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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. 

Male orangutans exhibit bimaturism—two mature morphs—flanged and unflanged males. Flanged males are larger, have cheek pads (flanges) and large throat sacs, and produce long calls. Previous orangutan paternity studies found variation between the reproductive success of each morph and in the degrees of reproductive skew. However, these studies were limited by a lack of behavioral maternity data, the inclusion of ex-captive orangutans, and/or the presence of feeding stations. Here we present the first paternity data from completely wild orangutans with known mothers. We hypothesized that (1) flanged males would have higher reproductive success than unflanged males due to flanged male dominance and female preference and (2) a single male would not monopolize paternity due to the temporal and spatial distribution of fecund females. We used fecal samples collected in Gunung Palung National Park from 2008-2019 to genotype orangutans (13 offspring born 2002-2015, their 10 mothers, and 19 candidate sires) using 12 microsatellites. MICROCHECKER 2.2.3 and CERVUS 3.0 were used to confirm the suitability of the microsatellite panel, fidelity of individual identities, and genetic maternity. Paternity analysis was performed with both CERVUS 3.0 and COLONY 2.0.6.7. We were able to identify paternity for six offspring. Four flanged males sired five offspring, and one sire’s morph was unknown at the time of conception. We found that flanged males have higher reproductive success and that females are not monopolizable in this completely wild setting. We discuss the implications of all published orangutan paternity results for understanding bimaturism in orangutans. 

When female chimpanzees, orangutans, and callitrichids share challenging‐to‐process resources with their offspring, they improve offspring access to foods and calories which would otherwise be unavailable. Adult chimpanzees share foods rarely, but when they do, sharing valuable resources solidifies inter‐individual bonds (e.g., when building coalitions or eliciting copulations). While maternal‐offspring food sharing has been studied in wild orangutans, the context in which adult orangutans share food and feed in proximity is poorly known. We use 27 years of research on orangutans in West Kalimantan, Indonesia, to examine this behavior. Food sharing and tolerance were observed during 2,131 follows between 1994‐2019. Mother‐infant food sharing occurred in 78%, of these follows, female‐female sharing in 22%, male‐female sharing in 32%, and male‐male in just 1%. Adult females shared foods at different rates with adult males than with offspring (Chi‐square = 49.27,p< .01,N= 589 events). Eighty‐one percent of mother‐offspring food sharing/tolerance was fruit, compared to only 71% of male‐female food sharing/tolerance. Durio, Lithocarpus, and Willughbeia (hard‐to‐process fruits) were most frequently shared by mothers. Twenty‐three percent of male‐female food sharing/tolerance occurred while eating termites; only 3% of mother‐infant sharing did. Only two of 350 mouth‐to‐mouth or hand‐to‐mouth transfers involved adult males and females. Mothers increase their offspring’s access to challenging resources, while food sharing/tolerance among adult males and females is not limited to valuable resources, but may indicate strong social tolerance or affiliation in generally solitary adults 

Orangutan habitats are characterized by fluctuations in the availability of ripe fruits. During non-fruiting periods orangutans typically incorporate more lower-quality foods such as pith and bark in their diet. Condensed tannins (CT) are secondary plant compounds that bind to proteins, thus impeding the digestibility of proteins, and tending to make foods bitter or unpalatable. We analyzed condensed tannin content in 129 plant samples collected from Gunung Palung National Park in Borneo, Indonesia between 1994 and 2001. We predicted that CT concentrations would be highest in bark, and that there would be a correlation between protein and condensed tannin content. We used ANOVA with Bonferonni’s method for post-hoc comparisons to test for differences in 11tannin content between plant parts, and Pearson’s correlation to test for relationships between tannin concentrations and other nutrients. There were significant differences in condensed tannin content (F(¬4)=2.70, p=0.03) but no differences after adjusting the alpha-level for post-hoc comparisons. Whole fruit (including the skin) tended to have the highest CT concentration. However, we found no correlation between CT and concentration of nutrients including crude protein (R=0.12, p=0.19, N=127), free simple sugars (R=-0.09, p=0.40, N=100), or fiber (R=-0.38, p=0.67, N=128). This underscores that plants rich in desirable nutrients may also be rich in antifeedants, posing challenges for orangutan consumption and digestion even as they provide a source of high-quality energy. Additionally, for some food categories where high tannin content is predicted, such as bark, orangutans may be choosing to eat species that are lower in these compounds. Funders: Boston University Undergraduate Research Opportunities Program, NSF (BCS-1638823, BCS-0936199); National Geographic; USFish/Wildlife (F18AP00898, F15AP00812, F13AP00920, 96200-0-G249, 96200-9-G110, 9414388); Leakey; Disney Wildlife Conservation; Wenner-Gren; Nacey-Maggioncalda; Orangutan Conservancy; Conservation, Food, and Health Foundation. 

