What Does an Orangutan Eat? Their Diet in the Wild

Wild orangutans eat mostly fruit, and when fruit runs low they switch to bark, leaves, insects, and other fallback foods to survive. This simple-sounding diet hides real complexity: orangutans navigate enormous seasonal swings in food availability, sometimes feasting on over 8,000 calories a day and other times burning through their own muscle tissue. What they eat, how much they eat, and even how they learn to eat it all vary by species, by population, and by the rhythms of the Southeast Asian rainforest.

Fruit Is the Foundation

Orangutans are the largest tree-dwelling mammals on Earth, and fruit is overwhelmingly their preferred food. In a study of orangutans in a degraded Bornean landscape, fruit accounted for roughly 46% of total feeding time on average, with some individuals spending up to 78% of their feeding time on fruit in good months.¹ In pristine forests with higher fruit availability, those figures tend to be even higher. All three species of orangutan, the Bornean, Sumatran, and Tapanuli, prefer ripe, pulpy fruit when it is available.²

Their fruit diet is not limited to a few species. Orangutans feed on hundreds of different fruit-bearing trees and lianas across their range. Figs are a particularly important part of the menu and show up in orangutan diets at virtually every study site. In Danum Valley, Sabah, orangutans fed intensively on figs throughout most of the study period, and fig consumption became especially heavy during fruit shortages.³ Figs fruit year-round in many tropical forests while most other trees do not, making them a critical bridge between periods of plenty.

The Feast-or-Famine Cycle

Southeast Asian rainforests are defined by something called mast fruiting, irregular events (often separated by years) when a huge proportion of trees fruit simultaneously. Between these events, fruit can become extremely scarce. Orangutans have evolved a remarkable strategy for dealing with this unpredictability: when fruit is abundant, they gorge.

During peak fruit months at one Bornean site, males consumed around 8,400 calories a day and females around 7,400. Those figures are far above daily energy requirements and likely allowed the orangutans to gain as much as two-thirds of a kilogram per day, packing on fat stores to ride out the lean times ahead.⁴ Among the great apes, orangutans appear to be especially adapted for fat storage, a trait that makes sense given the extreme boom-and-bust cycle of their forests.

A long-term study tracking orangutan nutrition confirmed that total calorie intake rose about 19% between low-fruit and high-fruit periods, and that this increase was driven almost entirely by carbohydrates and fats rather than protein. Protein intake stayed remarkably stable regardless of what fruit was available, accounting for about 10% of total calories.⁵ This suggests orangutans prioritize keeping protein intake within a consistent range and let their energy intake from sugars and fats fluctuate with what the forest offers.

When Fruit Disappears

The real test of orangutan dietary flexibility comes during prolonged fruit shortages. When preferred tree fruits dry up, orangutans shift to what researchers call fallback foods: items that are less nutritious or harder to process but available year-round. These include bark, young leaves, pith, flowers, and the stems and leaves of climbing plants called lianas.

At the Danum Valley site, orangutans turned primarily to the leaves of the liana Spatholobus and figs during fruit shortages, along with the bark of Spatholobus and dipterocarp trees.³ At another Bornean site, a different liana genus called Bowringia played an outsized role in orangutan nutrition, increasingly consumed when preferred fruits were less available.⁶ Lianas may not be glamorous food, but they are vital to orangutan survival. They are woody, persistent plants that remain accessible through the worst fruit droughts.

Bark is another staple during hard times. In one degraded forest, bark alone accounted for an average of 33% of feeding time, and some individuals spent over 90% of their feeding time on bark during the lowest-fruit months.¹ Bark is low in energy and hard to chew, but it is always there. The fact that orangutans will invest so heavily in such an unrewarding food source tells you something about how desperate periods of scarcity can become.

The Physical Cost of Lean Times

Fat storage is not always enough. When fruit shortages last long enough, orangutans burn through their fat reserves and begin breaking down muscle tissue for energy. Research at multiple Bornean sites has documented that orangutans show signs of muscle loss during extended low-fruit periods, entering what amounts to an early stage of protein deficit even as they continue foraging on fallback foods.⁷ Their bodies, in other words, are slowly cannibalizing themselves.

This is a striking finding because orangutans have so many apparent adaptations for coping with food scarcity: the fat storage, the dietary flexibility, the willingness to eat bark and soil. And yet prolonged fruit shortages still push them to a physiological edge. It underscores how tightly orangutan survival is linked to the health and productivity of their forest, and how vulnerable they are when that forest is degraded or fragmented.

