Testosterone upregulates lipoprotein status to control sexual attractiveness in a colorful songbird

Blind date: female fossorial amphisbaenians prefer scent marks of large and healthy males

Selecting a good mate is a decision with important fitness consequences. For this reason, mate choice has promoted the evolution of sexual ornaments signaling the quality of an individual. In fossorial animals, inhabiting visually restricted underground environments, chemical senses should be very important for mate choice. We examined whether sexual chemical signals (substrate scent marks) produced by males of the Iberian worm lizard, Blanus cinereus, a strictly fossorial blind amphisbaenian, provide information to females on morphological traits and health state. We administered corticosterone (CORT) to males simulating a continuous stressor affecting their health. Females preferred settling at sites scent-marked by males in comparison with similar sites with female scent or unmarked sites, but the attractiveness of males' scent differed between individuals. Females preferred scent marks of larger/older males and with a higher immune response, while their body condition and CORT treatment were unrelated to female preferences. Chemical analyses showed that proportions of some compounds in precloacal secretions of males (used to produce scent marks) were correlated with the morphological (body size) and health state (immune response and body condition, but not CORT treatment) of these males. These results suggest that females may make site-selection decisions based on assessing the chemical characteristics of males' scent marks, which were reliably related to some of the traits of the male that produced the scent. Therefore, females might use chemical senses to increase the opportunities to find and mate with males of high quality, coping with the restrictions of the subterranean environment.

Song system neuroanatomy, and immediate early gene expression in a finch species with extensive male and female song

Birdsong is a relatively well-studied behavior, both due to its importance as a model for vocal production learning and as an intriguing complex social behavior. Until the last few decades, work on birdsong focused almost exclusively on males. However, it is now widely accepted that female song not only exists, but is fairly common throughout the oscine passerines. Despite this, and the large number of researchers who have begun exploring female song in the field, researchers in the lab have been slow to adopt model species with female song. Studying female song in the lab is critical for our understanding of sex-specific factors in the physiology controlling this fascinating behavior. Additionally, as a model for vocal production learning in humans, understanding the mechanistic and neuroendocrine control of female song is clearly important. In this study, we examined the red-cheeked cordon bleu (RCCB), an Estrildid finch species with extensive female song. Specifically, we found that there were no significant sex differences in circulating levels of testosterone and progesterone, nor in song production rate. There were no significant differences in cell densities in the three nuclei of the song control system we examined. Additionally, the volume of the robust nucleus of the arcopallium was not significantly different and we report the smallest sex difference in HVC yet published in a songbird. Finally, we demonstrated similar levels of motor driven immediate early gene expression in both males and females after song production.

Captivity affects mitochondrial aerobic respiration and carotenoid metabolism in the house finch (Haemorhous mexicanus)

In many species of animals, red carotenoid-based coloration is produced by metabolizing yellow dietary pigments, and this red ornamentation can be an honest signal of individual quality. However, the physiological basis for associations between organism function and the metabolism of red ornamental carotenoids from yellow dietary carotenoids remains uncertain. A recent hypothesis posits that carotenoid metabolism depends on mitochondrial performance, with diminished red coloration resulting from altered mitochondrial aerobic respiration. To test for an association between mitochondrial respiration and red carotenoids, we held wild-caught, molting male house finches in either small bird cages or large flight cages to create environmental challenges during the period when red ornamental coloration is produced. We predicted that small cages would present a less favorable environment than large flight cages and that captivity itself would decrease both mitochondrial performance and the abundance of red carotenoids compared with free-living birds. We found that captive-held birds circulated fewer red carotenoids, showed increased mitochondrial respiratory rates, and had lower complex II respiratory control ratios - a metric associated with mitochondrial efficiency - compared with free-living birds, though we did not detect a difference in the effects of small cages versus large cages. Among captive individuals, the birds that circulated the highest concentrations of red carotenoids had the highest mitochondrial respiratory control ratio for complex II substrate. These data support the hypothesis that the metabolism of red carotenoid pigments is linked to mitochondrial aerobic respiration in the house finch, but the mechanisms for this association remain to be established.

Seasonal but not sex-biased gene expression of the carotenoid ketolase, CYP2J19, in the sexually dichromatic southern red bishop (Euplectes orix)

Intense red colours in birds are often owing to ketocarotenoids (KCs). In many land birds, KCs are oxidized from dietary yellow precursors, presumably by the avian carotenoid ketolase CYP2J19, the regulation and constraints of which have important implications for condition-dependence and honest signalling of carotenoid colour displays. We investigated hepatic CYP2J19 gene expression in the seasonally and sexually dichromatic southern red bishop (Euplectes orix) in relation to season, sex, progression of the prenuptial moult, testis size, body condition, redness and circulating sex steroids. A coloration function of CYP2J19 is supported by a seasonal upregulation prior to and during the carotenoid-depositing stage of the male prenuptial moult. However, CYP2J19 expression was similarly high in females (which do not moult prenuptially), and remained high in males after moult, suggesting additional or alternative roles of hepatic CYP2J19 or its products, such as detoxification or antioxidant functions. In males, the CYP2J19 upregulation preceded and was unrelated to the rise in plasma testosterone, but was correlated with androstenedione, probably of adrenal origin and compatible with luteinizing hormone-induced and (in females) oestrogen-suppressed moult. Finally, contrary to ideas that carotenoid ketolation rate mediates honest signalling of male quality, CYP2J19 expression was not related to plumage redness or male body condition.

