Androgen control of immunocompetence in the male house finch, Carpodacus mexicanus Müller

Control and adaptability of seasonal changes in behavior and physiology of latitudinal avian migrants: Insights from laboratory studies in Palearctic-Indian migratory buntings

Twice-a-year migrations, one in autumn and the other in spring, occur within a discrete time window with striking alterations in the behavior and physiology, as regulated by the interaction of endogenous rhythms with prevailing photoperiod. These seasonal voyages are not isolated events; rather, they are part of an overall annual itinerary and remain closely coupled to the other annual subcycles, called seasonal life history states (LHSs). The success of migration depends on appropriate timing of the initiation and termination of each LHS, for example, reproduction, molt, summer nonmigratory, preautumn migratory (fattening and weight gain), autumn migratory, winter nonmigratory (wnM), prevernal (spring) migratory (fattening and weight gain), and spring migratory LHSs. Migration-linked photoperiod-induced changes include the body fattening and weight gain, nocturnal Zugunruhe (migratory restlessness), elevated triglycerides and free fatty acids, triiodothyronine and corticosterone levels. Hypothalamic expression of the thyroid hormone-responsive dio2 and dio3, light-responsive per2, cry1, and adcyap1 and th (tyrosine hydroxylase, involved in dopamine biosynthesis) genes also show significant changes with transition from wnM to the vernal migratory LHS. Concurrent changes in the expression of genes associated with lipid metabolism and its transport also occur in the liver and flight muscles, respectively. Interestingly, there are clear differences in the behavioral and physiological phenotypes, and associated molecular changes, between the autumn and vernal migrations. In this review, we discuss seasonal changes in the behavior and physiology, and present molecular insights into the development of migratory phenotypes in latitudinal avian migrants, with special reference to Palearctic-Indian migratory buntings.

Elevated corticosterone levels are associated with increased immunocompetence in male toads, both when calling and under experimental conditions

Circulating androgens can influence immune responses and sexual traits in male vertebrates. However, in the last 20 years, glucocorticoids have also been implicated as a possible source of variation in male sexual traits and immunocompetence. In this context, we studied the relations between male vocal mating display, immunity, androgens, and glucocorticoids in the explosive breeding toad Rhinella granulosa. In the field, males with high calling effort display either high- or low-corticosterone (CORT) plasma levels, but only males with both high calling effort and high CORT plasma levels showed high bacterial killing ability (BKA), suggesting that the acute CORT elevation can be immunostimulatory. CORT treatments increased BKA in laboratory experiments, confirming the functional relationship observed in the field. However, toads treated with a low dose of CORT increased BKA for 10 h after the treatment, while toads that received a high dose increased BKA for only 1 h after the treatment. These results indicate that different CORT doses can result in temporal differences in the immune response. We did not find any relationship between calling effort, immune response (BKA and PHA swelling response), and testosterone plasma levels in the field, or any effects of testosterone treatment on immunocompetence. Our results suggest a complex relationship between calling effort and immunity, mediated by CORT plasma levels.

Involvement of steroid and antioxidant pathways in spleen-mediated immunity in migratory birds

The molecular underpinnings of the spleen-mediated immune functions during the period of heightened energetic needs in the year are not known in avian migrants. We investigated this, in Palearctic-Indian migratory male redheaded buntings, which exhibited vernal (spring) premigratory / early testicular maturation states under artificial long days. This was evidenced by increased dio2 and decreased dio3 mRNA expression in the hypothalamus, elevated levels of circulating corticosterone and testosterone, and enlarged testes in long-day-photostimulated birds, as compared to unstimulated controls under short days. The concomitant decrease in both mass and volume of the spleen, and increase in the heterophil/ lymphocyte ratio suggested the parallel innate immunity effects in photostimulated buntings. Importantly, we found increased mRNA expression of genes coding for the cytokines (il15 and il34), steroid receptors (nr3c2) and oxidative stress marker enzymes (gpx1 and sod1) in the spleen, suggesting the activation of both immune and antioxidant molecular pathways during the early photostimulated state. However, the splenic expressions of il1β, il6, tgfβ, ar and nos2 genes were not significantly different between long-day stimulated and short-day unstimulated birds. The negative correlation of plasma corticosterone levels with spleen mass further indicated a role of corticosterone in the modulation of the spleen function, probably via nr3c2 gene encoded mineralocorticoid receptors. These results suggest the activation of the spleen-mediated innate immunity in anticipation of the heightened energetic stress state of the photostimulated spring migratory/breeding period in migratory songbirds.

Disruption of energy homeostasis by food restriction or high ambient temperature exposure affects gonadal function in male house finches (Haemorhous mexicanus)

Reproductive success requires that individuals acquire sufficient energy resources. Restricting food availability or increasing energy expenditure (e.g., thermoregulation) inhibits reproductive development in multiple avian species, but the nature of the energy-related signal mediating this effect is unclear. To investigate this question, we examined reproductive and metabolic physiology in male house finches that either underwent moderate food restriction (FR) or were exposed to high temperature (HT), in which birds were held at a high, but not locally atypical, ambient temperature cycle (37.8 °C day, 29.4 °C night) compared to a control group (CT; 29.4 °C day, 21.1 °C night). We hypothesized that FR and HT inhibit reproductive development by lowering available metabolic fuel, in particular plasma glucose (GLU) and free fatty acids (FFA). Following FR for 4 weeks, finches lost body mass, had marginally higher plasma FFA, and experienced a 90% reduction in testis mass compared to CT birds. Four weeks of HT exposure resulted in reduced voluntary food consumption and muscle mass, as well as an 80% reduction in testis mass relative to CT birds. Both FR and HT birds expressed less testicular 17β-hydroxysteroid dehydrogenase (17β-HSD) mRNA than controls but the expression of other testicular genes measured was unaffected by either treatment. Neither treatment significantly influenced plasma GLU. This study is among the first to demonstrate a negative effect of HT on reproductive development in a wild bird. Further studies are needed to clarify the role of metabolic mediators and their involvement under various conditions of energy availability and demand.