Orangutans are large-bodied frugivores predicted to be important seed dispersers, however little is known about their seed dispersal effectiveness. To understand wild Bornean orangutan (Pongo pygmaeus wurmbii) seed dispersal effectiveness, we measured the quantity of seeds dispersed, and we considered the quality of dispersal by measuring germination rates of gut-passed and control seeds, gut transit times, and dispersal distances. Research was conducted in Gunung Palung National Park, Borneo, Indonesia (August 2018 to August 2019). We systematically collected orangutan fecal samples, feeding behavior, and GPS tracks during consecutive full-day focal follows. We sieved 549 fecal samples collected from 36 orangutans to count and identify seeds (>2mm). Out of the fecal samples collected, 413 (75.2%) contained seeds. A total of 24 genera were dispersed via endozoochory. Orangutan fecal samples contained a mean of 1.17 genera (range 0-7). Germination experiments were conducted with orangutan defecated seeds and seeds from fruit. A significantly higher percent of orangutan defecated seeds germinated for 5 out of 6 genera than control seeds with pulp (p<0.01). A significantly higher percent of orangutan defecated seeds germinated for 3 out of 6 genera compared to control seeds without pulp (p<0.01). Gut transit times in wild orangutans ranged from 39.5 to 87 hours (n=6). Finally, we modeled seed dispersal distances using orangutan movement tracks (n= 30) with gut passage durations of 45 and 60 hours. Gut retention times of 45 hours resulted in a mean dispersal distance of 507 ± 123m (range 69 - 1341), and 60 hours resulted in a mean distance of 592 ± 115m (range 83 - 1260). We conclude orangutans are effective seed dispersers with similar efficacy to other great apes. Orangutans disperse a wide variety of genera over medium to long distances and gut passed seeds germinate at higher rates compared with controls. Keywords: Ecology, Seed dispersal effectiveness, Movement, Tropical, Asia Funders: National Science Foundation (BCS-1638823); National Geographic Society; US Fish and Wildlife Services (F19AP00798; F18AP00898); Disney Wildlife Conservation Fund 

Our previous research showed that the distance between an orangutan mother-offspring dyad decreases when males are present. This change in proximity has been argued to be one aspect of a suite of female infanticide avoidance strategies. Therefore, we hypothesized that mothers are responsible for proximity maintenance in the presence of male conspecifics. In order to investigate this hypothesis, we examined Hinde Indices and Brown-modified Hinde Indices during full day follows of mother-offspring orangutans in Gunung Palung National Park, West Kalimantan, Indonesia, from May 2018-April 2019. We found that 91% of days (N=34) had a Hinde Index indicating offspring control of proximity and 88% of days (N=34) had a Brown-modified Hinde Index indicating greater offspring responsibility for proximity changes than mother responsibility. There was no difference in the Hinde Index (Nmale absent=32, Nmale present=14, t=0.979, df=19.122, p=0.334) or Brown-modified Hinde Index (Nmale absent=32, Nmale present=14, t=0.216, df = 22.424, p=0.831) when males were present compared to when males were absent. Counter to our hypothesis, offspring, not mothers, were still responsible for proximity maintenance in the presence of males. However, controlling for variation by analyzing the data on a per follow basis, we found that Hinde Indices shifted towards becoming more mother-maintained when males were present for 70% of the follows, but this difference was not significant (N=10, t=1.050, df=9, p=0.321). One limitation in interpreting these results is that all offspring included in our study were over three years old and past the age at which they are most vulnerable to infanticide. 

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. 