Colours of stress in male Indian rock agamas predict testosterone levels but not performance

Rapid physiological colour change offers dynamic signalling opportunities that can reveal distinct information to receivers in different contexts. Information content in dynamic colours, however, is largely unexplored. In males of the Indian rock agama (Psammophilus dorsalis), stressful events, including male-male agonistic interactions, induce a colour change, wherein the dorsal band turns yellow and the lateral bands turn orange. We aimed to determine whether these pigment-based dynamic colours convey information about individual quality. Using an agamid-specific visual model, we first quantified the chromatic and achromatic contrasts of each colour component displayed by males during handling stress, which induces the maximal response of aggression-typical colours. We then measured baseline testosterone levels, morphology (body mass and size), and performance measures (bite force and sprint speed) of these lizards. Chromatic contrasts of the dorsal yellow and lateral orange bands, individually and relative to each other (internal pair), were negatively correlated with testosterone levels, while the chromatic contrast of the internal pair was positively correlated with body condition. The lack of an association between colour contrasts and both bite force and sprint speed indicate that the conspicuousness of colours expressed during stressful events, such as agonistic interactions, do not reveal male performance ability. Despite our expectations of a positive relationship with testosterone, morphology (body condition), and performance (bite force, sprint speed), we find that for P. dorsalis, the conspicuousness of stress-induced colours provide only some information about individual quality. We speculate that the dynamicity of physiological colours may influence their function as content-containing signals in social interactions.

Seasonal but not sex-biased gene expression of the carotenoid ketolase, CYP2J19, in the sexually dichromatic southern red bishop (Euplectes orix)

Intense red colours in birds are often owing to ketocarotenoids (KCs). In many land birds, KCs are oxidized from dietary yellow precursors, presumably by the avian carotenoid ketolase CYP2J19, the regulation and constraints of which have important implications for condition-dependence and honest signalling of carotenoid colour displays. We investigated hepatic CYP2J19 gene expression in the seasonally and sexually dichromatic southern red bishop (Euplectes orix) in relation to season, sex, progression of the prenuptial moult, testis size, body condition, redness and circulating sex steroids. A coloration function of CYP2J19 is supported by a seasonal upregulation prior to and during the carotenoid-depositing stage of the male prenuptial moult. However, CYP2J19 expression was similarly high in females (which do not moult prenuptially), and remained high in males after moult, suggesting additional or alternative roles of hepatic CYP2J19 or its products, such as detoxification or antioxidant functions. In males, the CYP2J19 upregulation preceded and was unrelated to the rise in plasma testosterone, but was correlated with androstenedione, probably of adrenal origin and compatible with luteinizing hormone-induced and (in females) oestrogen-suppressed moult. Finally, contrary to ideas that carotenoid ketolation rate mediates honest signalling of male quality, CYP2J19 expression was not related to plumage redness or male body condition.

Seasonal but not sex-biased gene expression of the carotenoid ketolase, CYP2J19, in the sexually dichromatic southern red bishop (Euplectes orix)

Intense red colours in birds are often owing to ketocarotenoids (KCs). In many land birds, KCs are oxidized from dietary yellow precursors, presumably by the avian carotenoid ketolase CYP2J19, the regulation and constraints of which have important implications for condition-dependence and honest signalling of carotenoid colour displays. We investigated hepatic CYP2J19 gene expression in the seasonally and sexually dichromatic southern red bishop (Euplectes orix) in relation to season, sex, progression of the prenuptial moult, testis size, body condition, redness and circulating sex steroids. A coloration function of CYP2J19 is supported by a seasonal upregulation prior to and during the carotenoid-depositing stage of the male prenuptial moult. However, CYP2J19 expression was similarly high in females (which do not moult prenuptially), and remained high in males after moult, suggesting additional or alternative roles of hepatic CYP2J19 or its products, such as detoxification or antioxidant functions. In males, the CYP2J19 upregulation preceded and was unrelated to the rise in plasma testosterone, but was correlated with androstenedione, probably of adrenal origin and compatible with luteinizing hormone-induced and (in females) oestrogen-suppressed moult. Finally, contrary to ideas that carotenoid ketolation rate mediates honest signalling of male quality, CYP2J19 expression was not related to plumage redness or male body condition.

Developing a transcriptomic framework for testing testosterone-mediated handicap hypotheses

Sexually selected traits are hypothesized to be honest signals of individual quality due to the costs associated with their maintenance, development, and/or production. Testosterone, a sex steroid associated with the development and/or production of sexually selected traits, has been proposed to enforce the honesty of sexually selected traits via its immunosuppressive effects (i.e., the Immunocompetence Handicap Hypothesis) and/or by influencing an individual's exposure/susceptibility to oxidative stress (i.e., the Oxidation Handicap Hypothesis). Previous work testing these hypotheses has primarily focused on physiological measurements of immunity or oxidative stress, but little is known about the molecular pathways by which testosterone could influence immunity and/or oxidative stress pathways. To further understand the transcriptomic consequences of experimentally elevated testosterone in the context of handicap hypotheses, we used previously published RNA-seq data from studies that measured the transcriptome of individuals treated with either a testosterone-filled or an empty (i.e., control) implant. Two studies encompassing three species of bird and three tissue types fit our selection criteria and we reanalyzed the data using weighted gene co-expression network analysis. Testosterone-treated individuals exhibited signatures of immunosuppression and our results describe the molecular pathways underlying this effect. We also provide some evidence to suggest that the transcriptomic signature of immunosuppression is evolutionarily conserved between the three species. While our results provide no evidence to suggest testosterone mediates handicaps via pathways associated with oxidative stress, they do support the hypothesis that testosterone enforces the honesty of sexually-selected traits by influencing an individual's immunocompetence. Overall, this study develops a framework for testing testosterone-mediated handicap hypotheses and provides guidelines for future integrative and comparative studies focused on the proximate mechanisms mediating sexually selected traits.

