Developmental conditions have intergenerational effects on corticosterone levels in a passerine

Parental developmental experience affects vocal learning in offspring

Cultural and genetic inheritance combine to enable rapid changes in trait expression, but their relative importance in determining trait expression across generations is not clear. Birdsong is a socially learned cognitive trait that is subject to both cultural and genetic inheritance, as well as being affected by early developmental conditions. We sought to test whether early-life conditions in one generation can affect song acquisition in the next generation. We exposed one generation (F1) of nestlings to elevated corticosterone (CORT) levels, allowed them to breed freely as adults, and quantified their son's (F2) ability to copy the song of their social father. We also quantified the neurogenetic response to song playback through immediate early gene (IEG) expression in the auditory forebrain. F2 males with only one corticosterone-treated parent copied their social father's song less accurately than males with two control parents. Expression of ARC in caudomedial nidopallium (NCM) correlated with father-son song similarity, and patterns of expression levels of several IEGs in caudomedial mesopallium (CMM) in response to father song playback differed between control F2 sons and those with a CORT-treated father only. This is the first study to demonstrate that developmental conditions can affect social learning and neurogenetic responses in a subsequent generation.

The effects of pre-hatch elevated corticosterone and post-hatch restrictive food availability on the HPA axis development of mallard ducks (Anas platyrhynchos)

Environmental changes can be stressors (altered habitat and food supply, climate change, etc.) to wild animals. Stressors trigger the hypothalamic pituitary adrenal (HPA) axis to release corticosterone (CORT) which modifies energy homeostasis. During nesting, stressed females can deposit increased concentrations of CORT into eggs, altering egg viability and offspring characteristics, constituting a significant mechanism regulating population productivity in subsequent generations. In this study, increased maternal disposition of CORT was mimicked through a 15 ng/g in ovo injection of CORT into mallard duck eggs. Growth and HPA axis function were measured during post-hatch development. For growth, changes in mass were compared at hatch, 7 weeks and 11 weeks. The HPA axis was assessed at seven weeks by measuring CORT at baseline, followed by restraint stress, dexamethasone (negative feedback) and ACTH (maximal adrenal capacity) challenges. At eleven weeks of age, ducks were subjected to a 6-day 25% feed reduction to simulate a poor quality environment to evaluate response to a chronic stressor by comparing CORT at baseline and after restraint stress. Growth and CORT concentration did not differ between treatments at seven weeks or after feed restriction (11 weeks). The CORT dosage administered did not appear to affect HPA axis development in ducklings. Mallards are a highly adaptable species and may have overcome any early alterations to their phenotype. Further research is needed to determine the effects of increased maternal CORT on growth and the development of the HPA axis in ducks. SUMMARY STATEMENT: This study examines how maternal stress (simulated through elevated corticosterone in ovo) and post-hatch chronic stressors (food restriction) affect the development of the HPA axis in a precocial bird.

Mitochondria as the powerhouses of sexual selection: Testing mechanistic links between development, cellular respiration, and bird song

The developmental environment can affect the expression of sexually selected traits in adulthood. The physiological mechanisms that modulate such effects remain a matter of intense debate. Here, we test the role of the developmental environment in shaping adult mitochondrial function and link mitochondrial function to expression of a sexually selected trait in males (bird song). We exposed male zebra finches (Taeniopygia guttata) to corticosterone (CORT) treatment during development. After males reached adulthood, we quantified mitochondrial function from whole red blood cells and measured baseline CORT and testosterone levels, body condition/composition, and song structure. CORT-treated males had mitochondria that were less efficient (FCR_L/R) and used a lower proportion of maximum capacity (FCR_R/ETS) than control males. Additionally, CORT-treated males had higher baseline levels of CORT as adults compared to control males. Using structural equation modelling, we found that the effects of CORT treatment during development on adult mitochondrial function were indirect and modulated by baseline CORT levels, which are programmed by CORT treatment during development. Developmental treatment also had an indirect effect on song peak frequency. Males treated with CORT during development sang songs with higher peak frequency than control males, but this effect was modulated through increased CORT levels and by a decrease in FCR_R/ETS. CORT-treated males had smaller tarsi compared to control males; however, there were no associations between body size and measures of song frequency. Here, we provide the first evidence supporting links between the developmental environment, mitochondrial function, and the expression of a sexually selected trait (bird song).

Clutch Abandoning Parasitised Yellow Warblers Have Increased Circulating Corticosterone With No Effect of Past Corticosterone or Differences in Egg Maculation Characteristics

Parental care can be costly to parents’ fitness. As such, abandonment of the current reproductive attempt may benefit potential future opportunities, maximising lifetime reproductive success. Obligate brood parasitism, a reproductive strategy in which parasites lay their eggs in the nests of other species and rely solely on them to raise the parasitic young, is an ideal system to study brood abandonment. Some parasitised host species have evolved anti-parasitic defences, notably clutch abandonment (egg burial and nest desertion), that may mitigate negative consequences of parasitism. Abandonment of clutches due to parasitism is not unlike abandonment of reproduction in times of stress, suggesting that host responses to parasitism could be triggered at least partly by elevated stress hormones that mediate individual decisions. Yet, the mechanistic basis for clutch abandonment remains unclear. Here, we experimentally parasitised clutches of yellow warblers (Setophaga petechia), a common host of the brown-headed cowbird (Molothrus ater), with model cowbird eggs to examine whether host circulating corticosterone (CORT) differed among females that accepted parasitic eggs or rejected them through clutch abandonment. We also assessed whether feather CORT, a measure of past corticosterone exposure, differed between accepters and abandoners. Finally, we investigated whether egg visual signals, specifically differences in maculation characteristics between model cowbird and host eggs, predicted abandonment of experimentally parasitised clutches. Circulating CORT was higher in females who abandoned their parasitised clutches, but not in those who accepted, relative to controls with no egg addition. Past stress and differences in maculation characteristics did not predict whether individuals accepted or abandoned experimentally parasitised clutches. Moreover, differences in maculation characteristics between the host and model cowbird eggs did not predict CORT levels or nest abandonment. Thus, parasitism with subsequent clutch abandonment may be associated with elevated circulating CORT, but neither past stress nor differences in maculation characteristics influenced abandonment. The combination of these results contributes to our understanding of the roles of corticosterone and egg visual signals in the context of clutch abandonment in brood parasitism specifically, and of parental care more broadly.

Information theory in vertebrate stress physiology

Information theory has been applied productively across biology, but it has been used minimally in endocrinology. Here, we advocate for the integration of information theory into stress endocrinology. Presently, the majority of models of stress center on the regulation of hormone concentrations, even though what interests most endocrinologists and matters in terms of individual health and evolutionary fitness is the information content of hormones. In neuroscience, the free energy principle, a concept offered to explain how the brain infers current and future states of the environment, could be a guide for resolving how information is instantiated in hormones such as the glucocorticoids. Here, we offer several ideas and promising options for research addressing how hormones encode and cells respond to information in glucocorticoids.