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Testosterone is controlled while it circulates in the bloodstream 

Abstract The ability to cope with heat is likely to influence species success amidst climate change. However, heat coping mechanisms are poorly understood in wild endotherms, which are increasingly pushed to their thermoregulatory limits.We take an organismal approach to this problem, unveiling how behavioural and physiological responses may allow success in the face of sublethal heat. We experimentally elevated nest temperatures for 4 h to mimic a future climate scenario (+4.5°C) during a critical period of post‐natal development in tree swallows (Tachycineta bicolor).Heat‐exposed nestlings exhibited marked changes in behaviour, including movement to cooler microclimates in the nest. They panted more and weighed less than controls at the end of the four‐hour heat challenge, suggesting panting‐induced water loss. Physiologically, heat induced high levels of heat shock protein (HSP) gene expression in the blood, alongside widespread transcriptional differences related to antioxidant defences, inflammation and apoptosis.Critically, all nestlings survived the heat challenge, and those exposed to milder heat weremorelikely to recruit into the breeding population. Early life but sub‐lethal heat may therefore act as a selective event, with the potential to shape population trajectories.Within the population, individuals varied in their physiological response to heat, namely in HSP gene expression, which exhibited higher mean and higher variance in heat‐exposed nestlings than in controls. Heat‐induced HSP levels were unrelated to individual body mass, or among‐nest differences in brood size, temperature, and behavioural thermoregulation. Nest identity explained a significant amount of HSP variation, yet siblings in the same nest differed by an average of ~4‐fold and individuals in the population differed by as much as ~100‐fold in their HSP response. This massive variation extends previous laboratory work in model organisms showing that heat shock proteins may harbour cryptic phenotypic variation.These results shed light on oft‐ignored elements of thermotolerance in wild birds at a critical stage of post‐natal development. By highlighting the scope of heat‐induced HSP gene expression and coupling it with a suite of organismal traits, we provide a framework for future testing of the mechanisms that shape species success in the face of change. Read the freePlain Language Summaryfor this article on the Journal blog. 

Abstract In seasonally breeding animals, the costs and benefits of territorial aggression should vary over time; however, little work thus far has directly examined the scope and adaptive value of individual-level plasticity in aggression across breeding stages. We explore these issues using the tree swallow (Tachycineta bicolor), a single-brooded bird species in which females compete for limited nesting sites. We measured aggressiveness in nearly 100 females within 3 different stages: (1) shortly after territory-establishment, (2) during incubation, and (3) while caring for young chicks. Based on the timing, direction, and magnitude of behavioral changes between stages, we used k-means clustering to categorize each female’s behavior into a “plasticity type.” We then tested whether plasticity type and stage-specific aggression varied with key performance metrics. About 40% of females decreased aggressiveness across consecutive breeding stages to some degree, consistent with population-level patterns. 33% of females exhibited comparatively little plasticity, with moderate to low levels of aggression in all stages. Finally, 27% of females displayed steep decreases and then increases in aggression between stages; females exhibiting this pattern had significantly lower body mass while parenting, they tended to hatch fewer eggs, and they had the lowest observed overwinter survival rates. Other patterns of among-stage changes in aggressiveness were not associated with performance. These results reveal substantial among-individual variation in behavioral plasticity, which may reflect diverse solutions to trade-offs between current reproduction and future survival. 

Increasingly frequent and intense heatwaves generate new challenges for many organisms. Our understanding of the ecological predictors of thermal vulnerability is improving, yet, at least in endotherms, we are still only beginning to understand one critical component of predicting resilience: exactly how do wild animals cope with sub-lethal heat? In wild endotherms, most prior work focuses on one or a few traits, leaving uncertainty about organismal consequences of heatwaves. Here, we experimentally generated a 2.8°C heatwave for free-living nestling tree swallows (Tachycineta bicolor). Over a week-long period coinciding with the peak of post-natal growth, we quantified a suite of traits to test the hypotheses that (a) behavioral or (b) physiological responses may be sufficient for coping with inescapable heat. Heat-exposed nestlings increased panting and decreased huddling, but treatment effects on panting dissipated over time, even though heat-induced temperatures remained elevated. Physiologically, we found no effects of heat on: gene expression of three heat shock proteins in blood, muscle, and three brain regions; secretion of circulating corticosterone at baseline or in response to handling; and telomere length. Moreover, heat had a positive effect on growth and a marginal, but not significant, positive effect on subsequent recruitment. These results suggest that nestlings were generally buffered from deleterious effects of heat, with one exception: heat-exposed nestlings exhibited lower gene expression for superoxide dismutase, a key antioxidant defense. Despite this one apparent cost, our thorough organismal investigation indicates general resilience to a heatwave that may, in part, stem from behavioral buffering and acclimation. Our approach provides a mechanistic framework that we hope will improve understanding of species persistence in the face of climate change. 

Uncovering the genomic bases of phenotypic adaptation is a major goal in biology, but this has been hard to achieve for complex behavioral traits. Here, we leverage the repeated, independent evolution of obligate cavity-nesting in birds to test the hypothesis that pressure to compete for a limited breeding resource has facilitated convergent evolution in behavior, hormones, and gene expression. We used an integrative approach, combining aggression assays in the field, testosterone measures, and transcriptome-wide analyses of the brain in wild-captured females and males. Our experimental design compared species pairs across five avian families, each including one obligate cavity-nesting species and a related species with a more flexible nest strategy. We find behavioral convergence, with higher levels of territorial aggression in obligate cavity-nesters, particularly among females. Across species, levels of testosterone in circulation were not associated with nest strategy, nor aggression. Phylogenetic analyses of individual genes and co-regulated gene networks revealed more shared patterns of brain gene expression than expected by drift, but the scope of convergent gene expression evolution was limited to a small percent of the genome. When comparing our results to other studies that did not use phylogenetic methods, we suggest that accounting for shared evolutionary history may reduce the number of genes inferred as convergently evolving. Altogether, we find that behavioral convergence in response to shared ecological pressures is associated with largely independent gene expression evolution across different avian families, punctuated by a narrow set of convergently evolving genes.