LAUSR

Howler monkeys (genus Alouatta ) exhibit the most extensive distribution among platyrrhines, comprising Mesoamerican and South American species groups, with the South American group including the Brazilian endemic A. belzebul species complex encompassing A. belzebul , A. discolor , and A. ululata . We herein analyzed their phylogenetic relationship, nucleotide and haplotype diversity, and population demography based on the mitochondrial gene cytochrome b. The phylogenetic and median-joining network analyses distinguished A. discolor , distributed in the west bank of the Xingu River, from A. belzebul on the east bank. This river is a zoogeographic barrier for these species. We did not find evidence of phylogenetic structure between the A. belzebul populations of opposite banks of the Tocantins River, likely related to the changes in the position of this river to the northeast in the late Pleistocene. The A. belzebul along this river showed great morphologic and haplotype diversity, and A. belzebul from the Amazon have kept a larger population size than A. discolor . We herein describe the karyotype of A. discolor , which was similar to those described for A. ululata and A. belzebul . Our results showed two well-defined and supported clades for A. discolor and A. belzebul . However, a new assessment of A. ululata across a large distribution of sampling is required due to the lack of a clear phylogenetic structure.