Experimental manipulation of testosterone and condition during molt affects activity and vocalizations of male blue tits

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.

Dominance-related seasonal song production is unrelated to circulating testosterone in a subtropical songbird

Circulating testosterone (T) is widely considered to play a key role in the production of sexual displays by male vertebrates. While numerous studies support a role for circulating T in promoting the production of song in male birds, this understanding is based primarily on evidence from seasonally breeding northern temperate species, leaving it unclear whether this mechanism generalizes to other regions of the world. Here we investigate whether variation in circulating levels of T can explain the marked within- and among-individual variation in male song performance observed in a subtropical population of the year-round territorial white-browed sparrow weaver (Plocepasser mahali mahali). Our findings reveal that both circulating T and male song production peaked at a similar time point, halfway through the population-level breeding season. However, while dominant males were more likely to sing and sang for longer than subordinate males, within-group paired comparisons revealed no dominance-related differences in circulating T. Moreover, comparisons both among and within individual dominant males revealed that song duration, syllable rate and proportion of time spent singing were all unrelated to circulating T. Together, our findings suggest that natural variation in male song production, at least in this population of white-browed sparrow weavers, is achieved principally through mechanisms other than variation in circulating T concentration. More widely, our results are in line with the view that male song production is not exclusively regulated by gonadally synthesized steroids.

Innate immunity and testosterone rapidly respond to acute stress, but is corticosterone at the helm?

When faced with a stressor, vertebrates can rapidly increase the secretion of glucocorticoids, which is thought to improve the chances of survival. Concurrent changes in other physiological systems, such as the reproductive endocrine or innate immune systems, have received less attention, particularly in wild vertebrates. It is often thought that glucocorticoids directly modulate immune performance during a stress response, but, in many species, androgens also rapidly respond to stress. However, to our knowledge, no study has simultaneously examined the interactions between the glucocorticoid, androgen, and innate immune responses to stress in a wild vertebrate. To address this issue, we tested the hypothesis that the change in plasma corticosterone (CORT) in response to the acute stress of capture and restraint is correlated with the concurrent changes in plasma testosterone (T) and innate immune performance (estimated by the capacity of plasma to agglutinate and lyse foreign cells) in the Abert's Towhee (Melozone aberti). Furthermore, to broaden the generality of the findings, we compared male and female towhees, as well as males from urban and non-urban populations. Acute stress increased plasma CORT, decreased plasma T in males, and decreased innate immune performance, but the increase in CORT during stress was not correlated with the corresponding decreases in either plasma T or innate immunity. By contrast, the plasma T stress response was positively correlated with the innate immune stress response. Collectively, our results challenge the proposition that the glucocorticoid stress response is correlated with the concurrent changes in plasma T, a key reproductive hormone, and innate immunity, as estimated by agglutination and lysis.

Food availability, energetic constraints and reproductive development in a wild seasonally breeding songbird

In many organisms, food availability is a proximate cue that synchronizes seasonal development of the reproductive system with optimal environmental conditions. Growth of the gonads and secondary sexual characteristics is orchestrated by the hypothalamic–pituitary–gonadal (HPG) axis. However, our understanding of the physiological mechanisms by which food availability modulates activity of the HPG axis is limited.

It is thought that many factors, including energetic status, modulate seasonal reproductive activation. We tested the hypothesis that food availability modulates the activity of the HPG axis in a songbird. Specifically, we food‐restricted captive adult male Abert's Towhees Melozone aberti for 2 or 4 weeks during photoinduced reproductive development. A third group (control) received ad libitum food throughout. We measured multiple aspects of the reproductive system including endocrine activity of all three levels of the HPG axis [i.e. hypothalamic gonadotropin‐releasing hormone‐I (GnRH‐I), plasma luteinizing hormone (LH) and testosterone (T)], and gonad morphology. Furthermore, because gonadotropin‐inhibitory hormone (GnIH) and neuropeptide Y (NPY; a potent orexigenic peptide) potentially integrate information on food availability into seasonal reproductive development, we also measured the brain levels of these peptides.

At the hypothalamic level, we detected no effect of food restriction on immunoreactive (ir) GnRH‐I, but the duration of food restriction was inversely related to the size of ir‐GnIH perikarya. Furthermore, the number of ir‐NPY cells was higher in food‐restricted than control birds. Food restriction did not influence photoinduced testicular growth, but decreased plasma LH and T, and width of the cloacal protuberance, an androgen‐sensitive secondary sexual characteristic. Returning birds to ad libitum food availability had no effect on plasma LH or T, but caused the cloacal protuberance to rapidly increase in size to that of ad libitum‐fed birds.

Our results support the tenet that food availability modulates photoinduced reproductive activation. However, they also suggest that this modulation is complex and depends upon the level of the HPG axis considered. At the hypothalamic level, our results are consistent with a role for the GnIH and NPY systems in integrating information on energetic status. There also appears to be a role for endocrine function at the anterior pituitary gland and testicular levels in modulating reproductive development in the light of energetic status and independently of testicular growth.

