Density and population structure of owl monkeys (Aotus azarai) in the Argentinean Chaco.

Owl monkeys are small monogamous primates ranging over a wide area extending from Panama to the Chaco region of northern Argentina. The Chaco, an alluvial plain covering over one million km2 of Argentina, Bolivia, Brazil, and Paraguay, consists of a mosaic of grasslands, savannas, xeric thorn forests, and gallery forests. The region shows significant seasonal variation in climate, rainfall, and food availability. The goal of this study was to determine the density, size, and structure of a population of Aotus azarai in the seasonal gallery forests of the eastern Argentinean Chaco. Reported population density, as well as group size and composition are based on data collected from 11 groups contacted on approximately 900 occasions, and observed for over 2,000 hours during a three-year period. Group and individual densities were 16 groups/km2 and 64 individuals/km2, respectively. Approximately half of the groups (n = 5) were small groups which had three individuals most of the time and never more than four, whereas the remaining groups were large groups composed of four or five individuals, and sometimes even six or seven individuals. This is the first study of A. azarai based on monitoring of a relatively large number of distinct groups. Our data suggest that owl monkeys in the seasonal subtropical forests of Formosa live at a density as high as those reported for owl monkey populations observed in tropical forests. The data also show that the social groups in the owl monkey population are of comparable size and composition to those characteristic of populations in the tropics.