Group size and composition of brown howler monkeys (Alouatta guariba) in three fragments of Araucaria Pine Forest in southern Brazil

Abstract

Brown howler monkeys (Alouatta guariba) are increasingly threatened by habitat loss, sylvatic yellow fever, and negative human interactions. In southern Brazil, brown howlers still inhabit some fragments of the endangered Araucaria Pine Forest (APF), but basic information on their social organization remains scarce. We surveyed brown howler monkey groups to assess their group size and composition in three nearby similarly sized peri-urban APF fragments in Curitiba, southern Brazil. Using Generalized Linear Mixed Models, we investigated whether the number of males, females and immatures could be predicted by the number of individuals in each of these sex-age categories, along with group size. Monthly expeditions were conducted over 11-13 months, using multiple walks in each fragment to survey howler monkeys' groups. We surveyed 17 groups over 332 km of fieldwork. Group sizes ranged from two to eight individuals (mean 4.88 ± 1.65 SD), sex and age proportion was 1 adult male: 0.5 subadult male: 1.7 adult females: 1.2 juvenile: 0.4 infant, and the immature-to-female ratio was 0.96. We found that group size was a significant positive predictor for the number of adult females, juveniles and immatures overall, suggesting a general increase in fitness in larger groups, but not for the number of males and infants. Our findings indicate the presence of a small established population of brown howler monkeys in relatively small APFs, with population parameters in the typical range of the species, but an immature-to-female ratio more similar to populations of stagnanting growth, pointing to the necessity to implement measures to conserve and connect this population in an endangered habitat.