Group size and composition of black-and-gold howler monkeys (Alouatta caraya) on the Upper Paraná River, Southern Brazil.

In social mammals, group size, sex and age-class composition are important parameters that are required to understand population dynamics and determine conservation strategies. These parameters are known only poorly for the black-and-gold howler monkey (Alouatta caraya). Here, we studied groups of A. caraya on islands and adjacent banks of the Upper Paraná River of southern Brazil, to examine variability in group size and composition. This location is important for this species because of the high density of howlers. Group size was large, varying from 6 to 18 individuals (average = 11.5, standard deviation = 3.3, n = 13). Groups were multi-male, and adult females outnumbered adult males. On average, groups had the following ratios: 1 adult male: 0.5 subadult male: 1.9 adult females: 0.9 juveniles: 0.5 infants. The ratio of 0.2 infant: 1 adult female is less than that in other species, but typical of other studies of the black-and-gold howlers. Here, we discuss environmental and social pressures that may impose structure on large groups of howlers in terms of strategies for living in high densities. We also compare these data with those of the area of sympatry shared between A. caraya and A. clamitans, and suggest that competition may occur between the two species and that reduced fitness may be a consequence of mixed groups.