Wild animals face novel environmental challenges as natural habitats give way to urban areas, with numerous biotic and abiotic differences between the two. Urban ‘stressors’ may elicit a constant release of glucocorticoids via the hypothalamic–pituitary–adrenal (HPA) axis, and chronically elevated glucocorticoid levels can be associated with negative effects on health and reproduction. Reduced glucocorticoid secretion is proposed to facilitate adaptation to urban habitats by avoiding the negative health and reproductive effects of chronically elevated levels of circulating glucocorticoids. Here, we investigated this mechanism of adaptation to urban stress in a common species that occurs across the urban-rural gradient, and over a wide geographic range, Sciurus carolinensis (Eastern Gray Squirrel). We measured hair cortisol concentration (HCC), a long-term indicator of circulating glucocorticoids, of 192 squirrels from urbanized habitats and natural forest habitats in two study areas ~850 km apart, in the USA (N = 96 samples) and Canada (N = 96 samples). We examined the relationship between HCC and two correlated indices of urbanization, one reflecting vegetation (NDVI) and one reflecting human-made urban cover (NDBI). In the Canadian dataset, HCC showed quadratic relationships with NDVI and NDBI, indicating that squirrels have lower HCC in the most urbanized habitats (two university campuses). Males and females had similar HCC in the Canadian dataset. In the USA dataset, there was no relationship between HCC and either index, and males had higher HCC than females. These results suggest that urban habitats may be relatively benign for urban Eastern Gray Squirrels. Reduced glucocorticoid levels may represent a form of (phenotypic) plasticity that facilitates adaptation to and persistence in urban environments.

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Featured Image: iNaturalist Elliot Greiner, some rights reserved (CC-BY)

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