physiology and signals of quality, should be explored further.

Effect of an immune challenge on the functional performance of male weaponry

Theories of parasite-mediated sexual selection predict a positive association between immune function and the expression of sexually selected ornaments. Few studies, however, have investigated how an immune challenge affects the performance of sexually selected weaponry. Male Wellington tree weta (Hemideina crassidens) (Orthoptera: Anostostomatidae) possess enlarged mandibles that are used as weapons in fights for access to females residing in tree galleries. Intense sexual competition appears to have favoured the evolution of alternative male mating strategies in this species as males have a trimorphic phenotype in which weapon size varies across morphotype: 8th instar males have the smallest jaws, 10th instar males have the largest and 9th instar males being intermediate to the other two. After injecting males and females with either lipopolysaccharide (LPS; immune challenge) or saline (control) I measured over a 24h period each weta's body mass to assess whether they responded immunologically to the LPS and their bite force to assess the functional performance of their jaws. Both sexes responded immunologically to the immune-challenge as LPS-injected individuals lost significantly more body mass than saline-injected controls with females losing more mass than males. Female bite force was significantly reduced 8h after LPS-injection whereas male bite force did not significantly decline. Both sexes regained pre-injection functional performance of their jaws 24h after the immune challenge. My results suggest that females trade-off bite force for immune function whereas males do not. This article is part of a Special Issue entitled: insert SI title.

Steller sex: infidelity and sexual selection in a social Corvid (Cyanocitta stelleri)

Genetic analysis of avian mating systems has revealed that more than 70% of monogamous species show incidence of offspring parentage that does not match the social partner. Extra-pair parentage (EPP) has been linked to a variety of factors, including size and symmetry of ornamental traits, coloration, resource availability, and local conspecific density. We examined how ornamental plumage traits of individual Steller's jays (Cyanocitta stelleri) and territory characteristics influence genetic fidelity of socially monogamous pairs. We used seven highly polymorphic microsatellite markers to assign paternity to 79 offspring, and identified 12 (15.2%) as extra-pair young (EPY). Steller's jays with extra-pair young had significantly lower values of feather brightness and hue, indicating more ultraviolet-blue shifted coloration, and nested in closer proximity to the forest edge than Steller's jays with no detected EPY. Body size, crest height, asymmetry of ornamental crest stripes, as well as vegetative composition of territories and their proximity to supplemental feeders appeared to have little relationship to EPP. These results indicate that extra-pair parentage plays a role in the evolution of secondary sexual characteristics in both sexes, and suggest local density and availability of resources may influence Steller's jay mating dynamics.

Hot or not: the effects of exogenous testosterone on female attractiveness to male conspecifics in the budgerigar

An increasing number of studies indicate that not only females but also males can be selective when choosing a mate. In species exhibiting male or mutual mate choice, females may benefit from being attractive. While male attractiveness is often positively influenced by higher plasma levels of the androgenic hormone testosterone, it has been shown that testosterone can masculinise female behavior and morphology in several bird species, potentially rendering them less attractive. In this study, we investigated whether female budgerigars, Melopsittacusundulatus, suffer from increased plasma testosterone levels through a negative effect on their attractiveness to males. We experimentally increased plasma testosterone levels in testosterone-treated females (T-females) compared to controls (C-females) and allowed males to choose between a T- and a C-female in a two-way choice situation. Although testosterone treatment significantly affected female behavioral and morphological characteristics, males did not show a significant difference in preference between T- and C-females. These results suggest that experimentally increasing testosterone levels in females does not appear to influence male preference during initial mate choice. Our findings indicate that selection for higher levels of testosterone in male budgerigars is probably not constrained by a correlated response to selection causing negative effects on female attractiveness during initial mate choice. Evaluating whether or not a potential constraint may arise from negative testosterone-induced effects on other fitness related traits in females requires further work.

Female attractiveness affects paternal investment: experimental evidence for male differential allocation in blue tits

Introduction

The differential allocation hypothesis (DAH) predicts that individuals should adjust their parental investment to their current mate’s quality. Although in principle the DAH holds for both sexes, male adjustment of parental investment has only been tested in a few experimental studies, revealing contradictory results. We conducted a field experiment to test whether male blue tits (Cyanistes caeruleus) allocate their parental effort in relation to female ornamentation (ultraviolet colouration of the crown), as predicted by the DAH.

Results

We reduced the UV reflectance in a sample of females and compared parental care by their mates with that of males paired to sham-manipulated control females. As predicted by the DAH our results demonstrate that males paired with UV-reduced females invested less in feeding effort but did not defend the chicks less than males paired with control females.

Conclusions

To our knowledge, this is one of the first studies providing support for male differential allocation in response to female ornamentation.

Temperature, but not available energy, affects the expression of a sexually selected ultraviolet (UV) colour trait in male European green lizards

BACKGROUND

Colour signals are widely used in intraspecific communication and often linked to individual fitness. The development of some pigment-based (e.g. carotenoids) colours is often environment-dependent and costly for the signaller, however, for structural colours (e.g. ultraviolet [UV]) this topic is poorly understood, especially in terrestrial ectothermic vertebrates.

METHODOLOGY/PRINCIPAL FINDINGS

In a factorial experiment, we studied how available energy and time at elevated body temperature affects the annual expression of the nuptial throat colour patch in male European green lizards (Lacerta viridis) after hibernation and before mating season. In this species, there is a female preference for males with high throat UV reflectance, and males with high UV reflectance are more likely to win fights. We found that (i) while food shortage decreased lizards' body condition, it did not affect colour development, and (ii) the available time for maintaining high body temperature affected the development of UV colour without affecting body condition or other colour traits.

CONCLUSIONS/SIGNIFICANCE

Our results demonstrate that the expression of a sexually selected structural colour signal depends on the time at elevated body temperature affecting physiological performance but not on available energy gained from food per se in an ectothermic vertebrate. We suggest that the effect of high ambient temperature on UV colour in male L. viridis makes it an honest signal, because success in acquiring thermally favourable territories and/or effective behavioural thermoregulation can both be linked to individual quality.