Androgen modulation of Foxp1 and Foxp2 in the developing rat brain: impact on sex specific vocalization

Sex differences in vocal communication are prevalent in both the animals and humans. The mechanism(s) mediating gender differences in human language are unknown, although, sex hormones, principally androgens, play a central role in the development of vocalizations in a wide variety of animal species. The discovery of FOXP2 has added an additional avenue for exploring the origins of language and animal communication. The FOXP2 gene is a member of the forkhead box P (FOXP) family of transcription factors. Prior to the prenatal androgen surge in male fetuses, we observed no sex difference for Foxp2 protein levels in cultured cells. In contrast, 24 hours after the onset of the androgen surge, we found a sex difference for Foxp2 protein levels in cultured cortical cells with males having higher levels than females. Furthermore, we observed the potent nonaromatizable androgen dihydrotestosterone altered not only Foxp2 mRNA and protein levels but also Foxp1. Androgen effects on both Foxp2 and Foxp1 were found to occur in the striatum, cerebellar vermis, and cortex. Immunofluorescence microscopy and coimmunoprecipitation demonstrate Foxp2 and the androgen receptor protein interact. Databases for transcription factor binding sites predict a consensus binding motif for androgen receptor on the Foxp2 promoter regions. We also observed a sex difference in rat pup vocalization with males vocalizing more than females and treatment of females with dihydrotestosterone eliminated the sex difference. We propose that androgens might be an upstream regulator of both Foxp2 and Foxp1 expression and signaling. This has important implications for language and communication as well as neuropsychiatric developmental disorders involving impairments in communication.

Peripheral androgen action helps modulate vocal production in a suboscine passerine

Androgenic activation of intracellular androgen receptors (AR) influences avian vocal production, though this has largely been investigated at the level of the brain. We investigated the influence of predominantly peripheral AR on vocal output in wild Golden-collared Manakins (Manacus vitellinus). In this suboscine species, males court females by performing acrobatic displays and by producing relatively simple chee-poo vocalizations. To assess whether peripheral AR influences the acoustic structure of these vocal signals, we treated reproductively active adult males with the peripherally selective antiandrogen bicalutamide and then measured phonation performance. Inhibiting AR outside of the central nervous system increased the duration of the chee note and decreased the fundamental frequency of the poo note. This treatment caused no discernable change to chee-poo frequency modulation or entropy. Our results show that activation of peripheral AR mediates note-specific changes to temporal and pitch characteristics of the Golden-collared Manakin's main sexual call. Thus, our study provides one of the first demonstrations that androgenic action originating outside of the brain and likely on musculoskeletal targets can modulate avian vocal production.

Testosterone stimulates the expression of male-typical socio-sexual and song behaviors in female budgerigars (Melopsittacus undulatus): An experimental study

The hormonal control of sex differences in behavior has been extensively studied, particularly in mammals and birds. Studies have shown that the activational potential of the androgenic sex steroid testosterone (T) on male-typical behaviors in females seems to be species- as well as behavior-specific in birds. It is therefore important to study the activational effects of T in a great variety of bird species and on a wide range of behaviors, preferably in social conditions that favor their expression. Here, we investigated the activational effects of T on vocal, socio-sexual (i.e. affiliative and non-vocal courtship behaviors), aggressive and approach behavior in females of the budgerigar, Melopsittacus undulatus, a highly social monogamous parrot species. We experimentally supplemented T-females with male-like plasma T levels compared to controls. First, we observed females when they were individually housed. We found that T-females performed male-like levels of warbling song, sang significantly longer, but not more song bouts and produced more socio-sexual behaviors than controls. Then, we consecutively confronted females with a female, a dummy, and a male conspecific. T-females showed a significantly shorter latency to interact in all three social contexts. In both intrasexual and intersexual contexts, T-females performed significantly higher levels of approach and socio-sexual behavior, including "mounting (attempts)", a strictly male behavior, which was not observed in control females. Aggression in a non-reproductive context did not appear to be sensitive to T supplementation. Our data indicate that in the budgerigar even marked sex differences in socio-sexual behavior may depend on the activational effects of T, while this is generally not the case in other 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.

Zenk expression in auditory regions changes with breeding condition in male Black-capped chickadees (Poecile atricapillus)

