Abstract
Urbanization drives phenotypic variation in many animal species. This includes behavioral and physiological traits such as activity patterns, aggression, and hormone levels. A current challenge of urban evolutionary ecology is to understand the environmental drivers of phenotypic variation in cities. Moreover, do individuals develop tolerance to urban environmental factors, which underlie adaptative responses and contribute to the evolution of urban populations? Most available evidence comes from correlative studies and rare experiments where a single urban-related environmental factor has been manipulated in the field. Here we present the results of an experiment in which we tested for differences in the glucocorticoid (CORT) response of urban and rural blue tits nestlings (Cyanistes caeruleus) to artificial light at night (ALAN). ALAN has been suggested to alter CORT response in several animal species, but to date no study has investigated whether this effect of ALAN differs between urban and rural populations. Immediately after hatching, urban and forest broods were either exposed to 2 lux of ALAN (using an LED source mounted inside the nestbox) or received no treatment (dark control). The experiment lasted until the chicks fledged. When the chicks were 13 days old plasma samples were collected to measure baseline CORT concentrations, and feather samples to provide an integrative measure of CORT during growth. Forest birds had higher plasma CORT (pCORT) concentrations than their urban counterparts, irrespective of whether they were exposed to ALAN or not. Conversely, we found population-specific responses of feather CORT to ALAN. Specifically, urban birds that received ALAN had increased feather CORT compared with the urban dark controls, while the opposite was true for the forest birds. pCORT concentrations were negatively associated to fledging success, irrespective of population and treatment, while feather CORT was positively associated to fledging success in broods exposed to ALAN, but negatively in the dark control ones. Our results demonstrate that ALAN can play a role in determination of the glucocorticoid phenotype of wild animals, and may thus contribute to phenotypic differences between urban and rural animals.
Highlights
Urbanization represents a major modification of natural habitats and is considered as a threat to biodiversity (Grimm et al 2008)
Post hoc tests indicated that fCORT concentrations in forest populations were significantly higher in dark control compared with artificial light at night (ALAN) nestlings
ALAN nestlings in the city had significantly higher fCORT concentrations compared with the ALAN nestlings in the forest, while fCORT concentrations in the dark control birds did not differ between the two populations (Supplementary Table S3).plasma CORT (pCORT) concentrations were significantly higher in the forest than in the city (b 1⁄4 0.4, t 1⁄4 2.6, P 1⁄4 0.009, Fig. 1B and Supplementary Tables S4 and S5), were negatively associated with body mass (b 1⁄4 À0.23, t 1⁄4 À2.9, P 1⁄4 0.004, Supplementary Table S4), but were not affected by the ALAN treatment (b 1⁄4 À0.1, t 1⁄4 À0.7, P 1⁄4 0.473)
Summary
Urbanization represents a major modification of natural habitats and is considered as a threat to biodiversity (Grimm et al 2008). Within the species that do well in the urban environment, some readily exploit anthropogenic resources, such as food supplementation and artificial nesting sites, whereas others suffer from pressures imposed by city life, they might appear to breed successfully. Understanding the effects of such pressures, such as noise, artificial light at night (ALAN), impervious surface, air pollution on lifehistory traits, behavior, physiology, and population dynamics, has been a major focus of urban ecology ß The Author(s) 2021.
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