Testosterone regulates CYP2J19-linked carotenoid signal expression in male red-backed fairywrens (Malurus melanocephalus)

Carotenoid pigments produce most red, orange and yellow colours in vertebrates. This coloration can serve as an honest signal of quality that mediates social and mating interactions, but our understanding of the underlying mechanisms that control carotenoid signal production, including how different physiological pathways interact to shape and maintain these signals, remains incomplete. We investigated the role of testosterone in mediating gene expression associated with a red plumage sexual signal in red-backed fairywrens (Malurus melanocephalus). In this species, males within a single population can flexibly produce either red/black nuptial plumage or female-like brown plumage. Combining correlational analyses with a field-based testosterone implant experiment and quantitative polymerase chain reaction, we show that testosterone mediates expression of carotenoid-based plumage in part by regulating expression of CYP2J19, a ketolase gene associated with ketocarotenoid metabolism and pigmentation in birds. This is, to our knowledge, the first time that hormonal regulation of a specific genetic locus has been linked to carotenoid production in a natural context, revealing how endocrine mechanisms produce sexual signals that shape reproductive success.

Carotenoid-based coloration predicts both longevity and lifetime fecundity in male birds, but testosterone disrupts signal reliability

Sexual selection promotes the evolution of conspicuous animal ornaments. To evolve as signals, these traits must reliably express the “quality” of the bearer, an indicator of individual fitness. Direct estimates of individual fitness may include the contribution of longevity and fecundity. However, evidence of a correlation between the level of signal expression and these two fitness components are scarce, at least among vertebrates. Relative fitness is difficult to assess in the wild as age at death and extra-pair paternity rates are often unknown. Here, in captive male red-legged partridges, we show that carotenoid-based ornament expression, i.e., redness of the bill and eye rings, at the beginning of reproductive life predicts both longevity (1–7 years) and lifetime breeding output (offspring number and hatching success). The recently proposed link between the individual capacity to produce red (keto) carotenoid pigments and the efficiency of cell respiration could, ultimately, explain the correlation with lifespan and, indirectly, fecundity. Nonetheless, in males of avian species, carotenoid-based coloration in bare parts is also partially controlled by testosterone. We also manipulated androgen levels throughout life by treating males with testosterone or antiandrogen compounds. Treatments caused correlations between signal levels and both fitness components to disappear, thus making the signals unreliable. This suggests that the evolution of carotenoid-based sexual signals requires a tightly-controlled steroid metabolism.

Testosterone activates sexual dimorphism including male-typical carotenoid but not melanin plumage pigmentation in a female bird

In males it is frequently testosterone (T) that activates the expression of sexually selected morphological and behavioral displays, but the role of T in regulating similar traits in females is less clear. Here, we combine correlational data with results from T and gonadotropin-releasing hormone (GnRH) manipulations in both sexes to assess the role of T in mediating sexually dimorphic coloration and morphology in the red-backed fairy-wren (Malurus melanocephalus). We show that: (1) natural variation in female expression of ornamental traits (darkened bills and red back feathers) is positively associated with age and circulating androgen titres, (2) females have the capacity to express most male-typical traits in response to exogenous T, including carotenoid-pigmented body plumage, shorter feathers, darkened bill and enlarged cloacal protuberance, but (3) appear constrained in production of male-typical melanin-pigmented plumage, and (4) low androgen levels during the pre-nuptial molt, probably because of low ovarian capacity for steroid production (or luteinizing hormone sensitivity), prevent females from developing male-like ornamentation. Thus, females appear to retain molecular mechanisms for hormonally regulated male-typical ornamentation, although these are rarely activated because of insufficient production of the hormonal signal.

Steroids in house sparrows (Passer domesticus): Effects of POPs and male quality signalling

At high trophic levels, environmental contaminants have been found to affect endocrinological processes. Less attention has been paid to species at lower trophic levels. The house sparrow (Passer domesticus) may be a useful model for investigating effects of POPs in mid-range trophic level species. In male house sparrows, ornamental traits involved in male quality signalling are important for female selection. These traits are governed by endocrinological systems, and POPs may therefore interfere with male quality signalling. The aim of the present study was to use the house sparrow as a mid-range trophic level model species to study the effects of environmental contaminants on endocrinology and male quality signalling. We analysed the levels of selected PCBs, PBDEs and OCPs and investigated the possible effects of these contaminants on circulating levels of steroid hormones (4 progestagens, 4 androgens and 3 estrogens) in male and female adult house sparrows from a population on the island Leka, Norway. Plasma samples were analysed for steroid hormones by GC-MS and liver samples were analysed for environmental contaminants by GC-ECD and GC-MS. In males, we also quantified ornament traits. It was hypothesised that POPs may have endocrine disrupting effects on the local house sparrow population and can thus interfere with the steroid hormone homeostasis. Among female house sparrows, bivariate correlations revealed negative relationships between POPs and estrogens. Among male sparrows, positive relationships between dihydrotestosterone levels and PCBs were observed. In males, positive relationships were also found between steroids and beak length, and between steroids and ornamental traits such as total badge size. This was confirmed by a significant OPLS model between beak length and steroids. Although sparrows are in the mid-range trophic levels, the present study indicates that POPs may affect steroid homeostasis in house sparrows, in particular for females. For males, circulating steroid levels appears to be more associated with biometric parameters related to ornamental traits.

