Genetic inheritance and environment determine endocrine plasticity to urban living

Endocrine flexibility can facilitate or constrain the ability to cope with global change

Global climate change has increased average environmental temperatures world-wide, simultaneously intensifying temperature variability and extremes. Growing numbers of studies have documented phenological, behavioural and morphological responses to climate change in wild populations. As systemic signals, hormones can contribute to orchestrating many of these phenotypic changes. Yet little is known about whether mechanisms like hormonal flexibility (reversible changes in hormone concentrations) facilitate or limit the ability of individuals, populations and species to cope with a changing climate. In this perspective, we discuss different mechanisms by which hormonal flexibility, primarily in glucocorticoids, could promote versus hinder evolutionary adaptation to changing temperature regimes. We focus on temperature because it is a key gradient influenced by climate change, it is easy to quantify, and its links to hormones are well established. We argue that reaction norm studies that connect individual responses to population-level and species-wide patterns will be critical for making progress in this field. We also develop a case study on urban heat islands, where several key questions regarding hormonal flexibility and adaptation to climate change can be addressed. Understanding the mechanisms that allow animals to cope when conditions become more challenging will help in predicting which populations are vulnerable to ongoing climate change. This article is part of the theme issue 'Endocrine responses to environmental variation: conceptual approaches and recent developments'.

Future directions in urban endocrinology - The effects of endocrine plasticity on urban tolerance

After twenty years of studies of endocrine traits in animals living in cities, the field of urban endocrinology has built a robust literature including numerous studies looking for signatures of the effects of urban living, usually in mean circulating hormone concentrations. The findings of this past research have primarily demonstrated the absence of any generalizable endocrine responses to city life. In this opinion paper, I suggest that a strong route forward would include investigations of the role of variation in endocrine plasticity in determining the degree to which organisms tolerate urban challenges (i.e., urban tolerance). Achieving this research aim will require creative experimental and comparative studies, consideration of alternative study systems, and teasing apart of sources of variation in plastic phenotypes (plasticity, sorting, and contemporary evolution). Insight into the role of endocrine plasticity in influencing urban tolerance could help us better understand and predict impacts of expanding urbanization on biodiversity across the globe.

Egg viability and egg mass underlie immune tradeoffs and differences between urban and rural lizard egg yolk physiology

Urbanization can cause innumerable abiotic and biotic changes that have the potential to influence the ecology, behavior, and physiology of native resident organisms. Relative to their rural conspecifics, urban Side-blotched Lizard (Uta stansburiana) populations in southern Utah have lower survival prospects and maximize reproductive investment via producing larger eggs and larger clutch sizes. While egg size is an important predictor of offspring quality, physiological factors within the egg yolk are reflective of the maternal environment and can alter offspring traits, especially during energetically costly processes, such as reproduction or immunity. Therefore, maternal effects may represent an adaptive mechanism by which urban-dwelling species can persist within a variable landscape. In this study, we assess urban and rural differences in egg yolk bacterial killing ability (BKA), corticosterone (CORT), oxidative status (d-ROMs), and energy metabolites (free glycerol and triglycerides), and their association with female immune status and egg quality. Within a laboratory setting, we immune challenged urban lizards via lipopolysaccharide injection (LPS) to test whether physiological changes associated with immune system activity impacted egg yolk investment. We found urban females had higher mite loads than rural females, however mite burden was related to yolk BKA in rural eggs, but not urban eggs. While yolk BKA differed between urban and rural sites, egg mass and egg viability (fertilized vs. unfertilized) were strong predictors of yolk physiology and may imply tradeoffs exist between maintenance and reproduction. LPS treatment caused a decrease in egg yolk d-ROMs relative to the control treatments, supporting results from previous research. Finally, urban lizards laid a higher proportion of unfertilized eggs, which differed in egg yolk BKA, CORT, and triglycerides in comparison to fertilized eggs. Because rural lizards laid only viable eggs during this study, these results suggest that reduced egg viability is a potential cost of living in an urban environment. Furthermore, these results help us better understand potential downstream impacts of urbanization on offspring survival, fitness, and overall population health.

