Alouatta caraya: Population density and demography in Northern Argentina

A long-term study was carried out on a black howler monkey (Alouatta caraya) population living in 150 ha of forest patches spread out in a 3,000 ha rural area in Northern Argentina. A total of 22 different groups were located between 1980 and 1984 and 11 of these were monitored regularly during 28 months. Ecological density varied between 12.14 and 12.93 groups per sq km of thall dense forest (0.80 to 1.15 ind./ha; biomass 306 to 365 kg/sq km). Solitaries and small associations of adult or subadult individuals of both sexes were also recorded but occupied habitats of inferior quality. Mean sizes of reproductive groups varied from 6.4 to 8.4 and the range was 3 to 15 individuals. Reproductive groups had 1 to 3 adult males, 1 to 3 adult females, and several immatures. Sex ratio was biased toward females among the adults but it varied in other age classes. Births occurred throughout the year showing a peak in the colder and drier season. Subadult or young adult males and females dispersed from suspected natal groups and became solitaries or joined associations. Males invaded groups and displaced or coexisted with resident males. Infanticides and disappearances of infants were associated with male changes. Comparative censuses on an island not far from the main study area showed higher density and biomass (2.8 ind./ha; 1,117 kg/sq km) and different group sizes and composition. Habitat features as well as the history of each study site may account for the observed demographic differences.