Epaulet size and current condition in red-winged blackbirds: examining a semistatic signal, testosterone, immune function, and parasites

Some sexually selected signals are thought to convey information about the current condition and genetic/epigenetic quality of the individual signaling, including the ability to resist parasites. However, it is unclear whether semistatic sexual signals that develop periodically and remain stable over protracted periods, such as avian breeding plumage, can relate to measures of current condition and health. We examined a semistatic signal (wing epaulet size) in male red-winged blackbirds (Agelaius phoeniceus) during the breeding season and looked for relationships between this trait and circulating testosterone (T), hematocrit, bacteria-killing ability (BKA) of the blood, and the infection status, richness, and abundance of four functional categories of parasite. We found that epaulet size was positively related to circulating levels of T and ectoparasite infections. We found no relationships between T and parasite infections. In adult males there was a negative relationship between T and BKA, whereas in yearling males there was no relationship. We found no evidence for a general reduction in immunocompetence in males with larger epaulets but rather an increase in susceptibility to specific types of parasites. Our results suggest that semistatic signals can be linked to measures of current condition, and we postulate that these relationships are modulated via activity levels related to breeding-season activities.

An experimental test of the testosterone mediated oxidation handicap hypothesis in a wild bird

The oxidation handicap hypothesis (OHH) proposed that honesty in sexual signals is maintained when testosterone simultaneously promotes the development of elaborate signals and imposes an oxidative cost. Although there is evidence that testosterone enhances display traits in some cases, relatively few studies have tested the prediction that testosterone generates oxidative costs. We tested this prediction experimentally by administering testosterone (n=14) and control (n=14) implants to free-living common yellowthroat warblers (Geothlypis trichas) and quantifying testosterone and oxidative state before and 35±15days after implantation. We interpreted our experimental results in the context of a larger database of 83 unmanipulated males observed over five breeding seasons. In our observational data, testosterone was related to aspects of the carotenoid-based bib, but these relationships were age-dependent. Bib coloration was related to testosterone only for first time breeders, while bib size was positively and negatively associated with testosterone among experienced and inexperienced breeders, respectively. Two measures of oxidative metabolism-damage to DNA and total antioxidant capacity (TAC)-were unrelated to endogenous testosterone. Despite the correlation between endogenous testosterone and plumage, our experimental results failed to support the key prediction of the OHH. Testosterone treated males had higher levels of TAC upon recapture, but oxidative damage to DNA did not differ from controls. Because antioxidants can protect against the harmful effects of oxidative stress, one interpretation of our results is that males physiologically compensated for elevated testosterone, avoiding the honesty enforcing mechanism of the OHH. Taken together, our results suggest that testosterone is not a direct mediator of honest signaling in yellowthroats via its effects on oxidative stress.

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.

Gender Differences in the Association between Lipid Profile and Sexual Function among Patients with Coronary Artery Disease

BACKGROUND

Although several studies have been conducted on the association between lipid profile and sexual function among men with coronary artery disease, there is a paucity of knowledge about this association among women with coronary artery disease.

OBJECTIVES

Our study aimed to evaluate the link between lipid profile and sexual function in men and women with coronary artery disease.

METHODS

One hundred and twenty patients with documented coronary artery disease were consecutively sampled from an outpatient cardiovascular clinic. The patients were assessed for lipid profile and sexual relationship using the Relation and Sexuality Scale (RSS). In addition, the Hospital Anxiety and Depression Scale (HADS) was used to measure the symptoms of anxiety and depression. The characteristics of chest pain were also measured using the Rose Angina Questionnaire. The data were analyzed through linear regression analysis.

RESULTS

This study was conducted on 91 males (75.8%) and 29 females (24.2%). Multivariate analysis showed that low-density lipoprotein cholesterol was correlated with sexual function (B = 0.01, P = 0.010) and total sexual relationship (B = 0.01, P = 0.050). A correlation was also observed between the level of high-density lipoprotein and sexual frequency score (B = -0.02, P = 0.040). Gender moderated these correlations. Among males, serum cholesterol (r = 0.193, P = 0.047) and low-density lipoprotein (r = 0.224, P = 0.037) were correlated to sexual function. In females, however, low-density lipoprotein was correlated to the total sexual relationship (r = 0.426, P = 0.021) and high-density lipoprotein was correlated to sexual frequency (r = -0.334, P = 0.046).

CONCLUSIONS

The findings of this study showed a relationship between lipid profile and sexual relationship among both male and female patients with coronary artery disease. The link between lipid profile and sexual function of the patients with coronary artery disease is thus beyond just the effect of lipid profile on erectile dysfunction.

Is sexual ornamentation an honest signal of male quality in the Chinese grouse (Tetrastes sewerzowi)?

We examined the variation in sexual ornamentation of male Chinese grouse (Tetrastes sewerzowi) in the Gansu Province, China, seeking to identify factors involved in whether ornament size and brightness are honest signals of male quality. Compared to unmated males, mated males had significantly larger and redder combs and, although they did not have significantly larger territories, they defended them more vigorously. Mated males had significantly higher blood carotenoid and testosterone levels, significantly better body condition, and significantly lower parasite loads than unmated males. Our findings are thus consistent with the hypothesis that comb size and color are honest signals of better male quality in the grouse, mediated through lower parasite loads and/or higher testosterone levels.

