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1. Habituation, avoidance strategies, and social learning in wild Bornean orangutans in Gunung Palung National Park, Indonesia

   Brubaker-Wittman, Laura A; Kane, Erin E; Susanto, Tri Wahyu; Knott, Cheryl D (April 2020, American journal of physical anthropology)

   Habituation, or the process of an animal becoming comfortable with human observers, is an essential part of wild primate observational studies. Despite the importance of this process, questions remain as to what counts as habituated for a particular species, how individuals and species react to humans, and how age-sex classes differ in these responses. To address these questions, we analyzed data from over 25 years of research on wild Bornean orangutans from Gunung Palung National Park, Indonesia, drawing from 8,383 follows and 82,413 hours of observation. We categorized the degree of agitation with humans by totaling the number of alarm vocalizations, giving each follow a score of 1-10. We then looked at behavioral measures using a GLMM to control for individual and food availability. This revealed that individuals with the highest vocalization scores spent a greater percentage of the day traveling (b=40.5, p < 0.0001), stayed higher in the canopy (b=16, p < 0.0001) and spent less time eating (b=205, p < 0.0001) than did animals that did not vocalize. Our analysis also revealed a less common, but frequently observed, opposing response to humans, which was to hide, often inside of a day nest, and emit no vocalizations. Individual orangutans were observed to switch between these two ’strategies’ to evade human observers. We discuss the implications of this behavior as well as present evidence that the reaction of other orangutans mediates the response of focal individuals to humans, suggesting the importance of social learning in this behavior. 

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2. The influence of fruit availability on energy allocation in infant and juvenile Bornean orangutans

   Kane, Erin E.; Susanto, Tri Wahyu; Knott, Cheryl D. (February 2020, American journal of physical anthropology)

   The juvenile risk aversion hypothesis suggests an extended life history is an adaptation to unpredictable environments, such as those that characterize Bornean orangutans. We examined how changes in infant and juvenile activity budgets changed over time and with varying periods of food availability. We hypothesized that during periods of low food availability juvenile orangutans would trade-off developmentally important behavior, such as play, in order to conserve energetic resources. We tested this hypothesis using 924 follows of infant (0-4 years) and juvenile (5-8 years) orangutans, collected over a 25 year period, encompassing period fo high and low food availability at Gunung Palung National Park, Indonesia. Using a GLMM to control for individual and examining both age and food availability we found that overall, infant and juveniles played less during period of low food availability compared to medium and high food availability (b= -14.3, p < 0.0001) and that play behavior varied significantly by age (b=-5.9,m p < 0.0001). Infants in the 1-2 age category showed the highest percentage of play, comprising 42% of their overall activity budget, with play dropping to 14% by age 4. We also found signficant interaction effects between age and food availability on play behavior (b=2.1, p < 0.0001). We discuss the ways that infants and juvenile trade off energetically expensive behavior through different development periods as well as the role that maternal behavior may play in buffering infants and juveniles of coping with periods of low food availability. 

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3. 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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4. 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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5. Age-related change in adult chimpanzee social network integration

   https://doi.org/10.1093/emph/eoab040  

   Thompson González, Nicole; Machanda, Zarin; Otali, Emily; Muller, Martin N.; Enigk, Drew K.; Wrangham, Richard; Emery Thompson, Melissa (December 2021, Evolution, Medicine, and Public Health)

   Abstract BackgroundSocial isolation is a key risk factor for the onset and progression of age-related disease and mortality in humans. Nevertheless, older people commonly have narrowing social networks, with influences from both cultural factors and the constraints of senescence. We evaluate evolutionarily grounded models by studying social aging in wild chimpanzees, a system where such influences are more easily separated than in humans, and where individuals are long-lived and decline physically with age. MethodologyWe applied social network analysis to examine age-related changes in social integration in a 7+ year mixed-longitudinal dataset on 38 wild adult chimpanzees (22 females, 16 males). Metrics of social integration included social attractivity and overt effort (directed degree and strength), social roles (betweenness and local transitivity) and embeddedness (eigenvector centrality) in grooming networks. ResultsBoth sexes reduced the strength of direct ties with age (males in-strength, females out-strength). However, males increased embeddedness with age, alongside cliquishness. These changes were independent of age-related changes in social and reproductive status. Both sexes maintained highly repeatable inter-individual differences in integration, particularly in mixed-sex networks. Conclusions and implicationsAs in humans, chimpanzees appear to experience senescence-related declines in social engagement. However, male social embeddedness and overall sex differences were patterned more similarly to humans in non-industrialized versus industrialized societies. Such comparisons suggest common evolutionary roots to ape social aging and that social isolation in older humans may hinge on novel cultural factors of many industrialized societies. Lastly, individual and sex differences are potentially important mediators of successful social aging in chimpanzees, as in humans.Lay summary: Few biological models explain why humans so commonly have narrowing social networks with age, despite the risk factor of social isolation that small networks pose. We use wild chimpanzees as a comparative system to evaluate models grounded in an evolutionary perspective, using social network analysis to examine changes in integration with age. Like humans in industrialized populations, chimpanzees had lower direct engagement with social partners as they aged. However, sex differences in integration and older males’ central positions within the community network were more like patterns of sociality in several non-industrialized human populations. Our results suggest common evolutionary roots to human and chimpanzee social aging, and that the risk of social isolation with age in industrialized populations stems from novel cultural factors. 

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