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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. The development and maintenance of sex differences in dietary breadth and complexity in Bornean orangutans

   https://doi.org/10.1007/s00265-021-03014-3  

   Schuppli, Caroline; Atmoko, S. Suci; Vogel, Erin R.; van Schaik, Carel P.; van Noordwijk, Maria A. (May 2021, Behavioral Ecology and Sociobiology)

   null (Ed.)

   Abstract Orangutans show a pronounced sexual dimorphism, with flanged males (i.e., males with fully grown secondary sexual characteristics) reaching twice the size of adult females. Furthermore, adult orangutans show sex-specific dispersal and activity patterns. This study investigates sex differences in adult foraging behavior and sheds light on how these differences develop in immatures. We analyzed 11 years of feeding data on ten adult female, seven flanged male, and 14 immature Bornean orangutans ( Pongo pygmaeus wurmbii ) at Tuanan in Central Kalimantan, Indonesia. We found that the diets of the adult females were significantly broader and required more processing steps before ingestion than the diets of flanged males. We also found evidence for a similar difference in overall diet repertoire sizes. For the immatures, we found that whereas females reached 100% of their mothers’ diet spectrum size by the age of weaning, males reached only around 80%. From the age of 4 years on (i.e., years before being weaned) females had significantly broader daily diets than males. We found no difference in daily or overall diet processing intensity of immature males and females but found preliminary evidence that immature males included fewer items of their mother’s diet in their own diets that were processing-intensive. Overall, our results suggest that by eating a broader variety and more complex to process food items, female orangutans go to greater lengths to achieve a balanced diet than males do. These behavioral differences are not just apparent in adult foraging behavior but also reflected in immature development from an early age on. Significance Statement In many species, males and females have different nutritional needs and are thus expected to show sex-specific foraging behavior. Sex differences in several aspects of foraging behavior have been found in various species, but it remains largely unclear when and how those develop during ontogeny, which is especially relevant for long-lived altricial species that learn foraging skills over many years. In our study, we analyzed a cross-sectional and longitudinal data set containing more than 750,000 feeding events of adult and immature Bornean orangutans ( Pongo pygmaeus wurmbii ). We found that adult females had significantly broader and more complex diets than males. We also found that these differences started to develop during infancy, suggesting that immature orangutans prepare for their sex-specific foraging niches long before those become physiologically relevant while they are still in constant association with their mothers and before being frequently exposed to other role models. 

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3. Social party initiation, maintenance, and affiliative interaction by adolescent female orangutans in Gunung Palung National Park, West Kalimantan, Indonesia

   O’Connell, Caitlin A.; Knott, Cheryl (April 2018, 87th Annual Meeting of the American Association of Physical Anthropologists)

   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. 

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4. Ingestive Behavior of Bornean Orangutans (Pongo pygmaeus wurmbii): coping with mechanical challenges while foraging

   Kane, Erin; McGraw, Scott; Laman, Tim; Susanto, Tri Wahyu; Knott, Cheryl (April 2018, 87th Annual Meeting of the American Association of Physical Anthropologists)

   Bornean orangutans (Pongo pygmaeus wurmbii) preferentially eat pulp and mesocarp when fleshy fruit is abundant. However, during non-masting periods, orangutans rely on foods that can be mechanically challenging, including leaves, woody plant tissue, and seeds. Although adult orangutans’ jaws are well adapted for intense and, perhaps, repetitive loading during chewing, it may be easier for flanged adult males to process tough or hard foods than for adult females because of dramatic sexual dimorphism. Here, we use video data and in situ focalobservations from Gunung Palung National Park, West Kalimantan, Indonesia to test the hypotheses that orangutans exhibit food- and sex-specific oral processing profiles. Pilot data (n=94 feeding bouts; 76 adult females, 18 adult males) suggest no significant differences in use of incisors (F(3,71)=0.41, p=0.75), canines (F(3,71)=0.78, p=0.52), or molars (F(3,71)=0.88, p=0.46) per ingestive action while processing fruit, leaves, termites, or bark. Females used significantly more incisions per ingestive action (2.20) than males (1.01) (t=2.44, p=0.008), and, though differences were not significant, performed more canine bites (female mean=0.08, male mean=0.00, p=0.17) and mastications (female mean=4.88, male mean=3.95, p=0.24) per ingestive action than males. We detected no difference in the behaviors used by orangutans to process different food types, despite great variation in food mechanical properties. However, this may be because our pilot data did not capture the range of mechanical challenges in orangutan diets. Nevertheless, our preliminary results support the hypothesis that adult females work harder than flanged males during oral food processing, explicable due to sexual dimorphism. National Science Foundation (BCS-1638823, BCS-0936199); National Geographic Society; US Fish and Wildlife (F15AP00812, F12AP00369, 98210-8- G661); Leakey Foundation; Disney Wildlife Conservation Fund; Wenner-Gren Foundation; Nacey-Maggioncalda Foundation 

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5. Female choice and sexual coercion in wild orangutans (Pongo pygmaeus wurmbii)

   Scott, AM; Susanto, TW; Knott, CD (January 2021, 90th Annual Meeting of the American Association of Physical Anthropologists)

   null (Ed.)

   Due to extreme incongruence in parental investment, the potential for sexual conflict in orangutan reproductive strategies is high. Female orangutans prefer flanged males, but with intense male-male competition and sexual coercion, it is unclear how females are able to exert choice. We hypothesized that female orangutans use initiation and maintenance of associations with preferred males as a mechanism of female choice. We used encounter rates and behavioral measures of proximity maintenance to distinguish between the role of female choice and male coercion in male-female associations in Gunung Palung National Park, West Kalimantan, Indonesia. From May 2018-April 2019, we collected data on initiation, termination, and proximity maintenance during associations between males and cycling females (females without dependent offspring or with offspring over 6 years old). Encounters between cycling females and flanged males were more likely to be female-initiated (83.3%), while encounters with unflanged males were more likely to be male-initiated (80%) (N=16, p=0.035, Fisher’s exact test). Additionally, the Brown Index indicates significantly more female proximity maintenance when cycling females associated with flanged males than unflanged males (N=33, W = 198.5, p= 0.012). However, using long-term data on male-female associations, we found that dyads of flanged males and females were less likely to encounter other males compared to dyads of unflanged males and females (N=542, χ2= 3.3051, df=1, p=0.035). This indicates that flanged males may also use associations to mate guard females. These data indicate that there are behavioral manifestations of both female choice and male coercion in orangutan associations. Funders: NSF (DGE-1247312, BCS-1638823, BCS-0936199), Boston University, Leakey Foundation, Disney Conservation Fund, and US Fish and Wildlife Service (F18AP00898) 

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