Sources of variation in plasma corticosterone and dehydroepiandrosterone in the male northern cardinal (Cardinalis cardinalis): II. Effects of urbanization, food supplementation and social stress

VALIDATION OF ENZYME-LINKED IMMUNOSORBENT ASSAY TECHNIQUES TO MEASURE SERUM DEHYDROEPIANDROSTERONE (DHEA) AND DHEA-S IN NARWHALS (MONODON MONOCEROS)

Narwhals (Monodon monoceros) are increasingly exposed to anthropogenic disturbances that may increase their stress levels with unknown consequences for the overall population dynamics. The validation and measurement of chronic stress biomarkers could contribute toward improved understanding and conservation efforts for this species. Dehydroepiandrosterone (DHEA) and its sulfated metabolite DHEA-S are collectively referred to as DHEA(S). Serum DHEA(S) concentrations combined in ratios with cortisol [cortisol/DHEA(S)] have been shown to be promising indicators of chronic stress in humans, domestic animals, and wildlife. During field tagging in 2017 and 2018 in Baffin Bay, Nunavut, Canada, 14 wild narwhals were sampled at the beginning and end of the capture-tagging procedures. Serum DHEA(S) were measured with commercially available competitive enzyme-linked immunosorbent assays (ELISA) developed for humans. A partial validation of the ELISA assays was performed by the determination of the intra-assay coefficient of variation, confirmation of the DHEA(S) dilutional linearity, and the calculation of the percentage of recovery. Mean values (nanograms per milliliter ± standard error of the mean) of narwhal serum cortisol, DHEA(S), and cortisol/DHEA(S) ratios, at the beginning and at the end of handling, respectively, are reported (cortisol = 30.74 ± 4.87 and 41.83 ± 4.83; DHEA = 1.01 ± 0.52 and 0.99 ± 0.50; DHEA-S = 8.72 ± 1.68 and 7.70 ± 1.02; cortisol/DHEA = 75.43 ± 24.35 and 84.41 ± 11.76, and cortisol/DHEA-S = 4.16 ± 1.07 and 6.14 ± 1.00). Serum cortisol and cortisol/DHEA-S were statistically higher at the end of the capture (P= 0.024 and P= 0.035, respectively). Moreover, serum cortisol at the end of handling was positively correlated to total body length (P = 0.042) and tended to be higher in males (P = 0.086). These assays proved easy to perform, rapid, and suitable for measuring serum DHEA(S) of narwhals and that calculated cortisol/DHEA(S) are potential biomarkers for chronic stress in narwhals and possibly other cetaceans.

Past and future: Urbanization and the avian endocrine system

Urban environments are evolutionarily novel and differ from natural environments in many respects including food and/or water availability, predation, noise, light, air quality, pathogens, biodiversity, and temperature. The success of organisms in urban environments requires physiological plasticity and adjustments that have been described extensively, including in birds residing in geographically and climatically diverse regions. These studies have revealed a few relatively consistent differences between urban and non-urban conspecifics. For example, seasonally breeding urban birds often develop their reproductive system earlier than non-urban birds, perhaps in response to more abundant trophic resources. In most instances, however, analyses of existing data indicate no general pattern distinguishing urban and non-urban birds. It is, for instance, often hypothesized that urban environments are stressful, yet the activity of the hypothalamus-pituitary-adrenal axis does not differ consistently between urban and non-urban birds. A similar conclusion is reached by comparing blood indices of metabolism. The origin of these disparities remains poorly understood, partly because many studies are correlative rather than aiming at establishing causality, which effectively limits our ability to formulate specific hypotheses regarding the impacts of urbanization on wildlife. We suggest that future research will benefit from prioritizing mechanistic approaches to identify environmental factors that shape the phenotypic responses of organisms to urbanization and the neuroendocrine and metabolic bases of these responses. Further, it will be critical to elucidate whether factors affect these responses (a) cumulatively or synergistically; and (b) differentially as a function of age, sex, reproductive status, season, and mobility within the urban environment. Research to date has used various taxa that differ greatly not only phylogenetically, but also with regard to ecological requirements, social systems, propensity to consume anthropogenic food, and behavioral responses to human presence. Researchers may instead benefit from standardizing approaches to examine a small number of representative models with wide geographic distribution and that occupy diverse urban ecosystems.

