RESUMO
Two neighboring groups of masked titi monkeys (Callicebus personatus personatus) were observed for nearly 240 hr over a 5-mo period. Minimum home range estimates for the groups were 10.7 and 12.3 ha; daily path length averaged 1 km. Each group shared a minimum of nearly 20% of its range with neighbors, and encounters between groups were rare and exclusively vocal. There was no strong evidence that titis called at the edge of their ranges or used non-overlap areas more intensively. Masked titis at this site were therefore not markedly territorial, in contrast to other descriptions of this genus. However, this study was carried out at a time of unusually low rainfall, which may have influenced behavior. Further longer-term studies of adjacent groups are therefore required to assess environmental, specific, and subspecific factors affecting territoriality in this genus.