Changes in the rearing environment cause reorganization of molecular networks associated with DNA methylation

Disentangling the interaction between the genetic basis and environmental context underlying phenotypic variation is critical for understanding organismal evolution. Environmental change, such as increased rates of urbanization, can induce shifts in phenotypic plasticity with some individuals adapting to city life while others are displaced. A key trait that can facilitate adaptation is the degree at which animals respond to stressors. This stress response, which includes elevation of baseline circulating concentrations of glucocorticoids, has a heritable component and exhibits intra- and inter-individual variation. However, the mechanisms behind this variability and whether they might be responsible for adaptation to different environments are not known. Variation in DNA methylation can be a potential mechanism that mediates environmental effects on the stress response, as early-life stressors increase glucocorticoid concentrations and change adult phenotype. We used an inter- and intra-environmental cross-foster experiment to analyse the contribution of DNA methylation to early-life phenotypic variation. We found that at hatching, urban house wren (Troglodytes aedon) offspring had higher methylation frequencies compared with their rural counterparts. We also observed age-related patterns in offspring methylation, indicating the developmental effects of the rearing environment on methylation. At fledgling, differential methylation analyses showed that cellular respiration genes were differentially methylated in broods of different origins and behavioural and metabolism genes were differentially methylated in broods of different rearing environments. Lastly, hyper-methylation of a single gene (CNTNAP2) is associated with decreased glucocorticoid levels and the rearing environment. These differential methylation patterns linked to a specific physiological phenotype suggest that DNA methylation may be a mechanism by which individuals adjust to novel environments during their lifespan. Characterizing genetic and environmental influences on methylation is critical for understanding the role of epigenetic mechanisms in evolutionary adaptation.

Urbanization and maternal hormone transfer: Endocrine and morphological phenotypes across ontogenetic stages

The phenotypes observed in urban and rural environments are often distinct; however, it remains unclear how these novel urban phenotypes arise. Hormone-mediated maternal effects likely play a key role in shaping developmental trajectories of offspring in different environments. Thus, we measured corticosterone (Cort) and testosterone (T) concentrations in eggs across the laying sequence in addition to Cort concentrations in nestling and adult female house wrens (Troglodytes aedon) at one urban and one rural site. We found that egg T concentrations were not different between birds from urban and rural sites. However, across all life stages (egg, nestling, and adult female), Cort concentrations were higher at the urban site. Additionally, urban nestling Cort concentrations, but not rural, correlated with fine-scale urban density scores. Furthermore, rural egg volume increased over the laying sequence, but urban egg volume leveled off mid-sequence, suggesting either that urban mothers are resource limited or that they are employing a different brood development strategy than rural mothers. Our study is one of the first to show that egg hormone concentrations differ in an urban environment with differences persisting in chick development and adult life stages. We suggest that maternal endocrine programing may shape offspring phenotypes in urban environments and are an overlooked yet important aspect underlying mechanisms of urban evolution.

Long-term changes of plumage between urban and rural populations of white-crowned sparrows (Zonotrichia leucophrys)

Urbanization is one of the most extreme forms of land transformation and results in changes to ecosystems and species compositions. As a result, there are strong directional selection pressures compared to nearby rural areas. Despite a surge in research on the different selection pressures on acoustic communication in urban and rural areas, there has been comparatively little investigation into traits involved with visual communication. We measured the plumage of museum specimens of white-crowned sparrows (Zonotrichia leucophrys) from urban and adjacent rural habitats in San Francisco, CA, to assess the effects of divergent habitats on plumage. We found significant differences in dorsal plumage, but not crown plumage, between urban and rural populations that have been diverging over the past 100 years. Urban birds have increasingly darker and duller dorsal plumage, whereas rural birds in adjacent areas have plumage with richer hues and more color complexity. Our findings suggest a newly observed adaptation to urban environments by native species and suggest that many traits, in addition to acoustic signals, may be changing in response to urban selection pressures. Additional collections in urban areas are needed to explore likely divergences in plumage coloration between urban and rural environments.

Incubation Behavior Differences in Urban and Rural House Wrens, Troglodytes aedon

As global land surfaces are being converted to urban areas at an alarming rate, understanding how individuals respond to urbanization is a key focus for behavioral ecology. As a critical component of avian parental care, incubating adults face a tradeoff between maintaining an optimal thermal environment for the developing embryos while meeting their own energetic demands. Urban habitats are biotically and abiotically different from their rural counterparts, i.e., in food availability, predator compositions, and the thermal environment. Therefore, urban birds may face different incubation challenges than their natural counterparts. We measured incubation behavior of rural and urban house wrens, Troglodytes aedon, with temperature loggers throughout the 12-day period. We found that urban females had more incubation bouts of shorter duration and spent less total time incubating per day than rural females. Results could provide evidence of behavioral shifts of wrens in cities, which have implications for the evolution of parental care. Our findings contribute to our understanding of the behavioral traits needed for city life and possible environmental pressures driving urban adaptations.

