Bornean orangutans ( Pongo pygmaeus) are critically endangered, facing a very high risk of extinction. Their population levels have dropped more than 50 percent over the past 60 years, and their habitat has declined by over 80 percent in the past 20 years. The total population of wild Bornean orangutans is estimated to be approximately 70,000-100,000 individuals. These arboreal apes face threats such as habitat loss due to unsustainable timber extraction and land conversion for agriculture, hunting, and the illegal pet trade.
General Dietary Habits
Primarily frugivorous, orangutans can forage for up to 6 hours a day in their rainforest home. Largely frugivorous, when it is abundant, the fruit will make up as much as 90% of their diet, supplemented with leaves, shoots, seeds, buds, flowers, bark, insects and mineral-rich soil, and occasional instances of meat-eating. Plant food species represent 453 genera and 131 families, while invertebrate species eaten include 4 species of ants, 4 species of termites, 2 species of caterpillars, leeches, wasps, maggots, bee larvae, crickets, and ticks. Vertebrates consumed include slow lorises, gibbons, birds’ eggs, young birds and tree rats.
Food preferences
The fruits they eat most often include:
- Durians
- Figs
- Lychees
- Jackfruit
- Breadfruit
Several fruits that only have scientific names, including Nessia, Sarcotheca, Nephelium, Tetramerista, Mallotus, Gironniera, Lithocarpus, Antiaris, Tinomiscium and Eugenia. They also eat flowers. For example, the blooms of Xanthophyllum rufuum are a favorite. Leaves make up a large part of an orangutans diet, particularly those of Gironniera nervosa, which is also an important source of bark. Leaves of various species of the breadfruit group Artcarpus are consumed, as are those of the trees of the genus Baccaurea. Although both ripe and unripe fruit will be eaten, orangutans prefer young, soft plant parts to older ones, especially leaves, which develop toxins as they grow to discourage leaf-eaters.
Nutritional needs
Diet and nutrition are critical factors influencing energetics and health. They must meet these demands while experiencing fluctuations in the availability of food resources (1). Recognizing an animal’s flexibility to nutrient supply and demand is thus valuable for understanding ecological and evolutionary processes, which in turn can be applied to conservation strategies for a species (4). The multidimensional nutritional geometry framework has enabled a higher resolution characterization of interactions between nutrient intake and physiological response (5). It has become clear that specific combinations of different nutrients are required for maintenance, growth, and reproduction, and these dynamic nutritional needs are dependent on the changing physiological condition of the forager in response to variable resource availability (12-15).
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Foraging Behavior
Orangutans mainly forage and eat first thing in the morning and in the late-afternoon when the heat of the day is less intense. During this time, they will use tree branches to slowly move through the forest canopy searching for tasty fruit whilst grazing on young tree leaves.
Learning and Social Influences on Foraging
Studies of social learning in the wild are important to complement findings from experiments in captivity. In this field study, immature Bornean orangutans rarely foraged independently but consistently followed their mothers' choices. Their diets were essentially identical to their mothers' even though not all mothers had the same diet. This suggests vertical transmission of diet by enhancement. Also, immatures selectively observed their mothers during extractive foraging, which increased goal-directed practice but not general manipulation of similar objects, suggesting observational forms of learning of complex skills. Teaching was not observed. These results are consistent with the reported presence of food traditions and skill cultures in wild orangutans. We suggest that food traditions can develop wherever association commonly allows for social learning.
Tool Use in Foraging
Orangutans have high cognitive abilities comparable to the other great apes. This high level of intelligence manifests itself in tool-use and even the making of simple tools in the wild. Some tool use is idiosyncratic but other kinds of tool use represent cultural traditions in orangutan populations.
Termites make a delicious snack for orangutans and are a good source of protein. To make catching them easier orangutans use sticks as tools. They will poke a stick in a nest and wait for the tiny insects to cover it. Then they will pull the stick out and eat the termites off the stick a bit like a lolly pop!
Orangutans have been observed making simple tools to scratch themselves. They also use leafy branches to shelter themselves from rain and sun, and sometimes even drape large leaves over themselves like a poncho. They have also been observed using branches as tools during insect foraging, honey collection, and protection against stinging insects, and to “fish” for branches or fruit that is out of reach. In Sumatra wild orangutans use tools to extract seeds from a hard shelled species of fruit. In captivity an orangutan was taught to chip a stone handaxe.