The effect of food restriction on the regulation of gonadotropin-releasing hormone in male house finches (Haemorhous mexicanus)

Seasonal activation of the vertebrate hypothalamic-pituitary-gonadal (HPG) axis and gonadal development is initiated by gonadotropin-releasing hormone-I (GnRH) release from the hypothalamus. In photoperiodic species, the consistent annual change in photoperiod is the primary environmental signal affecting GnRH cell activity, including changes in the synthesis and secretion of this neuropeptide. Non-photoperiodic environmental cues such as energy availability also influence HPG axis activity, but the mechanisms mediating this influence, in particular on the GnRH system, are unclear. Understanding how the neuroendocrine system integrates environmental information is critical in determining the plasticity and adaptability of physiological responses to changing environments. The primary objective of this study was to investigate GnRH-mediated changes in HPG axis activity and gonadal development in response to energy availability in a wild bird. We hypothesized that negative energy balance inhibits HPG axis activity by affecting GnRH secretion. Moderate food restriction for several weeks in male house finches, Haemorhous mexicanus, decreased body condition and inhibited photoinduced testicular growth compared to birds fed ad libitum. Food restriction did not affect plasma luteinizing hormone (LH; a correlate of GnRH release) or plasma testosterone, but it enhanced the plasma LH response to an injection of the glutamatergic agonist, N-methyl-D-aspartate (NMDA). Thus, food restriction may decrease photoinduced HPG axis activation by acting centrally, in particular by attenuating the release of accumulated GnRH stores.

Melanin-based coloration and host-parasite interactions under global change

The role of parasites in shaping melanin-based colour polymorphism, and the consequences of colour polymorphism for disease resistance, remain debated. Here we review recent evidence of the links between melanin-based coloration and the behavioural and immunological defences of vertebrates against their parasites. First we propose that (1) differences between colour morphs can result in variable exposure to parasites, either directly (certain colours might be more or less attractive to parasites) or indirectly (variations in behaviour and encounter probability). Once infected, we propose that (2) immune variation between differently coloured individuals might result in different abilities to cope with parasite infection. We then discuss (3) how these different abilities could translate into variable sexual and natural selection in environments varying in parasite pressure. Finally, we address (4) the potential role of parasites in the maintenance of melanin-based colour polymorphism, especially in the context of global change and multiple stressors in human-altered environments. Because global change will probably affect both coloration and the spread of parasitic diseases in the decades to come, future studies should take into account melanin-based coloration to better predict the evolutionary responses of animals to changing disease risk in human-altered environments.

Are there synergistic or antagonistic effects of multiple maternally-derived egg components (antibodies and testosterone) on offspring phenotype?

Eggs are ‘multivariate’ in that they contain multiple maternally-derived egg components (e.g. hormones, antibodies, mRNA, antioxidants) which are thought to influence offspring phenotype. However, most studies have focused on single egg components and on short-term effects. Here, we simultaneously manipulated two egg components, maternally-derived antibodies (MAb) and yolk testosterone (T) to assess potential synergistic or antagonistic effects on offspring phenotype from hatching to sexual maturity. We found no evidence for short-or long-term effects of either MAb or yolk T alone, or their interaction, on hatching mass, size at fledging (tarsus), body mass at sexual maturity (day 82), chick survival, humoral immune function, or any measured female reproductive trait at sexual maturity. There was a positive effect of yolk T, but not MAb, on offspring PHA response at 26 days of age but at 82 days of age MAb, but not yolk T, had a positive effect on PHA response. There was also a MAb*sex interaction on 30 day chick mass, and a positive effect of yolk T on male courtship behaviour at sexual maturity. However, we found no evidence for synergy, i.e. where offspring treated both with MAb and yolk T had higher trait values than offspring treated with either MAb or yolk T alone for any measured trait. Similarly, evidence for antagonistic (compensatory) effects, where offspring treated both with MAb and yolk T had intermediate trait values compared with offspring treated with either MAb or yolk T alone, was equivocal.

Effects of temperature on the timing of breeding and molt transitions in house finches

Temperature-correlated shifts in reproductive timing are now well documented in numerous bird species. However, whether temperature directly influences reproductive timing or whether its effects are mediated by an intermediate environmental cue, such as plant phenology, remains poorly understood. In this study, we investigated the direct effects of temperature on reproductive timing in house finches (Haemorhous mexicanus), which have a range and breeding diet not well represented in previous studies of temperature and reproductive timing. We conducted experiments with captive male house finches in which temperature was elevated within realistic ranges and the effects on the timing of preparations for reproduction, as well as on the termination of reproduction and the onset of prebasic feather molt, were examined. We found no adjustments in the timing of reproductive preparations of males in direct response to temperature. However, elevated temperature did advance the breeding-molt transition. Our results suggest that elevated temperatures in the range tested here do not directly impact physiological preparations for reproduction in male house finches, but may constrain the timing of the breeding-molt transition in this species.

