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Ramiro Dip,¹ Daniel Hegglin,² Peter Deplazes,² Oscar Dafflon,³ Herbert Koch,³ and Hanspeter Naegeli¹

¹Institute of Pharmacology and Toxicology, University of Zürich-Tierspital, Zürich, Switzerland; ²Institute of Parasitology, University of Zürich, Zürich, Switzerland; ³Federal Veterinary Office, Bern, Switzerland

Abstract
The colonization of urban and suburban habitats by red foxes (Vulpes vulpes) provides a novel sentinel species to monitor the spread of anthropogenic pollutants in densely populated human settlements. Here, red foxes were collected in the municipal territory of Zürich, Switzerland, and their perirenal adipose tissue was examined for persistent organochlorine residues. This pilot study revealed an unexpected pattern of contamination by polychlorinated biphenyls (PCBs) , with significantly higher levels of the predominant congeners PCB-138, PCB-153, and PCB-180 in juvenile foxes relative to adult animals. Further data analysis demonstrated that the observed difference was attributable to an age-dependent reduction of PCB concentrations in females, whereas male foxes retained approximately the same PCB burden throughout their life span. A similar sex-related bias between population members has been observed, primarily in marine mammals. Interestingly, the reduction of organochlorine contents with progressive age is reminiscent of human studies, where an extensive maternal transfer of xenobiotics to the offspring has been shown to result in increased exposure levels of infants relative to adults. To our knowledge, this is the first example of an urban wildlife species that faithfully reflects the dynamic distribution of toxic contaminants in the corresponding human population. Suburban and urban foxes occupy habitats in close proximity to humans, depend on anthropogenic food supplies, are relatively long-lived and readily available for sampling, can be easily aged and sexed, have a limited home range, and, therefore, meet several important requirements to serve as a surrogate species for the assessment of toxic health hazards. Key words: DDT, dieldrin, PCB, pollutants, polychlorinated biphenyls, risk assessment, sentinel species. Environ Health Perspect 111:1608-1612 (2003) . doi:10.1289/ehp.6226 available via http://dx.doi.org/ doi:10.1289/ehp.6226 available via http://dx.doi.org/ [Online 28 July 2003]