Urban resources limit pair coordination over offspring provisioning

The amount of care parents provide to the offspring is complicated by an evolutionary conflict of interest (‘sexual conflict’) between the two parents. Recent theoretical models suggest that pair coordination of the provisioning may reduce this conflict and increase parent and offspring fitness. Despite empirical studies showing that pair coordination is common in avian species, it remains unclear how environmental and ecological conditions might promote or limit the ability of parents to coordinate care. We compared the level of pair coordination, measured as alternation and synchrony of the nest visits, of house wrens Troglodytes aedon pairs breeding in a rural (10 nests) and a suburban (9 nests) site and investigated how differences in parental behaviours were related to habitat composition, prey abundance and how they ultimately related to reproductive success. We found that parents alternated and synchronized their nest visits more in the rural site compared to the suburban one. The suburban site is characterized by a more fragmented habitat with more coniferous trees and less caterpillar availability. Offspring from the rural site were heavier at fledging than at the suburban site. Taken together, these results suggest that environmental conditions play an important role on the emergence of coordinated parental care and that considering environmental variables is pivotal to assess the fitness consequences of parental strategies.

Biologia Futura: adaptive changes in urban populations

Cities represent novel environments where altered ecological conditions can generate strong selection pressures leading to the evolution of specific urban phenotypes. Is there evidence for such adaptive changes in urban populations which have colonized their new environments relatively recently? A growing number of studies suggest that rapid adaptations may be widespread in wild urban populations, including increased tolerance to various anthropogenic stressors, and physiological, morphological and behavioural changes in response to the altered resources and predation risk. Some of these adaptive changes are based on genetic differentiation, although other mechanisms, such as phenotypic plasticity and epigenetic effects, are also frequently involved.

Differences in adrenocortical responses between urban and rural burrowing owls: poorly-known underlying mechanisms and their implications for conservation

The hypothalamus-pituitary-adrenal/interrenal (HPA) axis of vertebrates integrates external information and orchestrates responses to cope with energy-demanding and stressful events through changes in circulating glucocorticoid levels. Urbanization exposes animals to a wide variety of ever-changing stimuli caused by human activities that may affect local wildlife populations. Here, we empirically tested the hypothesis that urban and rural owls (Athene cunicularia) show different adrenocortical responses to stress, with urban individuals showing a reduced HPA-axis response compared to rural counterparts to cope with the high levels of human disturbance typical of urban areas. We applied a standard capture-restraint protocol to measure baseline levels and stress-induced corticosterone (CORT) responses. Urban and rural owls showed similar circulating baseline CORT levels. However, maximum CORT levels were attained earlier and were of lower magnitude in urban compared to rural owls, which showed a more pronounced and long-lasting response. Variability in CORT responses was also greater in rural owls and contained the narrower variability displayed by urban ones. These results suggest that only individuals expressing low-HPA-axis responses can thrive in cities, a pattern potentially mediated by three alternative and non-exclusive hypotheses: phenotypic plasticity, natural selection and matching habitat choice. Due to their different conservation implications, we recommend further research to properly understand wildlife responses to humans in an increasingly urbanized world.

Genetic inheritance and environment determine endocrine plasticity to urban living

As urban areas continue to expand globally, understanding how and why species respond to novel habitats becomes increasingly important. Knowledge of the mechanisms behind observed phenotypic changes in urban animals will enable us to better evaluate the impact of urbanization on current and future generations of wildlife. Physiological changes, such as those involved in the endocrine stress response, may allow individuals to inhabit and thrive in urbanized areas, but it is currently unknown how these changes arise in natural populations. In this study, we performed a four-way cross-foster experiment in free-living house wren chicks, Troglodytes aedon, to disentangle whether differences in baseline corticosterone between urban and rural individuals are a result of genetic and/or plastic mechanisms during development. We found that urban chicks already had higher corticosterone levels than their rural counterparts on the day they hatched, which suggests a possible genetic component to the corticosterone phenotype. However, rural offspring that were moved to an urban environment significantly increased their corticosterone levels, mimicking those of urban offspring. Our findings suggest that, although differences in baseline corticosterone concentrations between urban and rural individuals may have a genetic component, plasticity plays a pivotal role and can modify the corticosterone phenotype in response to the environment experienced in the first two weeks of life.