Seasonal Patterns of Variation in Steroid Plasma Levels and Immune Parameters in Anurans from Brazilian Semiarid Area

Elevated androgens and glucocorticoids displayed by males during the reproductive season have been proposed to mediate a possible trade-off between reproduction and immunocompetence. Anurans living in arid and semiarid environments display a strong seasonal reproduction, which could accentuate the variation in physiological, immunological, and behavioral parameters. We studied covariation between steroid plasma levels, morphometric variables associated with body condition and immunity, leukocyte profile, parasite load, and response to an immunological challenge across different phases of the annual life-history cycle of three anuran species from a Brazilian semiarid area. Our results showed a seasonal pattern of covariation among leukocyte parameters, kidney mass, and steroid plasma levels, with higher values measured during the reproductive season, particularly when males were sampled during calling activity. Moreover, these anurans showed a stronger response to an immunological challenge during the reproductive period. The immunosuppression during the dry period was particularly evident for the species that aestivate, indicating that the availability of energetic resources might be an important factor determining seasonal variation in inflammatory response. Intensity of the helminth infection was associated with eosinophil count but showed a more complex pattern with regard to androgens levels. These data emphasize that variations in the intensity of helminth infection might be more closely related to specific aspects of the immune response than to the general seasonal patterns of variation in steroid plasma levels, total circulating leukocytes, and inflammatory response.

The immunomodulatory role of the hypothalamus-pituitary-gonad axis: Proximate mechanism for reproduction-immune trade offs?

The present review discusses the communication between the hypothalamic-pituitary-gonad (HPG) axis and the immune system of vertebrates, attempting to situate the HPG-immune interaction into the context of life history trade-offs between reproductive and immune functions. More specifically, (i) we review molecular and cellular interactions between hormones of the HPG axis, and, as far as known, the involved mechanisms on immune functions, (ii) we evaluate whether the HPG-immune crosstalk serves as proximate mechanism mediating reproductive-immune trade-offs, and (iii) we ask whether the nature of the HPG-immune interaction is conserved throughout vertebrate evolution, despite the changes in immune functions, reproductive modes, and life histories. In all vertebrate classes studied so far, HPG hormones have immunomodulatory functions, and indications exist that they contribute to reproduction-immunity resource trade-offs, although the very limited information available for most non-mammalian vertebrates makes it difficult to judge how comparable or different the interactions are. There is good evidence that the HPG-immune crosstalk is part of the proximate mechanisms underlying the reproductive-immune trade-offs of vertebrates, but it is only one factor in a complex network of factors and processes. The fact that the HPG-immune interaction is flexible and can adapt to the functional and physiological requirements of specific life histories. Moreover, the assumption of a relatively fixed pattern of HPG influence on immune functions, with, for example, androgens always leading to immunosuppression and estrogens always being immunoprotective, is probably oversimplified, but the HPG-immune interaction can vary depending on the physiological and envoironmental context. Finally, the HPG-immune interaction is not only driven by resource trade-offs, but additional factors such as, for instance, the evolution of viviparity shape this neuroendocrine-immune relationship.

Helminth burden and ecological factors associated with alterations in wild host gastrointestinal microbiota

Infection by gastrointestinal helminths of humans, livestock and wild animals is common, but the impact of such endoparasites on wild hosts and their gut microbiota represents an important overlooked component of population dynamics. Wild host gut microbiota and endoparasites occupy the same physical niche spaces with both affecting host nutrition and health. However, associations between the two are poorly understood. Here we used the commonly parasitized European shag (Phalacrocorax aristotelis) as a model wild host. Forty live adults from the same colony were sampled. Endoscopy was employed to quantify helminth infection in situ. Microbiota from the significantly distinct proventriculus (site of infection), cloacal and faecal gastrointestinal tract microbiomes were characterised using 16S rRNA gene-targeted high-throughput sequencing. We found increasingly strong associations between helminth infection and microbiota composition progressing away from the site of infection, observing a pronounced dysbiosis in microbiota when samples were partitioned into high- and low-burden groups. We posit this dysbiosis is predominately explained by helminths inducing an anti-inflammatory environment in the proventriculus, diverting host immune responses away from themselves. This study, within live wild animals, provides a vital foundation to better understand the mechanisms that underpin the three-way relationship between helminths, microbiota and hosts.

Food restriction negatively affects multiple levels of the reproductive axis in male house finches, Haemorhous mexicanus

Nutrition influences reproductive functions across vertebrates, but the effects of food availability on the functioning of the hypothalamic-pituitary-gonadal (HPG) axis in wild birds and the mechanisms mediating these effects remain unclear. We investigated the influence of chronic food restriction on the HPG axis of photostimulated House Finches, Haemorhous mexicanus. Food-restricted birds had underdeveloped testes with smaller seminiferous tubules than ad libitum-fed birds. Baseline plasma testosterone (T) increased in response to photostimulation in ad libitum-fed but not in food-restricted birds. Food availability did not, however, affect the plasma T increase resulting from a gonadotropin-releasing hormone (GnRH) or a luteinizing hormone (LH) challenge. The number of hypothalamic GnRH-I immunoreactive (ir) but not proGnRH-ir perikarya was higher in food-restricted than ad libitum-fed finches, suggesting inhibited secretion of GnRH. Hypothalamic gonadotropin-inhibitory hormone (GnIH)-ir and neuropeptide Y (NPY)-ir were not affected by food availability. Plasma corticosterone (CORT) was also not affected by food availability, indicating that the observed HPG axis inhibition did not result from increased activity of the hypothalamic-pituitary-adrenal (HPA) axis. This study is among the first to examine multilevel functional changes in the HPG axis in response to food restriction in a wild bird. The results indicate that food availability affects both hypothalamic and gonadal function, but further investigations are needed to clarify the mechanisms by which nutritional signals mediate these effects.

