Hormonal adjustments to urban conditions: stress hormone levels in urban and rural populations of Apodemus agrarius

Animals' adaptation to life in cities is a widely studied issue in ecology. One such adaptation is the adjustment of stress response to new conditions and anthropogenic disturbances. There are various examples of such adjustments in urban birds, but the stress response of non-avian species has rarely been investigated. We studied an urban and a rural population of a small mammal, the striped field mouse Apodemus agrarius. We compared baseline corticosterone (CORT) concentration in faecal samples from mice, using ELISA tests with antibodies for this hormone. We found that although urban specimens have more anthropogenic stressors than their rural equivalents, they do not have an elevated level of CORT. This confirms the hypothesis that urban animals undergo hormonal adjustment to urban conditions. Moreover, the CORT concentrations of urban mice were lower compared to their rural conspecifics, while for body weight we observed an inverse relationship. A CORT level allows urban mammals to redirect more energy to weight gain (body fat); this is probably an effect of the lower pressure of competitors in urban areas, as well as better food availability (access to anthropogenic food). The results of this work show that species that have managed to overcome fear of humans and adapt to anthropogenic stressors can therefore achieve measurable ecological benefits.