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1. Developmental tradeoffs: Fruit availability and offspring age influence mother and offspring activity in Bornean orangutans (Pongo pygmaeus wurmbii)

   Kane, EE; Susanto, TW; Knott, CD (January 2021, American journal of physical anthropology)

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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 

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2. Foraging efficiency and ecological risk aversion in juvenile Bornean orangutans; Program of the 86th Annual Meeting of the American Association of Physical Anthropologists

   https://doi.org/10.1002/ajpa.23210  

   Knott, Cheryl; DeLong, K (April 2017, American Journal of Physical Anthropology)

   For species with low mortality and high reproductive costs, like primates, rather than maximize reproduction, natural selection should favor slow growth and slow reproductive rates. Orangutans, because of their slow life history, and the extreme fluctuations in their food supply, are hypothesized to have been selected for slow juvenile development to avoid ecological risk. Juveniles are predicted to be particularly vulnerable during periods of low food availability because of lower foraging success. Thus, we tested the hypothesis that juvenile orangutans are less efficient foragers than adults and that they are less able to both access and digest important fall-back foods. Data were collected on wild orangutans in Gunung Palung National Park, Borneo, Indonesia between 1994-2016. Analyses are drawn from 468 matched follows of mother-offspring pairs in which more than 75% of the diet has been analyzed. We found that juveniles ate fruit when their mother’s ate fruit during 98.3% of bouts. However, for other food items, juveniles were much less likely to eat the same food items (insects = 65.2%, leaves 76.5, bark 70.0%, flowers 75.0%, pith 65.8%). As expected, we found that juveniles ate significantly fewer calories than did adults overall, but this difference was particularly pronounced during periods of low food availability (p < 0.001). We show that these results were due to constraints on both the processing and digesting of fall-back foods. Thus, the long period of orangutan growth and dependency may reflect a risk-averse growth strategy in this forest characterized by dramatic fluctuations in preferred fruits. 

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3. Foraging efficiency and ecological risk aversion in juvenile Bornean orangutans

   Knott, Cheryl; DeLong, K (January 2017, American Association of Physical Anthropologists Annual Meeting)

   For species with low mortality and high reproductive costs, like primates, rather than maximize reproduction, natural selection should favor slow growth and slow reproductive rates. Orangutans, because of their slow life history, and the extreme fluctuations in their food supply, are hypothesized to have been selected for slow juvenile development to avoid ecological risk. Juveniles are predicted to be particularly vulnerable during periods of low food availability because of lower foraging success. Thus, we tested the hypothesis that juvenile orangutans are less efficient foragers than adults and that they are less able to both access and digest important fall-back foods. Data were collected on wild orangutans in Gunung Palung National Park, Borneo, Indonesia between 1994-2016. Analyses are drawn from 468 matched follows of mother-offspring pairs in which more than 75% of the diet has been analyzed. We found that juveniles ate fruit when their mother’s ate fruit during 98.3% of bouts. However, for other food items, juveniles were much less likely to eat the same food items (insects = 65.2%, leaves 76.5, bark 70.0%, flowers 75.0%, pith 65.8%). As expected, we found that juveniles ate significantly fewer calories than did adults overall, but this difference was particularly pronounced during periods of low food availability (p < 0.001). We show that these results were due to constraints on both the processing and digesting of fall-back foods. Thus, the long period of orangutan growth and dependency may reflect a risk-averse growth strategy in this forest characterized by dramatic fluctuations in preferred fruits. 

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4. Oral Processing Behavior in Bornean Orangutans of Different Age and Sex Classes

   Wyszynski, Sofia I; Kane, Erin E; Bakley, Tori D; Rinaldi, R; Susanto, Wahyu; Knott, Cheryl D. (November 2021, Northeastern Evolutionary Primatologists)

   Orangutan diets vary dramatically with food availability. Orangutans preferentially eat fruit when available, but due to dramatic and unpredictable fluctuations in fruit availability, orangutans often consume unripe fruit, bark, seeds, and leaves. Their robust craniodental structure suggests that they are well adapted to consume mechanically challenging foods. Since differences in jaw anatomy and body size pose physiological differences in terms of gape, exerted force, and resistance to wear and breakdown, growth and allometry likely affect an orangutan’s ability to process a mechanically challenging diet. Thus, we predict that orangutans of different ages and sexes process foods differently. Given juveniles' smaller and less powerful craniodental structure, and the time required to develop ecological competence, we hypothesized that juveniles may have more difficulty in processing foods than adults. We recorded the frequency that foods were introduced to the mouth, and chewed with different teeth (incisors, canines, and molars) in 561 feeding videos collected in Gunung Palung National Park in West Kalimantan, Borneo on wild orangutans (Pongo pygmaeus wurmbii). Videos were stratified by age and sex class and foods were categorized by type. Infants and juveniles use their canines significantly more frequently than adult females (p< 0.05) and flanged males (p< 0.05). Molar use also differed by age and sex class (F(3)=2.551, p=0.05), with juveniles chewing with their molars significantly more frequently than adult females (p=0.05). Differences in adult and juvenile oral processing profiles suggest juveniles may process some foods less efficiently than adults. 

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5. 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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