House finch responses to Mycoplasma gallisepticum infection do not vary with experimentally increased aggression

Aggression can alter infectious disease dynamics through two, non-exclusive mechanisms: 1) increasing direct contact among hosts and 2) altering hosts' physiological response to pathogens. Here we examined the latter mechanism in a social songbird by manipulating intraspecific aggression in the absence of direct physical contact. We asked whether the extent of aggression an individual experiences alters glucocorticoid levels, androgen levels, and individual responses to infection in an ecologically relevant disease model: house finches (Haemorhous mexicanus) infected with Mycoplasma gallisepticum (MG). Wild-caught male finches were housed in one of three settings, designed to produce increasing levels of aggression: 1) alone, with no neighbor ("no neighbor"), 2) next to a sham-implanted stimulus male ("sham neighbor"), or 3) next to a testosterone-implanted stimulus male ("testosterone neighbor"). Following one week of social treatment, focal males were experimentally infected with MG, which causes severe conjunctivitis and induces sickness behaviors such as lethargy and anorexia. While social treatment increased aggression as predicted, there were no differences among groups in baseline corticosterone levels, total circulating androgens, or responses to infection. Across all focal individuals regardless of social treatment, pre-infection baseline corticosterone levels were negatively associated with the severity of conjunctivitis and sickness behaviors, suggesting that corticosterone may dampen inflammatory responses in this host-pathogen system. However, because corticosterone levels differed based upon population of origin, caution must be taken in interpreting this result. Taken together, these results suggest that in captivity, although aggression does not alter individual responses to MG, corticosterone may play a role in this disease.

Body height, immunity, facial and vocal attractiveness in young men

Health, facial and vocal attributes and body height of men may affect a diverse range of social outcomes such as attractiveness to potential mates and competition for resources. Despite evidence that each parameter plays a role in mate choice, the relative role of each and inter-relationships between them, is still poorly understood. In this study, we tested relationships both between these parameters and with testosterone and immune function. We report positive relationships between testosterone with facial masculinity and attractiveness, and we found that facial masculinity predicted facial attractiveness and antibody response to a vaccine. Moreover, the relationship between antibody response to a hepatitis B vaccine and body height was found to be non-linear, with a positive relationship up to a height of 188 cm, but an inverse relationship in taller men. We found that vocal attractiveness was dependent upon vocal masculinity. The relationship between vocal attractiveness and body height was also non-linear, with a positive relationship of up to 178 cm, which then decreased in taller men. We did not find a significant relationship between body height and the fundamental frequency of vowel sounds provided by young men, while body height negatively correlated with the frequency of second formant. However, formant frequency was not associated with the strength of immune response. Our results demonstrate the potential of vaccination research to reveal costly traits that govern evolution of mate choice in humans and the importance of trade-offs among these traits.

In situ forming microparticle implants for delivery of sex steroids in fish: Modulation of the immune response of gilthead seabream by testosterone

Current knowledge on the sensitivity of marine fish to androgenic environmental chemicals is limited, despite the growing interest in the effects of endocrine disrupting chemicals. To study in vivo the effects of testosterone (T) on the fish immune response, we used a microencapsulation implant technique, the in situ forming microparticle system, containing 1 mg T/kg body weight (T-ISM), in adult specimens of gilthead seabream (Sparus aurata L.), a species of great economic interest. We demonstrated that implants themselves (without T) have no significant effect on most of the parameters measured. In T-ISM implanted fish, T serum levels reached supraphysiological concentrations accompanied by a slight increase in 11-ketotestosterone and 17β-estradiol levels 21 days post-implantation (dpi). Liver and head-kidney samples were processed 7 and 21 dpi to assess T-ISM effect on (i) the mRNA expression of genes involved in the metabolism of steroid hormones and in the immune response, and (ii) phagocyte activities. The expression profile of cytokines, chemokines and immune receptors was altered in T-ISM implanted animals that showed an early pro-inflammatory tendency, and then, a mixed pro-/anti-inflammatory activation during longer exposure. Furthermore, the enhancement of phagocytic activity and the production of reactive oxygen species by leukocytes 21 dpi in T-ISM implanted specimens suggest fine modulation of the innate immune response by T. Taken together, these data demonstrate for the first time the feasibility of using ISM implants in an aquatic species, and provide new data on the role played by T on the immune response in fish.

The impact of exposure to a novel female on symptoms of infection and on the reproductive axis

BACKGROUND/AIM

Sickness behaviors are the behavioral alterations animals exhibit during the course of an infection, often accompanied by reduced reproductive activity. Adopting sickness behaviors may aid in overcoming the infection, by diverting energy from routine activities towards enhancement of the immune system. Nonetheless, sickness behaviors are plastic, being influenced by specific environmental and social circumstances. Here, we tested whether the presentation of a novel female to males suffering from a simulated infection could impact the behavioral effects of sickness, the reproductive axis, or both.

METHODS

Male zebra finches were housed in isolation and injected intramuscularly with lipopolysaccharide or saline. Behaviors were recorded before (3 h before injection) and after (3.5 h after injection) addition of a novel female to the cage for 30 min. Four hours after injection, we collected the brain and testis for the analysis of important reproductive axis modulators, gonadotropin-releasing hormone, and gonadotropin-inhibitory hormone, and to quantify gene expression of a proinflammatory cytokine involved in the regulation of sickness behaviors [interleukin (IL)-1β]. Testosterone was quantified in the plasma.

