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The Marginal Value Theorem (MVT) is an integral supplement to Optimal Foraging Theory (OFT) as it seeks to explain an animal's decision of when to leave a patch when food is still available. MVT predicts that a forager capable of depleting a patch, in a habitat where food is patchily distributed, will leave the patch when the intake rate within it decreases to the average intake rate for the habitat. MVT relies on the critical assumption that the feeding rate in the patch will decrease over time. We tested this assumption using feeding data from a population of wild Bornean orangutans (Pongo pygmaeus wurmbii) from Gunung Palung National Park. We hypothesized that the feeding rate within orangutan food patches would decrease over time. Data included feeding bouts from continuous focal follows between 2014 and 2016. We recorded the average feeding rate over each tertile of the bout, as well as the first, midpoint, and last feeding rates collected. We did not find evidence of a decrease between first and last feeding rates (Linear Mixed Effects Model,n = 63), between a mid‐point and last rate (Linear Mixed Effects Model,n = 63), between the tertiles (Linear Mixed Effects Model,n = 63), nor a decrease in feeding rate overall (Linear Mixed Effects Model,n = 146). These findings, thus, do not support the MVT assumption of decreased patch feeding rates over time in this large generalist frugivore.

 

The primate adolescent period is characterized by a series of changes in physiology, behavior, and social relationships. Orangutans have the slowest life history and the longest period of dependency of all primates. As members of a semisolitary species with high levels of sexual coercion, adolescent female orangutans face a unique combination of challenges when achieving independence from their mother. This study examined the mating behavior of adolescent female orangutans and compared it with that of adult females to assess whether mating behavior reflects distinct strategies at these different points in the life cycle. Data were collected in Gunung Palung National Park on the island of Borneo over 20 years. Mating events from adolescent (n = 19) and adult females (n = 26) were scored and compared. Adolescent female mating events had significantly higher mating scores (indicating more proceptivity) than those of adult females (β = 1.948,p = .001). Adolescent females also engaged in elaborate sociosexual interactions with different flanged males, behaviors that were never observed during mating events of adult females. These interactions involved characteristic behavior on the part of both the adolescent females and the flanged males. Given these findings and the documentation of similar accounts of adolescent female–flanged male mating from the island of Sumatra, we propose that adolescent female orangutans display distinctive behavioral repertoires throughout the genusPongowhich serves to overcome male ambivalence toward nulliparous females, establish familiarity, and evaluate coercive tendencies in flanged males. We suggest that these behavioral patterns are an integral part of female social development in a female philopatric, but highly dispersed species where consistent social support is absent after ranging independence is achieved.

 

Orangutans challenge our dichotomous perception of phenotypic sex seeing as they have three sexuallymature adult morphs: females, unflanged males, and flanged males. In males, a significant increase in androgen levels is associated with numerous changes in physical characteristics and behavior that develop during the flanging process. While unflanged males lack these obvious secondary sexual characteristics, they still have higher testosterone levels compared to females. Here, we test whether captive unflanged males and females have similar facial ratios (facial width/bi-orbital distance), since flanges form from a facial compartment that contains androgen receptors and is present prior to flanging. When flanging, males deposit fat to these compartments. In the field, unflanged males can be hard to distinguish from female orangutans, without a clear view of the genitalia. Flanged males (3.9 ± 0.5, range=2.7-5.1, N=20) have significantly wider facial ratios compared to unflanged males (2.4 ± 0.3, range=2.1-3.0, N=9) and females (2.3 ± 0.3, range=1.9-2.7, N=22; F(2,48)= 98.18, p-value >0.0001), who are similar in their facial ratios. Interobserver reliability between measurements (N=74) of the bi-orbital distance (V=1171, p=0.3251), facial width (V=1711, p=0.04779), and facial ratio (V=496.5, p=0.2434) are low. While there are other morphological differences between them, unflanged males and females do not differ in their facial ratios. The lack of significant differences in facial ratios between unflanged males and females, despite the higher testosterone levels in unflanged males, is consistent with them having an alternative 'sneaker' male reproductive strategy that includes mimicking female size and appearance. 

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 8383 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, alternative 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 get rid of human observers. We discuss the implications of this behavior as well as present evidence that different strategies in response to observers may vary between age-sex classes due to differing energetic demands. 

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. 

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. 

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. 

The Gunung Palung Orangutan Project has conducted research on critically endangered wild Bornean orangutans (Pongo pygmaeus wurmbii) since 1994 in Gunung Palung National Park, West Kalimantan, Indonesia. A major goal of our broad-ranging research on orangutan behavior and ecology is to understand how the unique rainforest environment of Southeast Asia, characterized by dramatic changes in fruit productivity due to unpredictable mast fruiting, impacts orangutan behavior, physiology, and health. Much of our research has been devoted to the development of non-invasive techniques and an integrated biology approach – using hormonal assays, fecal processing, nutritional analysis, genetics, and behavioral ecology – and has led to an increased understanding of the ecological and evolutionary pressures shaping orangutan adaptations. Our results show that the extended life history and very slow reproductive rate of orangutans are adaptations to their environment. Orangutans in the Gunung Palung landscape, as elsewhere across Borneo and Sumatra, also face a series of conservation challenges, including extensive habitat loss and the illegal pet trade. We highlight how our investigations of orangutan health status, ecosystem requirements, and the assessment of orangutan density using ground and drone nest surveys have been applied to conservation efforts. We describe our project’s direct conservation interventions of public education and awareness campaigns, sustainable livelihood development, establishment of village-run customary forests, investigation of the illegal pet trade, and active engagement with Indonesian government organizations. These efforts, in concert with the development of local scientific and conservation capacity, provide a strong foundation for further conservation as orangutans face a challenging future. 

In 2015, Lambert and Rothman urged primate nutritional ecologists to revise the view of fallback and optimal foods from stable traits inherent in the food to variable qualities determined by the state of the consumer. Here we provide behavioral evidence to support this revision. In primates, fruit is often the preferred food category because it is typically high-energy, high-carbohydrate, and low in fiber. Orangutans in particular, are said to consume fruit preferentially and when it is available, whereas leaves, bark, and pith are often considered fallback foods. Using movement ecology, we ask if wild Bornean orangutans (Pongo pygmaeus wurmbii) navigate only to fruit, or whether navigation to non-fruit foods is supported by our data. We find that orangutans do deviate from a direct fruit-to-fruit path to consume non-fruit foods (n = 54, range 8% - 84%; p = 6.819e-07.) Next, we ask if orangutans consume non-fruit foods when in the proximity of fruit resources. We find that 25.5% of the time that orangutans eat a non-fruit food, there is an available fruit within 50m (n=308). Building on previous research finding that orangutans maintain a 10.1:1 NPE:P balance, we use this geospatial data showing that orangutans navigate to and choose non-fruit foods, even when fruit is available, to suggest that orangutans are seeking foods based on their current nutritional state and not only to maximize energy. This supports the claim that 'fallback' is not an inherent characteristic of a food, but rather is in the state-dependent eye of the consumer. Funders: National Science Foundation (BCS-1638823; BCS-1613393), NSF GRFP (DGE-1247312); Boston University; National Geographic Society, US Fish and Wildlife (F15AP00812), Leakey Foundation, Disney Wildlife Conservation Fund; Wenner-Gren Foundation; Nacey-Maggioncalda Foundation, Princeton University