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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)

   Abstract 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 data set 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 vs. 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. Facial Widths in the Three Adult Morphs of Orangutans

   Harwell, Faye S; Albert, Zoe A; Knott, Cheryl D. (November 2021, Northeastern Evolutionary Primatologists)

   Orangutans challenge our dichotomous perception of phenotypic sex seeing as they have three sexuallymature adult morphs: females, unflanged males, and flanged males. In males, a significant increase in androgen levels is associated with numerous changes in physical characteristics and behavior that develop during the flanging process. While unflanged males lack these obvious secondary sexual characteristics, they still have higher testosterone levels compared to females. Here, we test whether captive unflanged males and females have similar facial ratios (facial width/bi-orbital distance), since flanges form from a facial compartment that contains androgen receptors and is present prior to flanging. When flanging, males deposit fat to these compartments. In the field, unflanged males can be hard to distinguish from female orangutans, without a clear view of the genitalia. Flanged males (3.9 ± 0.5, range=2.7-5.1, N=20) have significantly wider facial ratios compared to unflanged males (2.4 ± 0.3, range=2.1-3.0, N=9) and females (2.3 ± 0.3, range=1.9-2.7, N=22; F(2,48)= 98.18, p-value >0.0001), who are similar in their facial ratios. Interobserver reliability between measurements (N=74) of the bi-orbital distance (V=1171, p=0.3251), facial width (V=1711, p=0.04779), and facial ratio (V=496.5, p=0.2434) are low. While there are other morphological differences between them, unflanged males and females do not differ in their facial ratios. The lack of significant differences in facial ratios between unflanged males and females, despite the higher testosterone levels in unflanged males, is consistent with them having an alternative 'sneaker' male reproductive strategy that includes mimicking female size and appearance. 

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5. Wild Bornean orangutans experience muscle catabolism during episodes of fruit scarcity

   https://doi.org/10.1038/s41598-021-89186-4  

   O’Connell, Caitlin A.; DiGiorgio, Andrea L.; Ugarte, Alexa D.; Brittain, Rebecca S.; Naumenko, Daniel J.; Utami Atmoko, Sri Suci; Vogel, Erin R. (December 2021, Scientific Reports)

   null (Ed.)

   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. 

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