RESULTS

The presence of a novel female diminished sickness behaviors and induced alterations in the reproductive axis within 30 min, with no associated changes in brain gene expression of IL-1β. Social environment itself altered brain gene expression of IL-1β.

CONCLUSIONS

Male zebra finches prioritize the opportunity to mate versus investment in recovery from an infection, as determined by reduced expression of sickness behaviors when a potential mate was present. The behavioral effects of IL-1β appear to be context dependent in this species.

Cues to sex- and stress-hormones in the human male face: functions of glucocorticoids in the immunocompetence handicap hypothesis

The stress-linked version of the immunocompetence handicap hypothesis has been proposed to account for inconsistencies in relationships between testosterone and immune response. The model has received some support from studies demonstrating roles of stress hormones in relationships between testosterone, immune function and secondary sexual ornamentation. Such work, however, has relied on artificial elevation of testosterone so may not reflect relationships in natural populations. We created human male facial stimuli on the basis of naturally co-occurring levels of salivary testosterone and the stress hormone cortisol. In Study 1 we tested female preferences for male faces with cues to combinations of the hormones across the menstrual cycle, and in Study 2 we tested perceptions of health and dominance in a novel set of facial stimuli. Females preferred cues to low cortisol, a preference that was strongest during the fertile phase of the menstrual cycle. The effects of cortisol on attractiveness and perceived health and dominance were contingent upon level of testosterone: the effects of the stress hormone were reduced when testosterone was high. We propose explanations for our results, including low cortisol as a cue to a heritable component of health, attractiveness as a predictor of low social-evaluative threat (and, therefore, low baseline cortisol) and testosterone as a proxy of male ability to cope efficiently with stressors.

Energetic cost of communication

Communication signals may be energetically expensive or inexpensive to produce, depending on the function of the signal and the competitive nature of the communication system. Males of sexually selected species may produce high-energy advertisement signals, both to enhance detectability and to signal their size and body condition. Accordingly, the proportion of the energy budget allocated to signal production ranges from almost nothing for many signals to somewhere in excess of 50% for acoustic signals in short-lived sexually selected species. Recent data from gymnotiform electric fish reveal mechanisms that regulate energy allocated to sexual advertisement signals through dynamical remodeling of the excitable membranes in the electric organ. Further, males of the short-lived sexually selected species, Brachyhypopomus gauderio, trade off among different metabolic compartments, allocating energy to signal production while reducing energy used in other metabolic functions. Female B. gauderio, by contrast, do not trade off energy between signaling and other functions. To fuel energetically expensive signal production, we expect a continuum of strategies to be adopted by animals of different life history strategies. Future studies should explore the relation between life history and energy allocation trade-offs.

Plasma testosterone levels decrease after activation of skin immune system in a free-ranging mammal

The immunocompetence handicap hypothesis (ICHH) postulates a trade-off between the display of androgen-related, sexually selected traits and investment in immune functions, often summarized as immunosuppression by testosterone (TE). We investigated the relationship between delayed type hypersensitivity (DTH) skin responses and the endocrine system in a free-ranging mammal and asked: (i) are high plasma TE levels related to a weak DTH response or (ii) does an activation of the cellular immune system cause a decrease in plasma TE secretion? and (iii) does baseline cortisol (CORT) and the capacity to release CORT in response to stress influence the DTH response? We studied these questions in the bat Carollia perspicillata and conducted an immune challenge (IC), a DTH skin test with the antigen 1,4-dinitrofluorobenzene (DNFB), as a measure for antigen-specific cell-mediated immunity. We found a high individual variance in DTH response irrespective of sex. In addition, pre-IC plasma TE did not affect the extent of DTH responses, but instead the IC lowered post-IC TE titres in both sexes. Also, pre-IC baseline CORT titres did not influence the DTH responses significantly, but the extent of an induced CORT response was negatively related to the DTH reaction. Our study does not support the presumption of an immunosuppressive effect of TE, but suggests that an activation of the immune system reduces plasma TE. This is still in line with the ICHH as originally formulated, because only immunocompetent animals are able to cope with pathogens effectively and maintain high TE levels at the same time.

Evidence for the stress-linked immunocompetence handicap hypothesis in human male faces

The stress-linked immunocompetence handicap hypothesis (SL-ICHH) of sexual selection incorporates a role of the stress hormone corticosterone (C; cortisol in humans) in relationships between testosterone (T), immunity and secondary sexual trait expression. In support of this, C has been shown to mediate and moderate relationships between T and immune response and to be inversely related to attractiveness in some avian species. We predicted that female preferences for cues to T in human male faces would be contingent upon co-occurring cortisol levels. In study 1, we tested relationships between T and cortisol and attractiveness, masculinity and health ratings of raw male faces. We found cortisol to be inversely related to attractiveness. In study 2, we tested female preferences for male faces that were parametrically manipulated on the basis of cues to naturally co-occurring levels of T and cortisol across the menstrual cycle. Women preferred cues to low cortisol in general and in the fertile phase of the cycle, and there was an interaction between T and cortisol in general and in the non-fertile phase. Results were consistent with the SL-ICHH but not the original immunocompetence handicap model: females expressed preferences for cues to cortisol but not for cues to T, except in interaction with the stress hormone. Results inform the SL-ICHH by demonstrating female preferences for low cortisol and the nature of its interaction with T in humans, as well as indicating the traits that may be signalled by different combinations of the hormones including immune response, current health and resource acquisition characteristics.