Signaling stress? An analysis of phaeomelanin-based plumage color and individual corticosterone levels at two temporal scales in North American barn swallows, Hirundo rustica erythrogaster

Sexually selected traits confer greater reproductive benefits to individuals with more elaborate forms of the signal. However, whether these signals convey reliable information about the physiology underlying trait development remains unknown in many species. The steroid hormone corticosterone (CORT) mediates important physiological and behavioral processes during the vertebrate stress response, and CORT secretion itself can be modulated by melanocortins. Thus, sexually selected melanin-based plumage coloration could function as an honest signal of an individual's ability to respond to stressors. This hypothesis was tested in North American barn swallows, Hirundo rustica erythrogaster, where males with darker ventral plumage color exhibit higher phaeomelanin content and are more successful at reproduction. Because reproductive behavior occurs months after plumage signals are developed, we also addressed the potential temporal disconnect of physiological state during trait development and trait advertisement by analyzing three different measurements of CORT levels in adult males during the breeding season (trait advertisement) and in nestling males while they were growing their feathers (trait development). Variation in adult plumage color did not predict baseline or stress-induced CORT, or stress responsiveness. Likewise, there was no relationship between nestling plumage color and any of the CORT measurements, but heavier nestlings had significantly lower baseline CORT. Our finding that a predominantly phaeomelanin-based trait is unrelated to circulating CORT suggests that phaeomelanin and eumelanin signals may convey different physiological information, and highlights the need for further study on the biochemical links between the hypothalamic-pituitary-adrenal (HPA) axis and the production of different melanin-based pigments.

Carotenoid supplementation during adulthood, but not development, decreases testis size in mallards

Nutritional constraints on reproduction are well-characterized in female animals, but rarely have particular nutrients been linked to male reproductive investments. Carotenoid pigments promote egg-laying and fertility in several animals, and are displayed externally within secondary sex traits by males of many colorful species to attract mates, but it is unclear if or how carotenoids affect male primary sex traits. We manipulated carotenoid availability in the diet of male mallards (Anas platyrhynchos) during both development and adulthood to determine effects on size and carotenoid content of the testes. We found that developmental carotenoid manipulations did not affect testis size or carotenoid concentration, but that increased carotenoid dietary levels at adulthood resulted in more carotenoid-rich, but smaller, testes. This latter result was surprising, given positive correlations in mammals between testicle size and carotenoid concentration. We also found negative correlations between testis size and carotenoid concentration for individual ducks, regardless of dietary treatment. These results suggest that carotenoid deposition into testis tissue can reduce investment in gonad size (and thus overall sperm count), although the functional consequences of this relationship remain to be tested.

Genetics of colouration in birds

Establishing the links between phenotype and genotype is of great importance for resolving key questions about the evolution, maintenance and adaptive function of phenotypic variation. Bird colouration is one of the most studied systems to investigate the role of natural and sexual selection in the evolution of phenotypic diversity. Given the recent advances in molecular tools that allow discovering genetic polymorphisms and measuring gene and protein expression levels, it is timely to review the literature on the genetics of bird colouration. The present study shows that melanin-based colour phenotypes are often associated with mutations at melanogenic genes. Differences in melanin-based colouration are caused by switches of eumelanin to pheomelanin production or by changes in feather keratin structure, melanoblast migration and differentiation, as well as melanosome structure. Similar associations with other types of colourations are difficult to establish, because our knowledge about the molecular genetics of carotenoid-based and structural colouration is quasi inexistent. This discrepancy stems from the fact that only melanin-based colouration shows pronounced heritability estimates, i.e. the resemblance between related individuals is usually mainly explained by genetic factors. In contrast, the expression of carotenoid-based colouration is phenotypically plastic with a high sensitivity to variation in environmental conditions. It therefore appears that melanin-based colour traits are prime systems to understand the genetic basis of phenotypic variation. In this context, birds have a great potential to bring us to new frontiers where many exciting discoveries will be made on the genetics of phenotypic traits, such as colouration. In this context, a major goal of our review is to suggest a number of exciting future avenues.

Androgens regulate gene expression in avian skeletal muscles

Circulating androgens in adult reproductively active male vertebrates influence a diversity of organ systems and thus are considered costly. Recently, we obtained evidence that androgen receptors (AR) are expressed in several skeletal muscles of three passeriform birds, the golden-collared manakin (Manacus vitellinus), zebra finch (Taenopygia guttata), and ochre-bellied flycatcher (Mionectes oleagieus). Because skeletal muscles that control wing movement make up the bulk of a bird’s body mass, evidence for widespread effects of androgen action on these muscles would greatly expand the functional impact of androgens beyond their well-characterized effects on relatively discrete targets throughout the avian body. To investigate this issue, we use quantitative PCR (qPCR) to determine if androgens alter gene mRNA expression patterns in wing musculature of wild golden-collared manakins and captive zebra finches. In manakins, the androgen testosterone (T) up-regulated expression of parvalbumin (PV) and insulin-like growth factor I (IGF-I), two genes whose products enhance cellular Ca²⁺ cycling and hypertrophy of skeletal muscle fibers. In T-treated zebra finches, the anti-androgen flutamide blunted PV and IGF-I expression. These results suggest that certain transcriptional effects of androgen action via AR are conserved in passerine skeletal muscle tissue. When we examined wing muscles of manakins, zebra finches and ochre-bellied flycatchers, we found that expression of PV and IGF-I varied across species and in a manner consistent with a function for AR-dependent gene regulation. Together, these findings imply that androgens have the potential to act on avian muscle in a way that may enhance the physicality required for successful reproduction.