Stress in the city: meta-analysis indicates no overall evidence for stress in urban vertebrates

As cities continue to grow it is increasingly important to understand the long-term responses of wildlife to urban environments. There have been increased efforts to determine whether urbanization imposes chronic stress on wild animals, but empirical evidence is mixed. Here, we conduct a meta-analysis to test whether there is, on average, a detrimental effect of urbanization based on baseline and stress-induced glucocorticoid levels of wild vertebrates. We found no effect of urbanization on glucocorticoid levels, and none of sex, season, life stage, taxon, size of the city nor methodology accounted for variation in the observed effect sizes. At face value, our results suggest that urban areas are no more stressful for wildlife than rural or non-urban areas, but we offer a few reasons why this conclusion could be premature. We propose that refining methods of data collection will improve our understanding of how urbanization affects the health and survival of wildlife.

De Novo Assembly of the Northern Cardinal (Cardinalis cardinalis) Genome Reveals Candidate Regulatory Regions for Sexually Dichromatic Red Plumage Coloration

Northern cardinals (Cardinalis cardinalis) are common, mid-sized passerines widely distributed in North America. As an iconic species with strong sexual dichromatism, it has been the focus of extensive ecological and evolutionary research, yet genomic studies investigating the evolution of genotype-phenotype association of plumage coloration and dichromatism are lacking. Here we present a new, highly-contiguous assembly for C. cardinalis We generated a 1.1 Gb assembly comprised of 4,762 scaffolds, with a scaffold N50 of 3.6 Mb, a contig N50 of 114.4 kb and a longest scaffold of 19.7 Mb. We identified 93.5% complete and single-copy orthologs from an Aves dataset using BUSCO, demonstrating high completeness of the genome assembly. We annotated the genomic region comprising the CYP2J19 gene, which plays a pivotal role in the red coloration in birds. Comparative analyses demonstrated non-exonic regions unique to the CYP2J19 gene in passerines and a long insertion upstream of the gene in C. cardinalis Transcription factor binding motifs discovered in the unique insertion region in C. cardinalis suggest potential androgen-regulated mechanisms underlying sexual dichromatism. Pairwise Sequential Markovian Coalescent (PSMC) analysis of the genome reveals fluctuations in historic effective population size between 100,000-250,000 in the last 2 millions years, with declines concordant with the beginning of the Pleistocene epoch and Last Glacial Period. This draft genome of C. cardinalis provides an important resource for future studies of ecological, evolutionary, and functional genomics in cardinals and other birds.

Physiological stress does not increase with urbanization in European blackbirds: Evidence from hormonal, immunological and cellular indicators

Urbanization changes the landscape structure and ecological processes of natural habitats. While urban areas expose animal communities to novel challenges, they may also provide more stable environments in which environmental fluctuations are buffered. Species´ ecology and physiology may determine their capacity to cope with the city life. However, the physiological mechanisms underlying organismal responses to urbanization, and whether different physiological systems are equally affected by urban environments remain poorly understood. This severely limits our capacity to predict the impact of anthropogenic habitats on wild populations. In this study, we measured indicators of physiological stress at the endocrine, immune and cellular level (feather corticosterone levels, heterophil to lymphocyte ratio, and heat-shock proteins) in urban and non-urban European blackbirds (Turdus merula) across 10 European populations. Among the three variables, we found consistent differences in feather corticosterone, which was higher in non-urban habitats. This effect seems to be dependent on sex, being greater in males. In contrast, we found no significant differences between urban and non-urban habitats in the two other physiological indicators. The discrepancy between these different measurements of physiological stress highlights the importance of including multiple physiological variables to understand the impact of urbanization on species' physiology. Overall, our findings suggest that adult European blackbirds living in urban and non-urban habitats do not differ in terms of physiological stress at an organismal level. Furthermore, we found large differences among populations on the strength and direction of the urbanization effect, which illustrates the relevance of spatial replication when investigating urban-induced physiological responses.

