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1. Social party initiation, proximity maintenance, and affiliation in wild Bornean adolescent female orangutans in Gunung Palung National Park

   O’Connell, Caitlin A.; Knott, Cheryl D. (October 2017, Northeastern Evolutionary Primatology)

   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 

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2. Orangutan Nesting Behavior in Gunung Palung National Park, West Kalimantan, Indonesia

   Brubaker-Wittman, L; Blackburn, A; DiGiorgio, A; Harwell, FS; Kane, EE:; Knott, CD. (March 2019, American journal of physical anthropology)

   Nesting behavior is unique to the great apes among primates and has wide ranging implications for understanding socioecology and conservation. While much is known about nesting in gorillas, chimpanzees, and some orangutan populations living in disturbed forest and peat swamp, the nesting behavior of orangutans living in primary forest is poorly understood. We studied the nesting behavior of Bornean orangutans (Pongo pygmaeus wurmbii) in Gunung Palung National Park, West Kalimantan, Indonesia, using observations of 4,526 nesting events collected between October 1994 and September 2018, testing hypotheses about nest height. We found a significant effect of age/sex on nest height (F(3)=106.1, p<0.001). Post-hoc comparisons (adjusted α-level= 0.008) showed that flanged males nested significantly lower than all other age/sex classes (p<0.001) while females nested significantly lower than juveniles and unflanged males (p<0.001). Flanged males and females tended to nest lower in the canopy when alone than in the presence of other orangutans (males: F(3)=24.25, p<0.001; females: F(3)=5.83, p=0.001). Our results help demonstrate that across forest types, flanged male orangutans prefer to nest lower in the canopy while all other age- and sex-classes prefer higher canopy positions for nesting. Furthermore, our finding that solitary individuals nest lower than individuals near other orangutans suggests that nesting higher in the canopy may allow individuals to space their nests optimally when in proximity of other orangutans. These results have significant conservation implications, as logging and deforestation fundamentally change the forest structure, disrupting the canopy and making preferred nesting locations unavailable. 

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3. The development of sex differences in play in wild white‐faced capuchins ( Cebus capucinus )

   https://doi.org/10.1002/ajp.23434  

   Winkler, Sasha L.; Perry, Susan E. (September 2022, American Journal of Primatology)

   Many mammalian species display sex differences in the frequency of play behavior, yet the animal literature includes few longitudinal studies of play, which are important for understanding the developmental timing of sex differences and the evolutionary functions of play. We analyzed social play, solitary play, and grooming using an 18-year dataset on 38 wild white-faced capuchin monkeys (Cebus capucinus) followed since infancy. Rates of each behavior were measured as the proportion of point samples taken during focal follows in which the individual engaged in each behavior. To determine sex differences in these rates, we ran a series of generalized linear mixed models, considering both linear and quadratic effects of age, and chose the optimal model for each of the three behavioral outcomes based on information criteria. Rates of both social play and solitary play decreased with age, with the exception of social play in males, which increased in the early juvenile period before decreasing. Male and female capuchins had different developmental patterns of social play, with males playing more than females during most of the juvenile period, but they did not display meaningful sex differences in solitary play rates. Additionally, males and females had different patterns of grooming over the lifespan: males participated in grooming at low rates throughout their lives, while adult females participated in grooming at much higher rates, peaking around age 11 years before declining. We suggest that male and female white-faced capuchins may adopt alternative social bonding strategies, including different developmental timing and different behaviors (social play for males versus grooming for females). Our results were consistent with two functional hypotheses of play, the practice and bonding hypotheses. This study demonstrates that play behavior may be critical for the development of sex-specific social strategies and emphasizes the importance of developmental perspectives on social behaviors. 

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4. Female Affiliation and Status in Semi‐Free‐Ranging Chimpanzees

   https://doi.org/10.1002/ajpa.70244  

   Bryer, Margaret_A H; Barnes, Paige; Sabbi, Kris H; Brodnan, Skylar; MacKenzie, Charlie; Monroe, Isabelle; Peña, Natalia Camargo; Sekulya, Phillip; Ampeire, Innocent; Okello, Amos; et al (April 2026, American Journal of Biological Anthropology)

   ABSTRACT ObjectivesSex differences in social behavior and status are pervasive across primates and other mammals. In the wild, chimpanzees (Pan troglodytes) exhibit many sex‐typed behaviors: adult female chimpanzees exhibit lower aggression, are subordinate to adult males, and also are generally less social than males. This pattern is thought to partly reflect the energetic constraints (e.g., feeding competition) that wild female chimpanzees face. Materials and MethodsWe test the drivers of sex differences in chimpanzee behavior by examining a semi‐free‐ranging African chimpanzee sanctuary group where individuals are provisioned, an environment that should relax ecological constraints on socializing. Using two years of focal observations on a group with 45 chimpanzees (17 males, 28 females), we examined sex differences in social proximity, grooming, aggression, coalition formation, and dominance. ResultsIn contrast to patterns in wild chimpanzees, we found that males and females in this population exhibited comparable rates of affiliative behavior. Males engaged in more aggressive behavior overall than females, as in the wild. However, females were equally likely to aggress either sex, and a high proportion of female aggression involved coalitions. Finally, David's scores revealed that a few of the adult females outranked some of the lower‐status adult males. DiscussionThese findings show that sex differences in chimpanzee social behavior are partially flexible, and females may show more affiliation, cooperation, and higher status when ecological conditions are favorable. More generally, some forms of female power can emerge even in a species with strong sex biases in behavior for male affiliation and status. 

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5. The socioecology of parasite infection in wild Bornean orangutans (Pongo pygmaeus wurmbii) in Gunung Palung National Park, West Kalimantan, Indonesia

   O’Connell, Caitlin A.; Robinson, Natalie; Karimah, Ishma Fatiha; Susanto, Tri Wahyu; Foitova, Ivona; Knott, Cheryl D. (February 2020, American journal of physical anthropology)

   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. 

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