On the basis of these different inferred phylogenetic histories, three main hypotheses concerning the general biogeographic history of spider monkeys have been proposed. First, on the basis of morphological data, Froehlich et al. (1991) suggested that the specieslevel diversity of most extant spider monkeys is largely explained by the distribution of rivers acting as barriers to gene flow, with the Río Tapajos forming the geographic boundary between the distributions of Ateles marginatus and A. chamek and the Río Amazonas/Río Ucayali forming the boundary between A. chamek and A. belzebuth. Under Froehlich et al.’s (1991) scenario, speciation of A. hybridus and A. belzebuth must be explained by the Pleistocene Forest Refugia hypothesis (Haffer, 1969) as there is no other apparent geographic barrier to gene flow between these species. For the speciation of Ateles geoffroyi, Froehlich et al. (1991) postulated that a population was isolated in a northern Colombian forest refugium during the Pleistocene and then migrated to Central America and dispersed as far north as Mexico.