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Bornean orangutans' extended life history may be an adaptation to their forest habitat, characterized by dramatically fluctuating fruit availability. We hypothesize that juveniles rest and are carried more when fruit availability is low, spending less time in developmentally important behavior including play and independent travel to conserve energy. We test this using generalized linear mixed models and data derived from 976 follows of infant and juvenile orangutans collected during periods of varying fruit availability. Age (p<0.001), but not fruit availability (p>0.05) significantly affected offsprings’ time being carried (β=-8,1) and resting (β=-2.6, p<0.001). Younger individuals spent more time clinging and resting regardless of fruit availability. Fruit availability and offspring age interact to affect the proportion of time juveniles play (β=1.5, p<0.001) and travel (β=-0.8, p<0.001). Fruit availability impacts younger juveniles’ play behavior more dramatically than older juveniles: younger juveniles play more when fruit availability is high than when it is low, while older juveniles exhibit little variation in time spent playing depending on fruit availability. Juveniles aged 6-8 travel more when fruit availability is medium and high, juveniles aged 2-5 travel slightly less when fruit availability is high, and juveniles under 2 rarely travel independently. We also examined relationships between fruit availability, offspring age, and maternal travel distance using 2065 mother-offspring follows, demonstrating that females travel shorter distances with offspring under four, and when fruit availability is low. Thus, orangutans shift activity in response to fruit availability and throughout development, buffering young orangutans against energy depletion but suppressing developmentally important activities. Funders: NSF (9414388, BCS-1638823, BCS-0936199); National Geographic; USFish/Wildlife (F18AP00898, F15AP00812, F13AP00920, 96200-0-G249, 96200-9-G110); Leakey; Disney Conservation Fund; Wenner-Gren; Nacey-Maggioncalda; Conservation-Food-Health; Orangutan Conservancy; Woodland Park Zoo 

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.