The costs and benefits of sociality explored in wild Bornean orangutans (Pongo pygmaeus wurmbii)

Abstract

The socioecological model offers a framework for attempting to explain variation in sociality based on differences in ecological and social factors such as resource distribution, predation pressure, and infanticide risk. Orangutans are unusual among higher primates in their low degree of sociality and are considered to be semi-solitary. Their limited social behavior is thought to be a result of weak predation pressure on these large-bodied apes, coupled with the low and unpredictable fruit productivity that is characteristic of their habitat. Orangutans do come together occasionally, and there appears to be variability in the gregariousness of different populations. Orangutans present a unique opportunity to examine both social and solitary conditions within a single population to test predictions regarding the costs and benefits of sociality. This study assesses the ecological and social context in which social parties occur in Gunung Palung National Park on the island of Borneo. The potential costs of these associations are evaluated using behavioral and physiological markers of stress and parasite infection patterns. Fruit availability is predicted to influence the frequency of social associations, with sociality increasing when fruit availability is high. While the socioecological model predicts that female orangutans display reduced sociality, this should affect females in different reproductive (and hence, energetic) states differently. The results of this study confirmed that fruit availability influences the occurrence of social events and revealed adolescent females to be the most social age-sex class. Adolescent females displayed the most affiliative behaviors and engaged in notable sexual rituals with flanged males. They sought and maintained social associations with others, particularly their mothers. Despite evident signs of anxiety, adolescent females did not display elevation in the stress hormone cortisol under social conditions, while adult females and flanged males did. Intestinal parasites were widespread in this population, and the prediction for elevated parasite prevalence in more social classes was unsupported. This study revealed a greater degree of gregariousness than orangutans are typically credited with, and highlights the adolescent period as behaviorally distinct and socially rich for female orangutans who face unique challenges as members of a semi-solitary species with high levels of sexual coercion.