Carotenoids in bird testes: links to body carotenoid supplies, plumage coloration, body mass and testes mass in house finches (Carpodacus mexicanus)

Carotenoid pigments can be allocated to different parts of the body to serve specific functions. In contrast to other body tissues, studies of carotenoid resources in the testes of animals are relatively scarce. We used high-performance liquid chromatography to determine the types and concentrations of carotenoids in the testes of house finches (Carpodacus mexicanus). Additionally, we examined the relationships between testes carotenoid concentrations and carotenoid pools in other body tissues, as well as body mass, testes mass and plumage coloration. We detected low concentrations of several carotenoids - lutein (the predominant carotenoid), zeaxanthin, anhydrolutein, β-cryptoxanthin, β-carotene and an unknown carotene - in the testes of wild house finches. We also found that testes lutein levels were significantly and positively associated with circulating lutein levels, while the concentration of zeaxanthin in testes was positively associated with zeaxanthin levels in liver, though in this instance the relationship was much weaker and only marginally significant. Furthermore, lutein levels in testes were significantly negatively associated with testes mass. Finally, plumage coloration was not associated with either the concentration of carotenoids in the testes or relative testes mass. These results suggest that testes carotenoids are reflective of the pool of circulating carotenoids in house finches, and that plumage coloration is unlikely to signal either the carotenoid content of testes tissue or a male's capacity for sperm production.

Hormonal organization and activation: evolutionary implications and questions

Comparative endocrinology is a fascinating field of science in part because it addresses both ultimate and proximate causation. Research on sexual dimorphism and sexual differentiation has excellent potential for this kind of integration. Vertebrate comparative endocrinologists have made many important discoveries about the role of genes and sex steroid hormones in the organization and activation of sexually differentiated behavior, brain function, anatomy and physiology. In addition to taxonomically general principles and conserved features, there is also striking diversity in sexual differentiation processes. Much of the evolutionary basis of this diversity (its phylogenetic history and adaptive functions) is not well understood. A set of questions is raised to illustrate this point, with an emphasis on mechanisms of sexual dimorphism in body size and ornamentation, sexual differentiation of avian behavior, particularly in Japanese quail and zebra finches, and the puzzle of the phylogenetic distribution of vertebrate sex determining mechanisms. Applying a comparative approach grounded in established phylogenies and concepts from evolutionary developmental biology such as developmental modules holds promise for generating and testing new hypotheses and eventually answering some of these questions.

Environmentally realistic exposure to the herbicide atrazine alters some sexually selected traits in male guppies

Male mating signals, including ornaments and courtship displays, and other sexually selected traits, like male-male aggression, are largely controlled by sex hormones. Environmental pollutants, notably endocrine disrupting compounds, can interfere with the proper functioning of hormones, thereby impacting the expression of hormonally regulated traits. Atrazine, one of the most widely used herbicides, can alter sex hormone levels in exposed animals. I tested the effects of environmentally relevant atrazine exposures on mating signals and behaviors in male guppies, a sexually dimorphic freshwater fish. Prolonged atrazine exposure reduced the expression of two honest signals: the area of orange spots (ornaments) and the number of courtship displays performed. Atrazine exposure also reduced aggression towards competing males in the context of mate competition. In the wild, exposure levels vary among individuals because of differential distribution of the pollutants across habitats; hence, differently impacted males often compete for the same mates. Disrupted mating signals can reduce reproductive success as females avoid mating with perceptibly suboptimal males. Less aggressive males are at a competitive disadvantage and lose access to females. This study highlights the effects of atrazine on ecologically relevant mating signals and behaviors in exposed wildlife. Altered reproductive traits have important implications for population dynamics, evolutionary patterns, and conservation of wildlife species.

Sexually selected male plumage color is testosterone dependent in a tropical passerine bird, the red-backed fairy-wren (Malurus melanocephalus)

Background

Sexual signals, such as bright plumage coloration in passerine birds, reflect individual quality, and testosterone (T) may play a critical role in maintaining signal honesty. Manipulations of T during molt have yielded mixed effects on passerine plumage color, in most cases delaying molt or leading to production of drab plumage. However, the majority of these studies have been conducted on species that undergo a post-nuptial molt when T is low; the role of T in species that acquire breeding plumage during a pre-nuptial molt remains largely unexplored.

Methodology/Principal Findings

We experimentally tested the effects of increased T on plumage color in second-year male red-backed fairy-wrens (Malurus melanocephalus), a species in which after-second-year males undergo a pre-nuptial molt into red/black (carotenoid and melanin-based) plumage and second-year males either assume red/black or brown breeding plumage. T treatment stimulated a rapid and early onset pre-nuptial molt and resulted in red/black plumage acquisition, bill darkening, and growth of the sperm storage organ, but had no effect on body condition or corticosterone concentrations. Control males molted later and assumed brown plumage. T treated males produced feathers with similar but not identical reflectance parameters to those of unmanipulated after-second-year red/black males; while reflectance spectra of red back and black crown feathers were similar, black breast feathers differed in UV chroma, hue and brightness, indicating a potentially age and plumage patch-dependent response to T for melanin- vs. carotenoid-pigmentation.

Conclusions/Significance

We show that testosterone is the primary mechanism functioning during the pre-nuptial molt to regulate intrasexually variable plumage color and breeding phenotype in male red-backed fairy-wrens. Our results suggest that the effects of T on plumage coloration may vary with timing of molt (pre- vs. post-nuptial), and that the role of T in mediating plumage signal production may differ across age classes, plumage patches, and between pigment-types.

