Endocrine phenotype, reproductive success and survival in the great tit, Parus major

Endocannabinoid Signaling in the Stress Response of Male and Female Songbirds

Endocannabinoid (eCB) signaling plays an important role in the stress response pathways of the mammalian brain, yet its role in the avian stress response has not been described. Understanding eCB signaling in avian species (such as the European starling, Sturnus vulgaris) allows a model system that exhibits natural attenuation of hypothalamic-pituitary-adrenal (HPA) responsiveness to stressors. Specifically, seasonally breeding birds exhibit the highest HPA activity during the breeding season and subsequently exhibit a robust HPA down-regulation during molt. Because eCB signaling in mammals has an overall inhibitory effect on HPA activity, we expected shifts in eCB signaling to regulate the seasonal HPA down-regulation during molt. However, our data did not support a role for eCB signaling in the molt-related suppression of HPA activity. For example, injection of the cannabinoid receptor (CB1) antagonist, AM251, did not potentiate molt-suppressed HPA activity. Instead, our data suggest eCB regulation of HPA plasticity as birds transition from breeding to molt. In support of this hypothesis, birds in the late breeding season demonstrated a more dynamic response at the level of avian amygdala eCB content in response to acute stress. The response and directionality of this effect match that seen in mammals. Overall, our data suggest that eCB signaling may allow for a dynamic range in HPA responsiveness (eg, breeding), but the signaling pathway's role may be limited when the HPA response is restrained (eg, molt). This first characterization of eCB signaling in the avian stress response also emphasizes that although the system functions similarly to other species, its exact role may be species specific.

Endocrine mechanisms, behavioral phenotypes and plasticity: known relationships and open questions

Behavior of wild vertebrate individuals can vary in response to environmental or social factors. Such within-individual behavioral variation is often mediated by hormonal mechanisms. Hormones also serve as a basis for among-individual variations in behavior including animal personalities and the degree of responsiveness to environmental and social stimuli. How do relationships between hormones and behavioral traits evolve to produce such behavioral diversity within and among individuals? Answering questions about evolutionary processes generating among-individual variation requires characterizing how specific hormones are related to variation in specific behavioral traits, whether observed hormonal variation is related to individual fitness and, whether hormonal traits are consistent (repeatable) aspects of an individual's phenotype. With respect to within-individual variation, we need to improve our insight into the nature of the quantitative relationships between hormones and the traits they regulate, which in turn will determine how they may mediate behavioral plasticity of individuals. To address these questions, we review the actions of two steroid hormones, corticosterone and testosterone, in mediating changes in vertebrate behavior, focusing primarily on birds. In the first part, we concentrate on among-individual variation and present examples for how variation in corticosterone concentrations can relate to behaviors such as exploration of novel environments and parental care. We then review studies on correlations between corticosterone variation and fitness, and on the repeatability over time of corticosterone concentrations. At the end of this section, we suggest that further progress in our understanding of evolutionary patterns in the hormonal regulation of behavior may require, as one major tool, reaction norm approaches to characterize hormonal phenotypes as well as their responses to environments.

In the second part, we discuss types of quantitative relationships between hormones and behavioral traits within individuals, using testosterone as an example. We review conceptual models for testosterone-behavior relationships and discuss the relevance of these models for within-individual plasticity in behavior. Next, we discuss approaches for testing the nature of quantitative relationships between testosterone and behavior, concluding that again reaction norm approaches might be a fruitful way forward.

We propose that an integration of new tools, especially of reaction norm approaches into the field of behavioral endocrinology will allow us to make significant progress in our understanding of the mechanisms, the functional implications and the evolution of hormone–behavior relationships that mediate variation both within and among individuals. This knowledge will be crucial in light of already ongoing habitat alterations due to global change, as it will allow us to evaluate the mechanisms as well as the capacity of wild populations to adjust hormonally-mediated behaviors to altered environmental conditions.

Experimental food restriction reveals individual differences in corticosterone reaction norms with no oxidative costs

Highly plastic endocrine traits are thought to play a central role in allowing organisms to respond rapidly to environmental change. Yet, not all individuals display the same degree of plasticity in these traits, and the costs of this individual variation in plasticity are unknown. We studied individual differences in corticosterone levels under varying conditions to test whether there are consistent individual differences in (1) baseline corticosterone levels; (2) plasticity in the hormonal response to an ecologically relevant stressor (food restriction); and (3) whether individual differences in plasticity are related to fitness costs, as estimated by oxidative stress levels. We took 25 wild-caught house sparrows into captivity and assigned them to repeated food restricted and control treatments (60% and 110% of their daily food intake), such that each individual experienced both food restricted and control diets twice. We found significant individual variation in baseline corticosterone levels and stress responsiveness, even after controlling for changes in body mass. However, these individual differences in hormonal responsiveness were not related to measures of oxidative stress. These results have implications for how corticosterone levels may evolve in natural populations and raise questions about what we can conclude from phenotypic correlations between hormone levels and fitness measures.