Wild primates have foraging goals beyond energy maximization. These foraging goals are often met by less energy dense, and more reliable, non-fruit foods. Are they able to meet these goals by navigating to high-energy fruit resources and consuming non-fruit (lower-energy) foods along the way, or do they navigate to non-fruit foods as well? Here we address this question in wild Bornean orangutans, a species known for consuming ripe fruit when it is available. Our research suggests that orangutans maintain a ratio of non-protein energy to protein and choose to consume non-fruit foods when in proximity to fruit foods. Using the movement ecology paradigm, we examine sequences of feeding bouts where an orangutan travels from one fruit patch to another, stopping at one or two non-fruit foods along the way (n = 54). By asking whether the direct path length between the two fruit patches is the same as, or less than, the actual travelled path length, we can determine whether the orangutans deviate from a fruit-to-fruit path and navigate to non-fruit foods. We find that orangutans do deviate from the direct fruit-to-fruit path by an average additional 18% of path length (range 8% - 84%; p = 6.819e-07). This is not impacted by fruit availability. This research suggests that orangutans are navigating to more constant non-fruit foods as well as the temporally and spatially patchy fruit resources. We discuss this in terms of which types of foods orangutans are navigating toward. 

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. 

Orangutans are large-bodied frugivores predicted to be effective seed dispersers. We studied Bornean orangutan (Pongo pygmaeus wurmbii) seed dispersal effectiveness, by measuring the quantity of seeds dispersed and the quality of dispersal in Gunung Palung National Park, Borneo, Indonesia (August 2018 to August 2019). For dispersal quality we conducted germination experiments, measured germination rates, and modeled dispersal distances. We systematically collected orangutan fecal samples, feeding behavior, and GPS tracks during focal follows. We sieved 549 fecal samples collected from 36 orangutans and identified the seeds, and of the fecal samples collected 75.2% contained seeds. A total of 24 genera were dispersed via endozoochory. Germination experiments were conducted with orangutan defecated seeds and seeds from fruits. A significantly higher percent of orangutan defecated seeds germinated for 5 out of 6 genera than control seeds with pulp (p<0.01). A significantly higher percent of orangutan defecated seeds germinated for 3 out of 6 genera compared to control seeds without pulp (p<0.01). Gut transit times in wild orangutans ranged from 39.5 to 87 hours. Finally, we modeled seed dispersal distances using orangutan movement tracks (n= 30) with gut passage durations of 45 and 60 hours. Gut retention times of 45 hours resulted in a mean dispersal distance of 507 ± 123m, and 60 hours resulted in a mean distances of 592 ± 115m. We conclude orangutans are effective seed dispersers, as orangutans disperse a wide variety of genera over medium to long distances and defecated seeds exhibit high germinability. Orangutans are large-bodied frugivores predicted to be effective seed dispersers. We studied Bornean orangutan (Pongo pygmaeus wurmbii) seed dispersal effectiveness, by measuring the quantity of seeds dispersed and the quality of dispersal in Gunung Palung National Park, Borneo, Indonesia (August 2018 to August 2019). For dispersal quality we conducted germination experiments, measured germination rates, and modeled dispersal distances. We systematically collected orangutan fecal samples, feeding behavior, and GPS tracks during focal follows. We sieved 549 fecal samples collected from 36 orangutans and identified the seeds, and of the fecal samples collected 75.2% contained seeds. A total of 24 genera were dispersed via endozoochory. Germination experiments were conducted with orangutan defecated seeds and seeds from fruits. A significantly higher percent of orangutan defecated seeds germinated for 5 out of 6 genera than control seeds with pulp (p<0.01). A significantly higher percent of orangutan defecated seeds germinated for 3 out of 6 genera compared to control seeds without pulp (p<0.01). Gut transit times in wild orangutans ranged from 39.5 to 87 hours. Finally, we modeled seed dispersal distances using orangutan movement tracks (n= 30) with gut passage durations of 45 and 60 hours. Gut retention times of 45 hours resulted in a mean dispersal distance of 507 ± 123m, and 60 hours resulted in a mean distances of 592 ± 115m. We conclude orangutans are effective seed dispersers, as orangutans disperse a wide variety of genera over medium to long distances and defecated seeds exhibit high germinability. Funders: National Science Foundation (BCS-1638823); National Geographic Society; US Fish and Wildlife Services (F19AP00798; F18AP00898); Disney Wildlife Conservation Fund