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Abstract Pronounced temporal and spatial variation in the availability of food resources can produce energetic deficits in organisms. Fruit-dependent Bornean orangutans face extreme variation in fruit availability and experience negative energy and protein balance during episodes of fruit scarcity. We evaluate the possibility that orangutans of different sexes and ages catabolize muscle tissue when the availability of fruit is low. We assess variation in muscle mass by examining the relationship between urinary creatinine and specific gravity and use the residuals as a non-invasive measure of estimated lean body mass (ELBM). Despite orangutans having a suite of adaptations to buffer them from fruit scarcity and associated caloric deficits, ELBM was lower during low fruit periods in all age-sex classes. As predicted, adult male orangutans had higher ELBM than adult females and immatures. Contrary to expectation, flanged and unflanged males did not differ significantly in ELBM. These findings highlight the precarity of orangutan health in the face of rapid environmental change and add to a growing body of evidence that orangutans are characterized by unique metabolic traits shaped by their unpredictable forest environment. 

The socioecological model predicts that food availability and risk of parasite transmission influence sociality in primates. As a semi-solitary ape inhabiting the masting forests of Southeast Asia, orangutans provide a unique opportunity to compare social and non-social periods and highly variable foraging conditions within one population. This study compared two data collection periods when fruit availability differed markedly to determine whether sociality and parasite prevalence decrease as expected during periods of fruit scarcity. Fecal samples were analyzed using direct smear and fecal concentration techniques on-site at Cabang Panti Research Station from 2013-2014 and 2018-2019. From the high fruit period to the low fruit period, sociality decreased from 54% of focal follows containing a social event to 29%, while overall parasite prevalence remained the same at 100%. Interesting differences arose for certain parasite species, however. Enterobius sp. prevalence decreased during the low fruit period for both sexes but even more so for males (50% to 29% for females; 56% to 0 for males). Prevalence of Trichuris sp. increased for females during the low fruit period (5% to 43%) while prevalence among males remained the same. These results lend support to the prediction that social contact influences transmission risk for some parasite species, while other parasites may be more responsive to factors such as changes in reproductive state. These findings suggest that differences in the behavioral strategies of the sexes and the differential energetic demands of life history stages have an influence on parasitic infection patterns. 

In an effort to understand orangutan sociality and the benefits of socializing for a semi-solitary ape, we explore the social lives of the most gregarious orangutan age-sex class - adolescent females. From 1994-2016 adolescent females in Gunung Palung National Park had a social encounter on 50% of their follow days, spending 31% of their time in the company of others. Adolescent females were responsible for initiating social parties (coming within 50 meters) with other age-sex classes 86% of the time. Once they were in a social party, the percentage of approaches (decreases in distance between individuals) performed by adolescent females was significantly predicted by the age-sex class of their social partner (F=4.086, p=0.02). Adolescent females performed most of the approaches when they associated with adult females (70%), while approaches were more equal when they associated with flanged males (46% performed by adolescent females) or unflanged males (56% performed by adolescent females). These findings, in combination with higher rates of agonistic interactions between adolescent and adult females and higher rates of affiliative behaviors between adolescent females and unflanged males, indicate that adolescent females actively seek social opportunities with all age-sex classes, but the benefits and risks associated with socializing vary based on the age-sex of their social partners. We argue that sociality is important during adolescence for female orangutans because they must establish themselves in the social landscape, and must seek social learning opportunities. Finally, we consider the adaptive significance of meaningful social bonds for a semi-solitary, sexually coercive ape. 

Infanticide as a male reproductive tactic is widespread across mammals, and is particularly prevalent in catarrhine primates. While it has never been observed in wild orangutans, infanticide by non-sire males has been predicted to occur due to their extremely long inter-birth intervals, semi-solitary social structure, and the presence of female counter-tactics to infanticide. Here, we report on the disappearance of a healthy four-month-old infant, along with a serious foot injury suffered by the primiparous mother. No other cases of infant mortality have been observed at this site in 30 years of study. Using photographic measurements of the injury, and information on the behavior and bite size of potential predators, we evaluate the possible causes of this injury. The context, including the behavior of the female and the presence of a new male at the time of the injury, lead us to conclude that the most likely cause of the infant loss and maternal injury was male infanticide. We suggest that in orangutans, and other species where nulliparous females are not preferred mates, these females may be less successful at using paternity confusion as an infanticide avoidance tactic, thus increasing the likelihood of infanticide of their first-born infants. 