Effects of methyltestosterone on immunity against Salmonella Pullorum in dwarf chicks

This study was conducted to determine effects of methyltestosterone on innate immunity and adaptive immunity against Salmonella Pullorum in dwarf chicks. In vivo experiment, comparisons of pathological sections, viable counts of bacteria, specific antibody levels, and subsets of T lymphocytes were set forth between chicks with or without 10(-7) M methyltestosterone treatment (2 d of age through 21 d of age) and challenged with 5 x 10(8) virulent Salmonella Pullorum (7 d of age), and in vitro experiment, phagocytic and killing abilities, reactive oxygen intermediate production, and reactive nitrogen intermediate production of monocytes-macrophages treated with high (10(-8) M/10(6) cell) or physiological (10(-14) M/10(6) cell) concentration of methyltestosterone were examined after Salmonella Pullorum infection. The results showed that (1) in vivo, administration of methyltestosterone enhanced susceptibility to Salmonella Pullorum infection and depressed cellular immunity against Salmonella Pullorum, whereas it had no effect on humoral immunity in dwarf chicks; (2) in vitro, at high concentration, methyltestosterone reduced (P < 0.05) monocytes-macrophages mediated reactive oxygen intermediate-dependent killing of Salmonella Pullorum, whereas low concentration of methyltestosterone enhanced (P < 0.05) reactive oxygen intermediate-dependent killing of Salmonella Pullorum in male dwarf chicks but not in females; and (3) although challenged with Salmonella Pullorum, phagocytic ability and monocytes-macrophages mediated reactive nitrogen intermediate-dependent killing were not affected by methyltestosterone in vitro. The results indicated that methyltestosterone affected the immune response to Salmonella Pullorum in dwarf chicks by changing monocytes-macrophages mediated reactive oxygen intermediate-dependent killing and cellular immunity, and the effects were dose-dependent; furthermore, the former 2 pathways played important roles in preventing Salmonella Pullorum infection in dwarf chicks, although the mechanism needs further study.

Blood parasitaemia in a high latitude flexible breeder, the white-winged crossbill,Loxia leucoptera: contribution of seasonal relapse versus new inoculations

We measured seasonal changes in the prevalence of haematozoa (Leucocytozoon fringillinarum, Haemoproteus fringillae, andTrypanosoma avium) in free-ranging White-winged Crossbills,Loxia leucoptera, over 1·5 year in Fairbanks, Alaska, USA. This prevalence was low during early winter.L. fringillinarumprevalence increased in late winter/early spring, in the absence of vectors, suggesting relapse of latent infection. By contrast, the prevalence ofT. aviumandH. fringillaedid not increase until mid-spring, coincident with the emergence of putative vectors and suggestive of new inoculations. The winter breeding period was not associated with lower body condition or elevated blood heterophil/lymphocyte ratios than the summer post-breeding period. Thus, birds unlikely perceived their breeding effort as particularly stressful. Adult males in May and June had low plasma testosterone and their blood prevalence ofL. fringillinarum, but not other haemoparasites, was higher than in adult females. This difference may have resulted from sex differences in behaviour and/or plumage colouration – bright red in males, dull green/yellow in females. Species in which reproduction and vector abundance are seasonally dissociated may constitute important models for investigating the respective contribution of reproductive hormones, breeding effort, and vector abundance to patent and latent hemoparasitic infections and to new inoculations.

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.

The effects of testosterone on immune function in quail selected for divergent plasma corticosterone response

The immunocompetence handicap hypothesis (ICHH) suggests that the male sex hormone testosterone has a dual effect; it controls the development and expression of male sexually selected signals, and it suppresses the immune system. Therefore only high quality males are able to fully express secondary sexual traits because only they can tolerate the immunosuppressive qualities of testosterone. A modified version of the ICHH suggests that testosterone causes immunosuppression indirectly by increasing the stress hormone corticosterone (CORT). Lines of Japanese quail (Coturnix japonica)selected for divergent responses in levels of plasma CORT were used to test these hypotheses. Within each CORT response line (as well as in a control stock) we manipulated levels of testosterone in castrated quail by treatment with zero (sham), low or high testosterone implants, before testing the birds'humoral immunity and phytohaemagglutinin (PHA)-induced immune response, as well as body condition. The PHA-induced response was not significantly affected by CORT selected line, testosterone treatment or their interaction. There was, however, a significant effect of CORT line on humoral immunity in that the control birds exhibited the greatest antibody production, but there was no significant effect of testosterone manipulation on humoral immunity. The males in the sham implant treatment group had significantly greater mass than the males in the high testosterone group, suggesting a negative effect of high testosterone on general body condition. We discuss these results in the context of current hypotheses in the field of sexual selection.

Dynamics of PHA-induced immune response and plasma carotenoids in birds: should we have a closer look?

Allocation trade-offs of limited resources are thought to ensure the honesty of sexual signals and are often studied using controlled immune challenges. One such trade-off between immunity and ornaments is that involving carotenoids. Phytohemagglutinin (PHA)-induced immune response is a widely used immune challenge, yet more details on the underlying physiological mechanisms and potential costs are needed. We investigated the temporal dynamics of PHA-induced immune response and associated changes in blood carotenoids, body mass and a carotenoid-based coloured signal. We found variation in individual response patterns to PHA after peak swelling was reached, with birds showing either a rapid or a slow subsequent decrease in swelling, suggesting variation in the duration of the immune response and/or inflammation. Body mass did not affect immune response. Plasma carotenoids followed a transient decrease closely matching the dynamics of the swelling. The peak of the immune response was negatively related to initial plasma carotenoid levels and positively correlated to the relative decrease in plasma carotenoids. Individual variation in duration of the swelling could be partly explained by plasma carotenoids; high initial carotenoid levels were associated with a slower decrease of the swelling. These contradictory effects of carotenoids suggest a complex role in the immune response. Bill colour was positively correlated to initial plasma carotenoid concentration but it did not predict or change as a consequence of immune response to PHA. Bill colour thus reflects medium- or long-term quality rather than immediate quality. Taking into account the dynamics of the immune response and that of associated physiological parameters would thus yield new insights into our interpretation of variation in PHA response.

