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The eastern fox squirrel (Sciurus niger) has continued to move westward along riparian corridors in the southwestern United States. This species is now established along the Arkansas River and its major tributaries in southeastern Colorado. In addition, a population of S. niger exists in northern New Mexico, and this population might be the result of natural expansion from populations along the Purgatoire River in southeastern Colorado.
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Keith Geluso (2004) studied this question.
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