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Diet Variation
The type and variety of food types eaten has been shown to be heavily influenced by a series other contributing factors, including island differences, seasons, climate, habitat type, and habitat quality, with differences seen in all major Sumatran and Bornean study sites.
Island and Regional Differences
The islands of Borneo and Sumatra differ in both forest type and forest productivity, and because of this, marked differences are observed in the diets of the Bornean and Sumatran species’. Figs are found in abundance in Sumatra, but are absent from large parts of Borneo, and, as such, play a much larger role in the diets of Sumatran orangutans than they do Borneans. In one four year study in Tanjung Putting in Indonesian Borneo, only 0.4% of foraging bouts involved figs, in comparison to 54% of all foraging bouts in a similar study in Gunung Leuser in Sumatra. Seeds are also eaten more widely by Bornean orangutans than they are by Sumatrans, due to the high frequency of trees from the Diptercarp family in Borneo’s forests. Trees from this family fruit irregularly, but produce large quantities of oil-rich seeds, which orangutans love.
Differences have also been noted at study sites on the same island. Orangutans eat from Shorea leprosula at four different sites, but only eat its fruit at one site. They eat from Durio kutejensis at four study sites in East Borneo, but eat different parts of the plant at each different site. Pith has also been observed being eaten at all sites except that in Tuanan in Borneo. While many of these anomalies may be explained by ecological factors and longer field studies, they nevertheless demonstrate the complexity of the orangutan diet.
Coping with Fruit Scarcity
The Bornean rainforest is renowned for having unpredictable seasons which can cause fruit availability to be scarce for long periods at a time. Bornean orangutans have evolved a unique digestive system which is better adapted than other primates for digesting fibre, storing fat and conserving energy.
Mast fruiting refers to periods of low fruit productivity that are punctuated by periods of high fruit availability, with 90% of canopy species producing fruits at the same time, followed by severe fruit scarcity. During mast fruiting, orangutans will gorge exclusively on fruit, build up fat reserves, and then diversify their diet when the mast is over, relying on different types of ‘fall-back’ foods. Because Sumatran forests produce higher numbers of energy-rich figs and fruit on a more consistent basis, mast fruiting has a greater effect on Bornean orangutans; For example, at Gunung Palung in west Kaliamantan, Indonesian Borneo, 37% of an orangutans diet after a mast fruiting is made up of low-quality food items such as bark.
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Metabolic Flexibility
We integrate macronutrient regulation and metabolic flexibility to elucidate how wild orangutans ( Pongo pygmaeus wurmbii) are buffered against natural fluctuations in nutritional resources. We found that these orangutans regulate protein and regularly switch between exogenous and endogenous nutritional substrates as preferred food resource availability declines. When total caloric, lipid, and carbohydrate intake declined during episodes of fruit scarcity, orangutans drew on fat and endogenous amino acids for energy. This strategy is beneficial only in the context of alternating periods of fruit scarcity and abundance.
Integrating macronutrient regulation and metabolic flexibility can help elucidate how animals may be buffered against resource instability in the wild. Bornean orangutans ( Pongo pygmaeus spp.) are a promising model species for examining this. They experience extreme and unpredictable variation in fruit availability relative to Neotropical or African primates (25), meeting their nutritional needs from multiple other food types (26), and exhibit many of the distinguishing characteristics that define generalist herbivores (27, 28). For example, while orangutans prefer fruit (29), within each single population they are known to feed on a large variety of foods, including fruits, flowers, insects, inner bark, and leaves that exhibit high overlap in their nutritional compositions (26, 30). During low fruit periods, several populations of orangutans have been shown to rely on fallback foods, such as leaves and inner cambium, that tend to have lower energy density compared to fruit (26, 31, 32). These dietary shifts result in striking variation in caloric intake (31-33), leading to negative energy status, as evidenced by increased fat catabolism (32, 34).
Mother-Infant Food Sharing
Baby orangutans rely on their mother's milk for the first 3-4 years of their life. As they grow they start to experiment with other, solid foods - much like human babies! Observing their mothers eat helps to teach them what foods are safe and what to avoid.
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.
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.