It has been suggested in the literature that primates of the genus Cacajao have been restricted to flooded-forest habitats of western Amazonia since their split from the Chiropotes line in the Tertiary. It has been proposed further that the differentiation of the two species of this genus, Cacajao melanocephalus and Cacajao calvus, occurred during the Pleistocene period as a result of the fragmentation of the Amazon forest and the isolation of populations in these forest fragments or refuges. However, recent evidence has shown that at least C. melanocephalus is not dependent on flooded-forest habitats, and molecular analysis of mitochondrial DNA shows that the two species had already differentiated during the Pliocene, thus Pleistocene glaciations do not explain the speciation in Cacajao. Considering that C. melanocephalus and its closest relative, Chiropotes, inhabit terra firme forests, it is suggested that preference for flooded-forest habitats may be an apomorphy in C. calvus.
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