Traffic noise exposure alters nestling physiology and telomere attrition through direct, but not maternal, effects in a free-living bird

Anthropogenic impacts, such as noise pollution from transportation networks, can serve as stressors to some wildlife species. For example, increased exposure to traffic noise has been found to alter baseline and stress-induced corticosterone levels, reduce body condition and reproductive success, and increase telomere attrition in free-living birds. However, it remains unknown if alterations in nestling phenotype are due to direct or indirect effects of noise exposure. For example, indirect (maternal) effects of noise may occur if altered baseline and stress-induced corticosterone in mothers results in differential deposition of yolk steroids or other components in eggs. Noise exposure may also alter nestling corticosterone levels directly, given that nestlings cannot escape the nest during development. Here, we examined maternal versus direct effects of traffic noise exposure on baseline and stress-induced corticosterone levels, and body condition (as measured by size-corrected mass) in nestling tree swallows (Tachycineta bicolor). We used a two-way factorial design and partially cross-fostered eggs between nests exposed to differing levels (i.e. amplitudes) of traffic noise. For nestlings that were not cross-fostered, we also investigated the effects of traffic noise on telomere dynamics. Our results show a positive relationship between nestling baseline and stress-induced corticosterone and nestling noise exposure, but not maternal noise exposure. While we did not find a relationship between noise and body condition in nestlings, nestling baseline corticosterone was negatively associated with body condition. We also found greater telomere attrition for nestlings from nests with greater traffic noise amplitudes. These results suggest that direct, rather than maternal, effects result in potentially long-lasting consequences of noise exposure. Reduced nestling body condition and increased telomere attrition have been shown to reduce post-fledging survival in this species. Given that human transportation networks continue to expand, strategies to mitigate noise exposure on wildlife during critical periods (i.e. breeding) may be needed to maintain local population health in free-living passerines, such as tree swallows.

Relaxing life of the city? Allostatic load in yellow-bellied marmots along a rural-urban continuum

Urban environments are expanding. As rural areas are urbanized, animals living in those environments must respond. Examinations of ecological responses to urbanization are abundant, but much less work has focused on the physiological responses driving those ecological patterns, particularly in mammals. Whether an animal interprets urbanized environments as stressful or not can help us understand, and even predict, the likelihood of individuals persisting in urbanized areas. Unpredictable events can cause stress and responses to such events can deplete limited stores of energy. Differences between required and available energy is termed allostatic load and is an indicator of stress. Allostatic load, and hence stress, is correlated with baseline levels of the metabolic hormones, glucocorticoids. We examined allostatic load in yellow-bellied marmots along a rural-urban gradient through analysis of fecal glucocorticoid metabolites (FGMs). We used GIS data and 'on-the-ground' measurements to quantify the degree of urbanization. We collected fecal samples from males and females of all age classes at six sites along this continuum. Female marmots had higher FGMs than males. All age groups of marmots exhibited a parabolic relationship between the degree of urbanization and FGM levels. In general, adult marmots had higher FGMs in more rural than urban environments, and both juveniles and yearlings had exhibited higher FGM levels in more urban environments.

Effects of experimental chronic traffic noise exposure on adult and nestling corticosterone levels, and nestling body condition in a free-living bird