The interplay between gonadal steroids and immune defence in affecting a carotenoid-dependent trait

The hypothesis that sexual ornaments are honest signals of quality because their expression is dependent on hormones with immune-depressive effects has received ambiguous support. The hypothesis might be correct for those signals that are carotenoid-dependent because the required carotenoid deposition in the signal, stimulated by testosterone, might lower the carotenoid-dependent immune defence of the organism. Two pathways underlying this androgen-dependent honest signaling have been suggested. Firstly, androgens that are needed for ornament expression may suppress immune defence, a cost that only high-quality animals can afford. Alternatively, immune activation may downregulate the production of androgens in low-quality individuals. Which of these alternatives is correct, and to what extent these effects are mediated by the different metabolites of androgens, remain open questions. To provide answers to these questions, we manipulated the levels of testosterone (T), 5α-dihydrotestosterone (DHT), and 17-β-estradiol (E2) in diamond doves Geopelia cuneata, a species in which both sexes exhibit a carotenoid-dependent, androgen-regulated red–orange periorbital ring of bare skin. On the first day of the experiment (day 0), we inserted steroid-releasing implants into groups of birds and on day 14, we subjected half of the birds to an immunological challenge by immunizing them with sheep red blood cells (SRBC). In females, but not in males, androgen but not estradiol treatments reduced antibody production to SRBC. In addition, the immunological challenge reduced redness and size of the trait as well as androgens levels in both sexes and in all treatments. This indicates that an immunological challenge can lower circulating T at the cost of the trait expression. These findings are in accordance with both pathways postulated in the immunocompetence-handicap hypothesis, but do not entirely support the idea that the immunosuppressive effect of androgens yields honest signaling since both T and DHT were not immunosuppressive in males, for which sexual signaling is supposed to be especially important.

Variation of a carotenoid-based trait in relation to oxidative stress and endocrine status during the breeding season in the Eurasian kestrel: a multi-factorial study

Carotenoid-based skin colorations vary seasonally in many bird species and are thought to be honest sexually selected signals. In order to provide more insight in the potential signal function and underlying mechanisms of such colorations we here quantified patterns of variation of leg coloration in adult male and female Eurasian kestrels (Falco tinnunculus tinnunculus) over the breeding season, and evaluated the relationship between coloration and levels of carotenoids, androgens and estrogens, oxidative damage and plasma non-enzymatic antioxidant capacity. We studied both reproducing wild and non-reproducing captive birds to test for the effect of diet and breeding effort. Males were more colored than females only during mating, and independently of diet, suggesting that leg-color is a sexually selected trait. Seasonal variation in leg color was associated with circulating carotenoids, but concentrations of these molecules were not related to antioxidant capacity, body condition or oxidative damage. These results indicate that carotenoid-based colorations may not be an honest signal of health status in this species. Production of carotenoid rich eggs coincided with low levels of circulating carotenoids in females, indicating that carotenoids might be a limited resource for laying female kestrels. Finally, young rearing males had higher levels of oxidative damage than females, and wild birds of both sexes had higher levels of these parameters than captive birds. These results may indicate that parental effort and physical activity are costly, independently from hormonal status. Since androgens did not explain carotenoid variation we suggest that multiple interacting factors can regulate carotenoid levels along the season.

Free radical exposure creates paler carotenoid-based ornaments: a possible interaction in the expression of black and red traits

Oxidative stress could be a key selective force shaping the expression of colored traits produced by the primary animal pigments in integuments: carotenoids and melanins. However, the impact of oxidative stress on melanic ornaments has only recently been explored, whereas its role in the expression of carotenoid-based traits is not fully understood. An interesting study case is that of those animal species simultaneously expressing both kinds of ornaments, such as the red-legged partridge (Alectoris rufa). In this bird, individuals exposed to an exogenous source of free radicals (diquat) during their development produced larger eumelanin-based (black) plumage traits than controls. Here, we show that the same red-legged partridges exposed to diquat simultaneously developed paler carotenoid-based ornaments (red beak and eye rings), and carried lower circulating carotenoid levels as well as lower levels of some lipids involved in carotenoid transport in the bloodstream (i.e., cholesterol). Moreover, partridges treated with a hormone that stimulates eumelanin production (i.e., alpha-melanocyte-stimulating hormone) also increased blood carotenoid levels, but this effect was not mirrored in the expression of carotenoid-based traits. The redness of carotenoid-based ornaments and the size of a conspicuous eumelanic trait (the black bib) were negatively correlated in control birds, suggesting a physiological trade-off during development. These findings contradict recent studies questioning the sensitivity of carotenoids to oxidative stress. Nonetheless, the impact of free radicals on plasma carotenoids seems to be partially mediated by changes in cholesterol metabolism, and not by direct carotenoid destruction/consumption. The results highlight the capacity of oxidative stress to create multiple phenotypes during development through differential effects on carotenoids and melanins, raising questions about evolutionary constraints involved in the production of multiple ornaments by the same organism.

Neuroendocrine contributions to sexual partner preference in birds

A majority of birds are socially monogamous, providing exceptional opportunities to discover neuroendocrine mechanisms underlying preferences for opposite-sex partners where the sexes form extended affiliative relationships. Zebra finches have been the focus of the most systematic program of research to date in any socially monogamous animal. In this species, sexual partner preference can be partially or largely sex reversed with hormone manipulations during early development, suggesting a role for organizational hormone actions. This same conclusion emerges from research with Japanese quail, which do not form long-term pairs. In zebra finches, social experience manipulations during juvenile development also can sex reverse partner preference, either alone or in combination with an early hormone manipulation. Although there are several candidate brain regions where neural mechanisms could underlie these effects of hormones or social experience, the necessary research has not yet been done to determine their involvement. The neuroendocrinology of avian sexual partner preference is still frontier territory.

