The heavy burden of city life: Factors affecting mercury concentration in urban red squirrels

• We compared THg levels in hair samples of urban forest and urban park squirrels. • THg was determined using atomic absorption spectrometry. • THg levels were lower in hair samples from sub-adults than from adults. • Higher values were reported for the park. • THg level increased with the body mass in the urban park but not in the forest. Mercury is one of the most harmful metal contaminants, potentially affecting population functioning. We assessed the use of the red squirrel as a bioindicator of Hg contamination in the urban habitats of Warsaw. We thus compared total mercury (THg) concentrations in hair samples of live-trapped squirrels inhabiting areas of different mercury pollution: the urban forest in Warsaw’s outer district and the urban park in the city center, including in the analysis factors that may influence mercury concentration, i.e., age, body mass, reproductive status, and season. Mercury determination was done in a laboratory using atomic absorption spectrometry. In general, the concentration of THg in hair samples from sub-adult individuals was lower than in adults. The site of a sample collection and the body mass of squirrels in the urban park positively affected THg contamination. The higher mercury levels in the urban park were in line with the pollution levels in the soil. They can be explained by current and/or historical pollution and possibly also by park squirrels’ altered behavior. On the contrary, the season when the samples were taken, sex, or reproductive status did not influence the THg level, suggesting that the red squirrel can be a useful bioindicator species of Hg contamination in urban areas. Although, in general, the mercury levels were moderate, elevated levels in the park suggest that mercury remains a problem and should be monitored in urban areas.