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Much of the Mexican state of Zacatecas lies in the area ascribed to the Mexican Plateau. Along the western border and in the southwest, however, the state encompasses a portion of the Sierra Madre Occidental, and perhaps more importantly, from a zoogeographic view, the Rio Juchipila, a major tributary of the Rio Grande de Santiago, drains the southern "panhandle." The valley of the Juchipila supports tropical vegetation as perhaps do the valleys of several other smaller tributaries of the Rio Grande that barely reach the southern part of the state. The setting of southern Zacatecas suggests, therefore, considerable faunal diversity.