Testosterone treatment diminishes sickness behavior in male songbirds

Males of many vertebrate species are typically more prone to disease and infection than female conspecifics, and this sexual difference is partially influenced by the immunosuppressive properties of testosterone (T) in males. T-induced immunosuppression has traditionally been viewed as a pleiotropic handicap, rather than an adaptation. Recently, it has been hypothesized that suppression of sickness behavior, or the symptoms of infection, may have adaptive value if sickness interferes with the expression of T-mediated behaviors important for male reproductive success. We conduct a classic hormone replacement experiment to examine if T suppresses sickness behavior in a seasonally-breeding songbird, Gambel's white-crowned sparrow (Zonotrichia leucophrys gambelii). Triggered experimentally by bacterial lipopolysaccharide (LPS), sickness behavior includes decreased activity, anorexia, and weight loss. Gonadectomized (GDX) males that were treated with silastic implants filled with T exhibited suppression of behavioral and physiological responses to LPS compared to GDX and sham-GDX controls given empty implants. Sickness responses of control groups were statistically indistinguishable. T-implanted birds had significantly higher plasma T than control groups and levels were within the range associated with aggressive interactions during male-to-male contests. These findings imply that suppression of sickness behavior could occur when T is elevated to socially-modulated levels. Alternatively, it is possible that this suppressive effect is mediated through a stress-induced mechanism, as corticosterone levels were elevated in T-implanted subjects compared to controls. We propose that males wounded and infected during contests may gain a brief selective advantage by suppressing sickness responses that would otherwise impair competitive performance. The cost of immunosuppression would be manifested in males through an increased susceptibility to disease, which is presumably offset by capitalizing upon limited reproductive opportunities.

Immunomodulatory parasites and toll-like receptor-mediated tumour necrosis factor alpha responsiveness in wild mammals

Background

Immunological analyses of wild populations can increase our understanding of how vertebrate immune systems respond to 'natural' levels of exposure to diverse infections. A major recent advance in immunology has been the recognition of the central role of phylogenetically conserved toll-like receptors in triggering innate immunity and the subsequent recruitment of adaptive response programmes. We studied the cross-sectional associations between individual levels of systemic toll-like receptor-mediated tumour necrosis factor alpha responsiveness and macro- and microparasite infections in a natural wood mouse (Apodemus sylvaticus) population.

Results

Amongst a diverse group of macroparasites, only levels of the nematode Heligmosomoides polygyrus and the louse Polyplax serrata were correlated (negatively) with innate immune responsiveness (measured by splenocyte tumour necrosis factor alpha responses to a panel of toll-like receptor agonists). Polyplax serrata infection explained a strikingly high proportion of the total variation in innate responses. Contrastingly, faecal oocyst count in microparasitic Eimeria spp. was positively associated with innate immune responsiveness, most significantly for the endosomal receptors TLR7 and TLR9.

Conclusion

Analogy with relevant laboratory models suggests the underlying causality for the observed patterns may be parasite-driven immunomodulatory effects on the host. A subset of immunomodulatory parasite species could thus have a key role in structuring other infections in natural vertebrate populations by affecting the 'upstream' innate mediators, like toll-like receptors, that are important in initiating immunity. Furthermore, the magnitude of the present result suggests that populations free from immunosuppressive parasites may exist at 'unnaturally' elevated levels of innate immune activation, perhaps leading to an increased risk of immunopathology.

Plasticity of the Rufous-winged Sparrow, Aimophila carpalis, song control regions during the monsoon-associated summer breeding period

In most temperate zone songbirds, exposure to increasing photoperiod in the spring stimulates the reproductive system and induces reproductive behaviors. Additionally, the brain regions that control singing (song control regions; SCRs) are larger during the breeding season, thus paralleling changes in the activity of the reproductive system. However, in some birds, environmental factors other than photoperiod initiate breeding. For example, free-living male Rufous-winged Sparrows develop their testes in March due to increasing photoperiod, but have relatively low plasma T until after they begin to breed, usually in July, during the monsoon period when day length is declining. We tested the hypothesis that SCRs grow and singing behavior increases after the monsoon rains begin. We captured adult male Rufous-winged Sparrows in July 2002, 7 days before and 20 days after the monsoon rains began, euthanized birds in the field, collected their brains, and measured SCR volumes from sections immunostained for the neuronal marker NeuN. In June and July 2006, we measured song rates in the field before and after the monsoon rains. SCR volumes were larger and singing behavior increased after the onset of the monsoon rains, coinciding with the initiation of breeding. Unlike in other species studied so far, SCR volumes grew as day length was decreasing. Comparative studies utilizing species that do not breed when day length is increasing may provide information on the relative contributions of various environmental factors to SCR neuroplasticity.

