Oil fields in the southern San Joaquin Valley of California have been identified as important habitat for the federally-designated endangered San Joaquin kit fox (Vulpes macrotiss mutica). However, the potential effects of oil field-related contaminants on kit foxes have not been evaluated. From 1992 to 1993 we collected blood samples from kit foxes inhabiting the Midway-Sunset oil field and from kit foxes inhabiting a reference site, the undeveloped Lokern Natural Area (Kern County, California), for hematology and serum chemistry analysis. We also collected tissue samples from deer mice (Peromyscus maniculatus), one of their prey species, for histologic analysis. A significantly higher proportion of kit foxes from the oil field had circulating immature red blood cells than foxes from the undeveloped reference area. A significantly higher proportion of deer mice from the oil field exhibited extramedullary hematopoiesis and adrenocortical vacuolation than deer mice inhabiting the undeveloped reference area. We conclude that kit foxes and deer mice inhabiting oil fields may be exposed to conditions that lead to potentially pathologic changes, although other factors unrelated to the oil field environment must also be considered. The potential detrimental effects of oil field exposure a terrestrial wildlife should be considered in the recovery planning process given the importance of oil fields as habitat for the endangered San Joaquin kit fox.
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