Physiological flexibility in an avian range expansion

The mechanisms that enable animals to colonize new areas are little known, but growing evidence indicates that the regulation of stress hormones is important. Stress hormones probably influence invasions because they enable organisms to adjust their phenotypes depending on environmental context. Often, studies of stress hormones are based on single or a few samples from individuals even though the flexibility in the regulation of such hormones is what enables them to achieve homeostasis and facilitate performance. Here, we asked whether flexibility in the regulation of one stress hormone, corticosterone, was related to colonization success in one of the world's most successful avian invaders, the house sparrow (Passer domesticus). We studied Kenyan house sparrows, as the species was recently introduced there (around 1950) and has since expanded northwestward. Previous work in this system revealed that younger populations released more corticosterone during a restraint stressor than older populations. Our first goal was to discern whether such population differences were fixed or flexible in adulthood; our second goal was to determine whether individual identity explained any variation in corticosterone regulation. As before, we found that corticosterone responses to short-term restraint (i.e., stress responses), but not baseline corticosterone, were larger in younger populations. We also found that both baseline and stress-induced corticosterone measures were flexible; both metrics became similar among sites after one week of captivity. For stress responses, we also found that individual identity was important. Altogether, the present data suggest that the colonization of Kenya by house sparrows might have been facilitated by stress hormone regulatory flexibility.

Stress, captivity, and reproduction in a wild bird species

In seasonal species, glucocorticoid concentrations are often highest during the breeding season. However, the role of increased hypothalamic-pituitary-adrenal (HPA) activity in the regulation of reproduction remains poorly understood. Our study is the first, to our knowledge, to document reproductive consequences of a non-pharmacological hindrance to seasonal HPA fluctuations. Using wild-caught male and female European starlings (Sturnus vulgaris) housed in an outdoor, semi-natural environment, we divided birds into two mixed-sex groups. One group remained in the outdoor aviary, where starlings breed at the appropriate time of year. The other group was transferred into an indoor flight aviary, where we predicted reproductive suppression to occur. We measured changes in corticosterone (CORT) at baseline and stress-induced concentrations prior to group separation and at the experiment's conclusion. After ten days, the birds showed remarkable differences in breeding behavior and HPA activity. Outdoor birds exhibited increases in baseline and stress-induced CORT and progressed into active breeding (pairing, nest building, egg laying, etc.). In contrast, indoor birds displayed no change in baseline or stress-induced CORT and few signs of active breeding. We found significant sex and treatment effects on expression of HPA and hypothalamic-pituitary-gonadal (HPG) axis elements, suggesting sex-specific regulatory mechanisms. Our data suggest a novel, facilitating role for the HPA axis in the transition between early breeding and active breeding in a wild, seasonal avian species. In addition, understanding how changes in housing condition affect seasonal HPA fluctuations may help alleviate barriers to breeding wild animals in captivity.

Evidence for baseline glucocorticoids as mediators of reproductive investment in a wild bird

Determining the mechanisms that mediate investment decisions between current and future reproductive attempts is still a key goal of life-history studies. Since baseline levels of stress hormones (glucocorticoids - GCs) act as predictive and labile regulators of daily energetic balance in vertebrates they remain excellent candidates for mediating investment decisions both within and across reproductive attempts. Using free-living female European starlings (Sturnus vulgaris) we experimentally reduced investment in current reproduction (number of offspring raised in the first brood) to examine whether baseline corticosterone (CORT) acted as a hormonal mediator preparing individuals for a predictable increase in future investment (number of offspring raised in the second brood). Although treatment and control birds raised the same total amount of offspring across two broods, the experimental birds increased reproductive investment in second broods to compensate for the reduced investment in the first brood. Data on both mean and intra-individual changes in baseline CORT support the idea that an increase in baseline CORT between the incubation stages in treatment birds strongly predicted this increase in investment. Importantly, we measured the increase in baseline CORT during late incubation prior to the increase in energetic demand associated with increased reproductive investment in offspring, indicating that flexible within-individual changes in baseline GCs can act as a labile mechanism preparing individuals for predictable increases in reproductive investment. As such, our experimental results indicate that elevated baseline GCs can prepare individuals for investment in energetically expensive life-history stages, rather than simply being elevated as a consequence of increased effort or demand. This suggests that short-term preparative increases in baseline GCs benefit individuals by successfully allowing them to maximize fitness under varying environmental conditions.

Parental care, loss of paternity and circulating levels of testosterone and corticosterone in a socially monogamous song bird

Introduction

In biparental birds testosterone levels of males are typically high during the mating phase and decrease during the parental phase. Testosterone implants may enhance mating behaviors, increase the likelihood of males to engage in extra-pair mating behavior and may reduce paternal care. Thus, sex steroids such as testosterone influence reproductive behaviors. Little is known, however, as to whether the more subtle differences in physiological concentrations of testosterone that occur between individuals are related to differences in paternal care, extra-pair behavior, and genetic paternity between those males. Here, we investigate these relationships in the male black redstart (Phoenicurus ochruros), a socially monogamous songbird with a low breeding synchrony. We used nestling provisioning as a proxy for parental care behavior and genetic paternity loss as a proxy for the efficiency of mate-guarding.

Results

There was no relationship between nestling provisioning and paternity loss of males. Baseline and gonadotropin releasing hormone (GnRH)-induced levels of testosterone, but not baseline corticosterone, were significantly higher during the mating than during the provisioning phase. Males fed more often when temperatures decreased and fed less when they sang more, but we found no correlation between parental behavior and baseline or GnRH-induced testosterone, and baseline corticosterone – both measured during either the mating or the parental phase. However, males that experienced loss of paternity had lower levels of testosterone during the provisioning phase than males that did not lose paternity. Further, males that lost paternity also expressed higher baseline levels of corticosterone.

Conclusions

Physiological differences in testosterone or baseline corticosterone were not related to differences in parental care, suggesting that the variation of testosterone within a physiological range may not relate to the degree of paternal care in this species. However, the profile of both hormones may indicate quality traits that influence the likelihood of the respective male to lose paternity.