Transportation noise affects urbanized, rural, and otherwise unaltered habitats. Given expanding transportation networks, alterations in the acoustic landscapes experienced by animals are likely to be pervasive and persistent (i.e. chronic). It is important to understand if chronic noise exposure alters behavior and physiology in free-living animals, as it may result in long-lasting impacts, such as reduced reproductive success. Here, we experimentally tested the effects of chronic traffic noise on baseline and stress-induced corticosterone (the primary avian glucocorticoid), parental feeding behavior, and fitness proxies in breeding tree swallows (Tachycineta bicolor). Our results show that chronic traffic noise is related to altered corticosterone in both adult female and nestling tree swallows, suggesting that noise may be a stressor in both groups. In adult females, our results suggest that traffic noise is related to a limited ability to respond to subsequent acute stressors (i.e. reduced stress-induced corticosterone levels after handling). Further, our results show no evidence of habituation to noise during the breeding season, as the negative relationship between traffic noise and adult female stress-induced corticosterone became stronger over time. In nestlings, we found a positive relationship between traffic noise exposure and baseline corticosterone. Finally, we found a negative relationship between traffic noise and nestling body condition, despite no detectable effects of noise on nestling provisioning (e.g. parental feeding rate, or insect bolus size/composition). These results highlight the potential long-term consequences of chronic noise exposure, as increased baseline corticosterone and reduced nestling body condition in noise-exposed areas may have negative, population-level consequences.

Corticosterone profiles in northern cardinals (Cardinalis cardinalis): Do levels vary through life history stages?

As animals move through life history stages, energy requirements for each stage will vary. Both daily and annual variation in the glucocoriticoid hormones (specifically corticosterone, or CORT, in birds) helps provide the variable energy needed through life history stages. In many bird species, CORT is higher in the breeding season when energy demands can be quite high and is often lower in the non-breeding season. Additionally, CORT has a role to play in the response to stressful stimuli and the level to which CORT is elevated following stressful events can vary through the annual cycle as well. Here we report on baseline and stress-induced CORT levels in both sexes of northern cardinals, Cardinalis cardinalis, a non-migrating, year-round territorial species across life history stages. Corticosterone is overall higher in the non-breeding season than the breeding season in both sexes. Males tend to have higher levels of stress-induced CORT than females, although the observed patterns are complex. Our findings differ from one of the more common profile reported in songbirds where breeding season CORT tends to be higher than non-breeding levels. A strong influence may be the prolonged breeding season seen in cardinals; lower levels of CORT during breeding may guard against adverse maternal effects, interruptions in breeding behaviors, or egg production. Additional investigation of species with similar ecologies to northern cardinals, and more populations of cardinals, may show that annual glucocorticoid profiles are more labile than previously appreciated.

Sources of variation in plasma corticosterone and dehydroepiandrosterone in the male northern cardinal (Cardinalis cardinalis): I. Seasonal patterns and effects of stress and adrenocorticotropic hormone

The secretion of steroids from the adrenal gland is a classic endocrine response to perturbations that can affect homeostasis. During an acute stress response, glucocorticoids (GC), such as corticosterone (CORT), prepare the metabolic physiology and cognitive abilities of an animal in a manner that promotes survival during changing conditions. Although GC functions during stress are well established, much less is understood concerning how adrenal androgens, namely dehydroepiandrosterone (DHEA) are influenced by stress. I conducted three field studies (one experimental and two descriptive) aimed at identifying how both CORT and DHEA secretion in free-living male northern cardinals (Cardinalis cardinalis), vary during acute stress; across different circulations (brachial vs. jugular); in response to ACTH challenge; and during the annual cycle. As predicted, restraint stress increased plasma CORT, but unexpectedly DHEA levels decreased, but the latter effect was only seen for blood sampled from the jugular vein, and not the brachial. The difference in DHEA between circulations may result from increased neural uptake of DHEA during stress. Injection with exogenous adrenocorticotropic hormone (ACTH) increased CORT concentrations, but failed to alter DHEA levels, thus suggesting ACTH is not a direct regulator of DHEA. Monthly field sampling revealed distinct seasonal patterns to both initial and restraint stress CORT and DHEA levels with distinct differences in the steroid milieu between breeding and non-breeding seasons. These data suggest that the CORT response to stress remains relatively consistent, but DHEA secretion is largely independent of the response by CORT. Although CORT functions have been well-studied in wild animals, little research exists for the role of DHEA and their variable relationship sets the stage for future experimental research addressing steroid stress responses.