Black-capped chickadees (Poecile atricapillus) produce different vocalizations at different times of year: the fee-bee song is produced by males primarily in spring, whereas the chick-a-dee call is produced year-round but most frequently in the fall and winter. We wanted to determine if neural response to playback in auditory regions of the songbird brain varied with season in parallel to production. We captured adult male black-capped chickadees from the wild in either breeding condition or non-breeding condition and within 24-48 h of bringing them into the laboratory setting, played them recordings of either conspecific vocalizations (fee-bee songs or chick-a-dee calls), heterospecific vocalizations (Song Sparrow, Melospiza melodia, song), or silence. We then measured ZENK protein immunoreactivity (Zenk-ir) in caudomedial nidopallium (NCM) and caudomedial mesopallium (CMM), two regions important for perception of conspecific vocalizations. We found that, overall, non-breeding birds had greater Zenk-ir than breeding birds. In addition, we found that birds in non-breeding condition had significantly greater Zenk-ir to heterospecific song than birds in breeding condition, but this difference was not seen in birds that heard conspecific songs or calls. Finally, in NCMd chickadees had greater response to playback of conspecific vocalizations (when combining song and call groups) than playback of heterospecific vocalizations but only while in breeding condition. Our results qualify the claim that Zenk-ir is biased toward conspecific vocalizations, and indicate that specificity of neural response varies with season. Variation could be a result of increased production and perceptual demand in spring, or hormonal changes in breeding birds, possibly because chickadees display vocal plasticity in chick-a-dee calls in the fall, requiring a degree of neural plasticity across seasons.

No evidence for general condition-dependence of structural plumage colour in blue tits: an experiment

Condition-dependence is a central but contentious tenet of evolutionary theories on the maintenance of ornamental traits, and this is particularly true for structural plumage colour. By providing diets of different nutritional quality to moulting male and female blue tits, we experimentally manipulated general condition within the natural range, avoiding deprivation or stressful treatments. We measured reflectance of the structural-coloured UV/blue crown, a sexually selected trait in males, and the white cheek, a nonpigmented structural colour, directly after moult and again during the following spring mating season. We employed a variety of colour indices, based on spectral shape and avian visual models but, despite significant variation in condition and coloration, found no evidence for condition-dependence of UV/blue or white plumage colour during either season. These and previously published results suggest that structural colour might be sensitive to stress, rather than reduced body condition, during moult.

The carotenoid conundrum: improved nutrition boosts plasma carotenoid levels but not immune benefits of carotenoid supplementation

Carotenoids are widely heralded as central to honest signaling due to their dual roles as pigments and antioxidants/immunostimulants. The aim of this study is to test if diet quality and carotenoids alone or in an interaction influence condition, carotenoid availability in plasma and immune responsiveness. Therefore, a diet experiment during the moult of great tits, Parus major, was performed. In a two-way design, we manipulated general quality (digestibility, protein and vitamin content) as well as carotenoid (lutein) content of semi-synthetic diets. Higher quality diet improved individual condition since birds had greater body mass, and to a lesser extent, higher hematocrit. In addition to the expected positive effect of carotenoid supplementation and individual lutein consumption on circulating lutein, there was a positive effect of enhanced diet quality on plasma carotenoid levels. Carotenoid supplementation, but not diet quality, improved the local inflammatory response and maintenance of body mass during a humoral immune reaction. The enhancement of circulating carotenoid levels by improved general quality of the diet or individual condition could provide a testable, mechanistic explanation for the variation in effects of carotenoid supplementation studies.

Enhanced testosterone levels affect singing motivation but not song structure and amplitude in Bengalese finches

Song is a fundamental component of territory defense and mate attraction in birds, and androgens (like testosterone) are known to play a key role in controlling it. However, little is known about how differences in testosterone levels between males translate into inter-individual song variation. Indeed, testosterone could affect both the motivation to sing and the structure of song itself. Here, we tested whether experimentally elevated testosterone levels in adult Bengalese finches (Lonchura striata var. domestica), an oscine bird species, have an activational effect on 1) song performance, and 2) song structure. Our results show that testosterone-treated males, in contrast to sham-control males, sang more when confronted with a female. Other performance-related traits, however, such as latency to sing and song amplitude, were not affected. Testosterone-treated males also showed no differences in our two measures of song structure: fundamental element frequency and mean song frequency. Because song structure is known to be organizationally affected by testosterone, our results, synthesized together with findings from the current literature, suggest that in oscine birds, song contains multiple messages about the signaler's hormonal status. First, song performance may reflect current hormonal condition, and second, song structure may reflect the past hormonal state.

Is testosterone immunosuppressive in a condition-dependent manner? An experimental test in blue tits

In this experiment we manipulated testosterone (T) and condition in juvenile male blue tits (Cyanistes caeruleus) during the moult, to test whether T's supposed immunosuppressive qualities are condition-dependent. To achieve this, we used T and control implants in combination with a dietary manipulation. We measured responses to both phytohaemagglutinin (PHA) and humoral immune challenges during the period of the treatments (moult) and also in the following breeding season (spring). During moult, males fed the enhanced diet were in better condition but there was no difference in humoral response between the dietary groups. T males produced a greater humoral antibody response than control (C) males. In the spring, males that had been previously treated with high T again exhibited higher antibody responses than C males. High T levels during moult were associated with a low PHA response but only in males with low body mass: heavier males that had high T exhibited the highest PHA responses. In the spring, the pattern of PHA responses was reversed; responses were highest in males that had low body mass but also had high T levels, and the lowest responses were by males that had both high T and were relatively heavy. Our results suggest that the effects of T on immunity can be either immunoenhancing or immunosuppressive, depending upon the condition of the individual, its life history stage, as well as on the immune challenge employed.