Hormonal and environmental control of song control region growth and new neuron addition in adult male house finches,Carpodacus mexicanus

In songbirds, testosterone (T) mediates seasonal changes in the sizes and neuroanatomical characteristics of brain regions that control singing (song control regions; SCRs). One model explaining the mechanisms of the growth of one SCR, the HVC, postulates that in the spring increasing photoperiod and circulating T concentrations enhance new neuron survival, thus increasing total neuron number. However, most research investigating the effects of T on new neuron survival has been done in autumn. The present study investigated the effects of photoperiod and T treatment on SCR growth and new neuron survival in the HVC in photosensitive adult male House Finches, Carpodacus mexicanus, under simulated spring-like conditions. Birds were castrated, given T-filled or empty Silastic capsules and maintained on short days (SD; 8L:16D) or long days (LD; 16L:8D). To mark new cells, birds received bromodeoxyuridine injections 11 days after experimental manipulations began and were sacrificed 28 days later. Testosterone treatment increased the sizes of two SCRs, the HVC and Robust nucleus of the arcopallium (RA). Exposure to LD did not affect HVC volume, but did increase RA volume. Testosterone treatment increased the total number of HVC neurons, but did not affect the number of new HVC neurons. Thus, T initiates SCR growth and increases neuron survival, but effects of T on new neuron incorporation may be limited in photosensitive birds under spring-like conditions. These results provide new insight into the effects of photoperiod and T treatment on vernal SCR growth and new neuron incorporation and support current models explaining this growth.

Behavioral and physiological responses to experimentally elevated testosterone in female dark-eyed juncos (Junco hyemalis carolinensis)

Testosterone mediates the expression of many fitness-related traits in male vertebrates and is thought to account for numerous sex differences in trait expression. Testosterone is also secreted by females; however, far less is known regarding its effects on female physiology and behavior. Using a bird species in which the effects of testosterone on males are well characterized, the dark-eyed junco (Junco hyemalis), we tested whether an increase in exogenous testosterone in females would alter the phenotypic expression of a suite of behavioral and physiological traits. We found that increased testosterone levels in female dark-eyed juncos led to decreased cell-mediated immune function and increased intrasexual aggression, hypothalamo-pituitary-adrenal (HPA) axis responsiveness, baseline corticosterone and corticosterone-binding globulin (CBG) levels. Furthermore, immunosuppression following testosterone implantation was negatively correlated with total and free testosterone but did not appear to be related to either total or free corticosterone. These results demonstrate that the phenotypic impact of elevated testosterone is not confined to males in dark-eyed juncos, and that the impact in adults can be similar in males and females. We discuss these results in the context of potential endocrine-immune interactions and the evolution of sexual dimorphism.

Control of luteinizing hormone and testosterone secretion in a flexibly breeding male passerine, the Rufous-winged Sparrow, Aimophila carpalis

Rufous-winged Sparrows, Aimophila carpalis, reside in the Sonoran desert and although testicular development is initiated in the spring under the influence of increasing day length, breeding occurs opportunistically in summer in association with heavy rainfall or "monsoon". The aim of this study in free-living male Rufous-winged Sparrows was to establish the relationship between concentrations of plasma luteinizing hormone (LH) and testosterone (T), and breeding associated with heavy rainfall, and to investigate whether breeding is mediated by changes in pituitary gland sensitivity to gonadotropin releasing hormone-I (GnRH) and the recently discovered avian gonadotropin-inhibitory hormone (GnIH). Concentrations of plasma LH and T were relatively low until mid-summer, but increased rapidly and transiently immediately prior to the monsoon which occurred after the summer solstice, when day lengths were decreasing. At this time the birds came into full breeding condition. An injection of chicken GnRH (10 ng) increased plasma LH within 2 min when given before or during the monsoon. An injection of GnIH (1 microg) did not affect plasma LH within 2 min during the monsoon and did not decrease GnRH-elicited LH secretion before or during the monsoon. No experimental treatment affected plasma T concentrations. The data suggest in male Rufous-winged Sparrows that the seasonal increase in plasma LH associated with summer monsoon results from increased stimulation of the pituitary gland by GnRH, rather than from a change in the responsiveness of the gland to GnRH, and that GnIH does not play an acute role in this mechanism. However, a possible chronic role for GnIH in the seasonal control of LH synthesis and secretion through an inhibitory effect on the hypothalamic GnRH system remains to be investigated.

Testosterone induces testicular development but reduces GnRH-I fiber density in the brain of the House Finch, Carpodacus mexicanus

Testosterone (T) in male birds generally inhibits the activity of the hypothalamo-pituitary-gonadal axis, but the androgen can also be gonadostimulatory. The mechanisms responsible for this stimulation are unknown. To address this question, adult male House Finches, Carpodacus mexicanus, held on short day length to inhibit reproductive function, received 20-mm long T-filled Silastic capsules either for 74 days (T+; n=5; Day 1-74) or for 66 days followed by eight days without capsules (T-; n=5), or empty capsules for 74 days (controls, C, n=10). At sacrifice on Day 74, plasma T concentrations were elevated in T+ males and low, and similar in T- and C males. Testes were enlarged in T+ males (136.5+/-57.2mg), small in T- males (16.2+/-9.5 mg), and undeveloped in C males (5.4+/-3.6 mg). The three groups had similar numbers and sizes of hypothalamic gonadotropin-releasing hormone-I immunoreactive (GnRH-I ir) perikarya, but perikaryon GnRH-I ir labelling was lighter in T+ than C males. T+ males also had lower density of GnRH-immunolabelled fibers in the preoptic area and lower intensity of GnRH immunolabelling in the median eminence than C males, suggesting an inhibition of GnRH-I neuronal activity. Plasma luteinizing hormone in the three experimental groups was uniformly low and did not differ. Thus, T administration-induced testicular development was associated with an inhibition of the reproductive neuroendocrine system, demonstrating a direct stimulatory effect of T on testes. Stimulation of testicular function by endogenous T in seasonal breeders may contribute to the rapid gonadal maturation that takes place in response to photostimulation.