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

Owl monkeys are small monogamous primates ranging over a wide area extendin g from Panama to the Chaco region of northern Argentina. The Chaco, an allu vial plain covering over one million km(2) of Argentina, Bolivia, Brazil, a nd Paraguay, consists of a mosaic of grasslands, savannas, xeric thorn fore sts, and gallery forests. The region shows significant seasonal variation i n climate, rainfall, and food availability. The goal of this study was to d etermine the density, size, and structure of a population of Aotus azarai i n the seasonal gallery forests of the eastern Argentinean Chaco. Reported p opulation density, as well as group size and composition are based on data collected from 11 groups contacted on approximately 900 occasions, and obse rved for over 2,000 hours during a three-year period. Group and individual densities were 16 groups/km(2) and 64 individuals/km(2), respectively. Appr oximately half of the groups (n = 5) were small groups which had three indi viduals most of the time and never more than four, whereas the remaining gr oups 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 o n monitoring of a relatively large number of distinct groups. Our data sugg est 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 mon key population are of comparable size and composition to those characterist ic of populations in the tropics. Am. J. Primatol. 53:99-108, 2001. (C) 200 1 Wiley-Liss, Inc.