In an effort to understand orangutan sociality and what benefits might be gained from socializing for a semi-solitary ape, we explore the details of the social lives of the most gregarious orangutan age-sex class - adolescent females. From 1994-2016 adolescent females in Gunung Palung National Park spent 50% of their focal follow days with at least one social association, and 31% of their focal follow time with at least one other independently ranging individual. Adolescent females are the party responsible for initiating their social associations with other age-sex classes 86% of the time. The percentage of approaches performed by adolescent females within dyadic associations is significantly predicted by the age-sex class of their social partner (F=4.086, p=0.02), with adolescents performing most of the approaches in associations with adult females (70%), and a more mutual responsibility for proximity maintenance when they associate with flanged males (46%) or unflanged males (56%). These findings, in combination with rates of agonistic interactions with adult females and affiliative behaviors with unflanged males, indicate that adolescent females actively seek social opportunities with all age-sex classes, but the benefits and risks associated with socializing vary based on the age-sex of their social partners. We discuss the importance of sociality at this life stage for orangutans, and the potential that there are meaningful social bonds beyond the mother-offspring dyad in the wild. Funders: BU GRAF, NSF (BCS-1638823, BCS-0936199, 9414388), National Geographic Society, US Fish and Wildlife (F15AP00812, F12AP00369, 98210-8-G661), Leakey Foundation, Disney Wildlife Conservation Fund, Wenner-Gren, Nacey-Maggioncalda Foundation, Conservation, Food and Health Foundation 

Primate field projects are often under the direction of foreign researchers, who live and work alongside members of local communities. Here we discuss the impact of religion on orangutan research and conservation in Gunung Palung National Park, Indonesia over the past 25 years. We demonstrate how our local staff integrates orangutan conservation with religious practices. This includes sharing Islamic prayers on the responsibility of humans to safeguard nature, and the breaking of the fast during Ramadan with villages that are our conservation partners. Islam prohibits the eating of orangutans (as well as forest pigs). In contrast, the indigenous Dayak communities of Borneo historically do hunt and eat orangutans. This impacts the current orangutan distribution. At the Cabang Panti Research Site we have been recording the presence of hunters within our trail system, as well as the number of gunshots heard, since 2008. We find that there is a significant association between these occurrences and the end of the fasting month of Ramadan. We discuss changing religious values and how increasingly conservative practices affect foreign researchers, particularly women. We also detail how social media can help researchers become better informed about important local issues. For example, through our data collection we became aware that local people may be consuming wild pig meat around religious holidays, potentially unknowingly. Social media made us aware that local communities were concerned about this issue, which also impacts wildlife conservation, and thus we can tailor our outreach efforts to meet this intersection between religion and conservation. Funders: NSF (BCS-1638823, BCS-0936199); National Geographic; USFish/Wildlife (F15AP00812, F13AP00920, 96200-0-G249, 96200-9-G110); Leakey; Disney Wildlife Conservation; Wenner-Gren; Nacey-Maggioncalda; Orangutan Conservancy; Conservation-Food-Health; Woodland Park Zoo; Holloman Price; AZA; Ocean Park Conservation; USAID; Arcus 

The Gunung Palung Orangutan Project has conducted research on critically endangered wild Bornean orangutans (Pongo pygmaeus wurmbii) since 1994 in Gunung Palung National Park, West Kalimantan, Indonesia. A major goal of our broad-ranging research on orangutan behavior and ecology is to understand how the unique rainforest environment of Southeast Asia, characterized by dramatic changes in fruit productivity due to unpredictable mast fruiting, impacts orangutan behavior, physiology, and health. Much of our research has been devoted to the development of non-invasive techniques and an integrated biology approach – using hormonal assays, fecal processing, nutritional analysis, genetics, and behavioral ecology – and has led to an increased understanding of the ecological and evolutionary pressures shaping orangutan adaptations. Our results show that the extended life history and very slow reproductive rate of orangutans are adaptations to their environment. Orangutans in the Gunung Palung landscape, as elsewhere across Borneo and Sumatra, also face a series of conservation challenges, including extensive habitat loss and the illegal pet trade. We highlight how our investigations of orangutan health status, ecosystem requirements, and the assessment of orangutan density using ground and drone nest surveys have been applied to conservation efforts. We describe our project’s direct conservation interventions of public education and awareness campaigns, sustainable livelihood development, establishment of village-run customary forests, investigation of the illegal pet trade, and active engagement with Indonesian government organizations. These efforts, in concert with the development of local scientific and conservation capacity, provide a strong foundation for further conservation as orangutans face a challenging future.