RESUMEN
Variability in mitochondrial DNA sequences was analyzed in the howler monkey, Alouatta caraya, in order to delineate evolutionary relationships among populations in the most southerly distributed New World monkey. Based on new and previously published sequence data, fourteen cytochrome b haplotypes were observed among 33 howlers sampled in Argentina, Paraguay and Brazil, and grouped in two main haplogroups. In northeastern Argentina and southern Paraguay, new sequence data on 73 specimens sampled from six localities gave 34 control region haplotypes that also clustered in two main haplogroups. At this southern distribution, both mitochondrial markers revealed the presence of two sympatric and differentiated clades that we interpret to be the consequence of a secondary contact between previously allopatric populations. Given evidence for a demographic expansion at the beginning of the Holocene 15,500-7000 years ago (Fu's test, F(S)=-12.137; P<0.001), we suggest that atleast two populations of A. caraya have colonized the southernmost range since the Holocene employing forested corridors on the Paraná and Paraguay Rivers.