Short-term exposure to a synthetic estrogen disrupts mating dynamics in a pipefish

Sexual selection is responsible for the evolution of some of the most elaborate traits occurring in nature, many of which play a vital role in competition over access to mates and individual reproductive fitness. Because expression of these traits is typically regulated by sex-steroids there is a significant potential for their expression to be affected by the presence of certain pollutants, such as endocrine disrupting compounds. Endocrine disruptors have been shown to alter primary sexual traits and impact reproduction, but few studies have investigated how these compounds affect secondary sexual trait expression and how that may, in turn, impact mating dynamics. In this study we examine how short-term exposure to a synthetic estrogen impacts secondary sexual trait expression and mating dynamics in the Gulf pipefish, a species displaying sex-role reversal. Our results show that only 10days of exposure to 17α-ethinylestradiol results in adult male pipefish developing female-like secondary sexual traits. While these males are capable of reproduction, females discriminate against exposed males in mate choice trials. In natural populations, this type of discrimination would reduce male mating opportunities, thus potentially reducing their long-term reproductive success. Importantly, the effects of these compounds on mating dynamics and mating opportunity would not be observed using the current standard methods of assessing environmental contamination. However, disrupting these processes could have profound effects on the viability of exposed populations.

Differential effects of testosterone, dihydrotestosterone and estradiol on carotenoid deposition in an avian sexually selected signal

Recent studies have demonstrated that carotenoid-based traits are under the control of testosterone (T) by up-regulation of carotenoid carriers (lipoproteins) and/or tissue-specific uptake of carotenoids. T can be converted to dihydrotestosterone (DHT) and estradiol (E2), and variation in conversion rate may partly explain some contradictory findings in the literature. Moreover, most studies on the effect of T on sexual signals have focused on the male sex only, while in many species females show the same signal, albeit to a lesser extent. We studied the effects of T, DHT, and E2 treatment in male and female diamond doves Geopelia cuneata in which both sexes have an enlarged red eye ring, which is more pronounced in males. We first showed that this periorbital ring contains very high concentration of carotenoids, of which most are lutein esters. Both T and DHT were effective in enhancing hue, UV-chroma and size in both sexes, while E2 was ineffective. However, E2 dramatically increased the concentration of circulating lipoproteins. We conclude that in both sexes both color and size of the secondary sexual trait are androgen dependent. The action of androgens is independent of lipoproteins regulation. Potential mechanisms and their consequences for trade-off are discussed.

Short-term exposure to testosterone propionate leads to rapid bill color and dominance changes in zebra finches

Testosterone (T) can influence both male-male competition and mate choice displays. In zebra finches, female mate choice is based in part on bill color, and bill color has been shown to be enhanced by long-term testosterone supplementation. However, it is not clear whether bill color plays a role in male-male interactions and how bill color responds to shorter-term changes in T. We tested whether a single injection of testosterone propionate (TP) would influence male-male dominance interactions and lead to rapid (over a three-day period) changes in bill color. In addition, we tested whether bill color predicted aggression and dominance. We allowed birds in triads to establish hierarchies and then injected either dominant or subordinate individuals with TP, in addition to establishing sham control triads. We found that red chroma, but not hue, predicted aggressiveness of males. Exposure to TP led both dominant and subordinate birds to increase dominance scores over three days, longer than the <24h period in which injected TP stays active. In addition, exposure to TP increased red chroma and hue in three days showing the dynamic nature of allocation of pigments to the bill. Our results suggest that zebra finches can modulate T and bill color levels over short time periods and these changes may occur through positive feedback between T-levels and dominance.

Yolk testosterone reduces oxidative damages during postnatal development

Conditions experienced during early life can influence the development of an organism and several physiological traits, even in adulthood. An important factor is the level of oxidative stress experienced during early life. In birds, extra-genomic egg substances, such as the testosterone hormone, may exert a widespread influence over the offspring phenotype. Interestingly, testosterone can also upregulate the bioavailability of certain antioxidants but simultaneously increases the susceptibility to oxidative stress in adulthood. However, little is known about the effects of maternally derived yolk testosterone on oxidative stress in developing birds. Here, we investigated the role of yolk testosterone on oxidative stress of yellow-legged gull chicks during their early development by experimentally increasing yolk testosterone levels. Levels of antioxidants, reactive oxygen species and lipid oxidative damage were determined in plasma during nestlings' growth. Our results revealed that, contrary to control chicks, birds hatched from testosterone-treated eggs did not show an increase in the levels of oxidative damage during postnatal development. Moreover, the same birds showed a transient increase in plasma antioxidant levels. Our results suggest that yolk testosterone may shape the oxidative stress-resistance phenotype of the chicks during early development owing to an increase in antioxidant defences and repair processes.

Impact of season and social challenge on testosterone and corticosterone levels in a year-round territorial bird

Plasma testosterone increases during breeding in many male vertebrates and has long been implicated in the promotion of aggressive behaviors relating to territory and mate defense. Males of some species also defend territories outside of the breeding period. For example, the European nuthatch (Sitta europaea) defends an all-purpose territory throughout the year. To contribute to the growing literature regarding the hormonal correlates of non-breeding territoriality, we investigated the seasonal testosterone and corticosterone profile of male (and female) nuthatches and determined how observed hormone patterns relate to expression of territorial aggression. Given that non-breeding territoriality in the nuthatch relates to the reproductive context (i.e., defense of a future breeding site), we predicted that males would exhibit surges in plasma testosterone throughout the year. However, we found that males showed elevated testosterone levels only during breeding. Thus, testosterone of gonadal origin does not appear to be involved in the expression of non-breeding territoriality. Interestingly, territorial behaviors of male nuthatches were stronger in spring than in autumn, suggesting that in year-round territorial species, breeding-related testosterone elevations may upregulate male-male aggression above non-breeding levels. In females, plasma testosterone was largely undetectable. We also examined effects of simulated territorial intrusions (STIs) on testosterone and corticosterone levels of breeding males. We found that STIs did not elicit a testosterone response, but caused a dramatic increase in plasma corticosterone. These data support the hypothesis that corticosterone rather than testosterone may play a role in the support of behavior and/or physiology during acute territorial encounters in single-brooded species.