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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. Fiber Digestibility in Wild Bornean Orangutans: Age-Sex Class Differences and Implications for Slow Juvenile Growth

   Zdanowicz, Victoria R ; Kane, Erin E ; Susanto, Tri Wahyu ; Knott, Cheryl D. ( November 2021 , Northeastern Evolutionary Primatologists)

   Bornean orangutans (Pongo pygmaeus wurmbii), with their long lifespans and inter-birth intervals, and late age at first birth, have the most extended life history of all primates. According to the Ecological Risk Aversion Hypothesis, slow growth is a strategy to avoid starvation in unpredictable environments. When fruits are unavailable, bark and leaves, high in indigestible fiber, dominate orangutan diets. Orangutan hindgut fermentation is critical in the extraction of nutrients from fibrous fallback foods. However, body size and gut passage times are predicted to influence digestive efficiency. We investigated age-sex class differences in fiber digestibility in response to fluctuations in fiber intake. Researchers at Gunung Palung National Park, Indonesia, collected fecal samples during focal follows from 2016-2019 which were analyzed at Boston University’s Primate Ecology Lab. Samples (N=143) were weighed and dried overnight before grinding in a Wiley Mill. Seeds separated from the fecal matter were counted, measured, photographed and identified using our reference database. Dry matter and percent organic matter were determined through drying, ashing, and hot weighing. We used an ANKOM 200 Fiber Analyzer to determine NDF and compared the percent fiber excreted by mother/offspring pairs with feeding data. While adult female and juvenile diets showed similar percentages of fiber intake, juveniles excreted 50% more fiber than their mothers. Our results highlight the influence of body size, with smaller individuals demonstrating a lower capacity for fiber digestion. Juvenile digestive efficiency likely constrains growth, development, and survivorship during periods of low food availability when orangutan diets are high in fiber. 

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5. 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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