Baseline and stress-induced corticosterone levels across birds and reptiles do not reflect urbanization levels

Abstract

Rates of human-induced environmental change continue increasing with human population size, potentially altering animal physiology and negatively affecting wildlife. Researchers often use glucocorticoid concentrations (hormones that can be associated with stressors) to gauge the impact of anthropogenic factors (e.g. urbanization, noise and light pollution). Yet, no general relationships between human-induced environmental change and glucocorticoids have emerged. Given the number of recent studies reporting baseline and stress-induced corticosterone (the primary glucocorticoid in birds and reptiles) concentrations worldwide, it is now possible to conduct large-scale comparative analyses to test for general associations between disturbance and baseline and stress-induced corticosterone across species. Additionally, we can control for factors that may influence context, such as life history stage, environmental conditions and urban adaptability of a species. Here, we take a phylogenetically informed approach and use data from HormoneBase to test if baseline and stress-induced corticosterone are valid indicators of exposure to human footprint index, human population density, anthropogenic noise and artificial light at night in birds and reptiles. Our results show a negative relationship between anthropogenic noise and baseline corticosterone for birds characterized as urban avoiders. While our results potentially indicate that urban avoiders are more sensitive to noise than other species, overall our study suggests that the relationship between human-induced environmental change and corticosterone varies across species and contexts; we found no general relationship between human impacts and baseline and stress-induced corticosterone in birds, nor baseline corticosterone in reptiles. Therefore, it should not be assumed that high or low levels of exposure to human-induced environmental change are associated with high or low corticosterone levels, respectively, or that closely related species, or even individuals, will respond similarly. Moving forward, measuring alternative physiological traits alongside reproductive success, health and survival may provide context to better understand the potential negative effects of human-induced environmental change.

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

“…For example, a meta-analysis on fishes showed that anthropogenic noise can have negative effects on their behaviour and physiology, but the fitness consequences depended on the noise source [72]. Similarly, a recent comparative analysis found that noise, but not light, pollution was negatively associated with baseline corticosterone levels in birds [73]. The generic use of 'urban area' as a proxy for the presence of anthropogenic stress might be too crude an index if it includes cities that vary in their extent of urbanization (e.g.…”

Life history and environment predict variation in testosterone across vertebrates

“…2019; Injaian et al. 2020). Hormone data and other continuous variables were natural log‐transformed.…”

Influence of growing up in the city or near an airport on the physiological stress of tree sparrow nestlings (Passer montanus)

“…In the case of noise, whereas some studies revealed that traffic noise could elicit an increment or descend in baseline CORT levels (Crino et al 2011;Kleist et al 2018, respectively), others detected no change in CORT levels in experimental (Injaian et al 2019;Angelier et al 2016) and correlational (Casasole et al 2017) set ups. The fact that rural-airport nestlings exhibited an overall good body condition and oxidative status may indicate that they do not perceive aircraft noise as a stressor, or that tree sparrow nestlings are habituated to this stressor (Dickens and Romero 2013;Injaian et al 2020).…”

“…In the absence of perturbations, the organism keeps CORT concentrations at basal levels through modulations of the hypothalamus-pituitary-adrenal (HPA) axis (Sapolsky et al 2000). Chronic exposure to stressors can disrupt the correct functioning of the HPA axis, altering basal CORT levels either by increasing or reducing them (Dickens and Romero 2013;Injaian et al 2020). Therefore, long-term exposure to situations that may alter HPA responses can result in the impairment of different aspects of animal physiology if habituation is not achieved.…”