Life in the city is stressful, for humans and wildlife! One of the main physiological traits thought to be impacted by the novel pressures of urban life, such as noise and light pollution, is stress hormone concentration. Nevertheless, no patterns have emerged across species. Allison Injaian and co-authors fill this gap in knowledge with their large-scale comparative analysis of baseline and stress-induced corticosterone concentrations across reptile and bird species globally. They report their findings in their recent paper “Baseline and stress-induced corticosterone levels across birds and reptiles do not reflect urbanization levels“.
What does urbanization have to do with stress?
Anthropogenic stressors may impact organismal stress levels and consequently stress hormone concentrations, via complex pathways (Fig. 1, above). And these stressors may extend far beyond the limits of the city. For example, even in remote rural locations, the glow of faraway cities can often be seen.
Studies of physiological effects of anthropogenic habitat modification on animals often focus on the stress hormones known as glucocorticoids. Specifically, researchers commonly measure both “baseline” hormone concentrations that reflect the everyday background level of stress experienced by an animal, and the “stress-induced” concentrations that reflect a response to an acute stressor such as a sudden noise or appearance of a predator. Research on baseline corticosterone concentrations (the main glucocorticoid stress hormone in reptiles and birds) suggest that stress hormone levels often increase with increasing artificial light at night, but decrease with increasing anthropogenic noise (due to a complex interaction with chronic noise exposure and ability to respond to acute stressors). These responses likely differ not only by species, but with non-urban habitat features (e.g., breeding season, weather) and urban-adaptability of a species.
Examining global trends in corticosterone
Measures of stress response can be valuable in assessing population trends in response to habitat modifications. But beyond the population level, are there emergent trends in how animals respond physiologically to anthropogenic habitat change? Using data from published studies aggregated in HormoneBase, Injaian et al. test to see if there are any relationships between baseline and stress-induced levels of corticosterone and various measures of urbanization: human footprint index, human population density, anthropogenic noise, and artificial light at night (ALAN). The authors also took into account the adaptability of a species, with the hypothesis that urban-avoiding species likely have stronger stress responses to urban stressors than urban-adapting species.
What they found was surprising. Across this global dataset of over 1000 populations of more than 100 species of birds and reptiles, they found no consistent relationship between features of the urban habitat and stress hormone concentrations. The only significant trend they observed was in urban-avoiding species, which had decreased concentrations of baseline corticosterone in habitats with high levels of anthropogenic noise. However, this relationship did not hold for urban-adapted species and no relationships were detected for the other measures of anthropogenic disturbance and stress hormone concentrations, nor for the non-avian reptiles they examined.
No universal stress response
This study demonstrates that stress responses to urbanization across birds and reptiles are not universal and appear to be species- and context-specific. This finding is consistent with a general impression I had that the relationship between anthropogenic stressors and stress hormones seemed to be highly variable and uncertain. But why is this the case? The authors provide some possible explanations as to why we don’t see a universal stress response:
- Physiological effects of chronic stress may vary by taxa depending on the nature of the disturbance (e.g., light is a visual disturbance), how it impacts activities (e.g., impacting visual acuity or visual cues), and the consequences of that disruption (e.g., altered nutritional state).
- Other anthropogenic stressors may act in concert or in opposition to the effects on stress hormones that may be population or species specific (e.g., predation risk or parasite load), imposing variable selection pressures and/or masking the effects of stressors such as ALAN and anthropogenic noise.
- Fluctuations in corticosterone concentrations may be highly individualized, depending on previous exposure events (e.g., habituation) or urban adaptation at the population level.
Given the ubiquitousness of glucocorticoid sampling with respect to anthropogenic stressors and environments, these results are surprising and have major implications. There does not appear to be a basis (at least in reptiles and birds) for the often assumed idea that urban animals experience shifts in stress hormone levels in response to urban stressors. Why does this matter? Well, to start, this means that studies based on the premise that stress hormone levels are correlated with urban stressors may be based on a flawed assumption and future studies should approach this hypothesis with caution. What does it mean then, if anything, when a correlation is or isn’t found between stress hormones and urban stressors? Does it actually say anything about the long- or short-term health implications of life in the city? The results of this study suggest they do not, and suggest that stress hormone levels alone are not valuable indicators of anthropogenic impacts.
As unsettling as this is, the authors offer some ways forward. Life in the city certainly is stressful, these results just suggest that measuring corticosterone to assess the effects of stress may not be the most informative approach, particularly when comparing across taxa. Future studies, they suggest, should consider corticosterone concentrations along with additional traits that may be impacted by stress levels such as physiological metrics, behavior, reproductive success, health, and survival.
Featured image: Troglodytes aedon aedon by John Benson from Madison WI, Madison WI 1, CC BY 2.0
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