Bornean Versus Sumatran Diets

The three recognized orangutan species live on different islands (Borneo and Sumatra, with the Tapanuli orangutan occupying a small range in northern Sumatra), and their diets reflect the different forests they inhabit. Sumatran forests tend to be more productive year-round, offering more consistent fruit availability. As a result, Sumatran orangutans eat a higher proportion of fruit on average and rely less heavily on tough vegetation during lean periods. Some populations of Bornean orangutans, by contrast, depend more on bark and other hard-to-process plant materials when fruit is scarce.²

This difference has left physical traces. Bornean orangutans tend to have more robust jaws and teeth than their Sumatran relatives, consistent with a diet that regularly includes hard, tough foods. The Tapanuli orangutan, the most recently described species and the rarest great ape on Earth, has received less dietary study, but preliminary work in the Batang Toru Forest found its diet dominated by a small number of tree species, with Campnosperma auriculatum being the most consumed.⁸ That study noted a low diversity index for the Tapanuli orangutan’s diet, which could reflect the species’ restricted range or the limited productivity of its habitat.

Insects, Meat, and Other Surprises

Orangutans are not strictly herbivorous. Insects, particularly ants, termites, and bee larvae, make regular appearances in their diet, though they rarely account for a large share of feeding time. Orangutans poke into tree holes, peel back bark, and raid insect nests to get at these protein-rich snacks. The contribution of invertebrates to total nutrition is hard to measure because insect feeding bouts tend to be short and opportunistic, but they occur across all known orangutan populations.

Actual meat-eating, though, is vanishingly rare. Sumatran orangutans have been observed hunting and eating slow lorises, a small nocturnal primate. Seven such incidents were documented at one Sumatran site starting in 1989, all involving adult females.⁹ Bornean orangutans, by contrast, have essentially no meat in their diet. Prior to a single documented observation, only two anecdotal reports of vertebrate consumption existed across more than 40 years of Bornean orangutan field research.¹⁰ So while the capacity for predation exists, meat is essentially an asterisk in the orangutan diet, not a meaningful food source.

Eating Dirt on Purpose

One of the more unusual items on the orangutan menu is soil. Orangutans at several sites have been observed visiting specific spots to eat earth, a behavior called geophagy. This is not accidental ingestion; they seek out particular soils and ignore others. Analysis of soils consumed by Bornean orangutans found that they preferred earth with a pH near or above 4.0 and higher clay content compared to surrounding control soils. Salt was not a factor, as chloride levels in the consumed soils were below detection limits.¹¹

Why eat clay-rich soil? The leading explanation is detoxification. Many wild fruits and leaves contain tannins or other secondary compounds that can irritate the gut or interfere with digestion. Clay particles bind to these compounds and neutralize them, essentially acting as an antacid. The higher pH of the preferred soils would also help buffer stomach acidity. The consumed soils were higher in aluminum, iron, and potassium, though the iron was mostly in a form that may not be easily absorbed. Geophagy has been documented in many primate species and other animals, and the detoxification hypothesis is the most widely supported explanation across the board.

Tools for Getting at Hard-to-Reach Foods

Orangutans are famous tool users, and some of their most impressive tool use is food-related. The classic example involves the fruit of Neesia trees. These large capsules contain nutritious seeds surrounded by irritating hairs that can embed in skin. At some Sumatran swamp-forest sites, orangutans fashion sticks to pry open the fruits and extract the seeds while avoiding the hairs. Researchers have compared Neesia-processing techniques between Sumatran and Bornean orangutans and found geographic variation in whether and how tools are used.¹² At some Bornean sites, orangutans simply bite the fruits open without tools, despite the irritating hairs. This kind of population-level difference in technique is one of the strongest cases for cultural variation in any non-human animal.

Tool-assisted foraging is not limited to Neesia. Orangutans have been observed using sticks to extract honey from bee nests, to fish insects out of tree holes, and to probe into crevices for seeds. These behaviors tend to be more common and more varied in Sumatran populations, possibly because Sumatran orangutans are more social and have more opportunity for observational learning.

How Young Orangutans Learn What to Eat

Orangutans have the longest childhood of any land mammal. Infants stay with their mothers for seven to nine years, and much of that time is spent learning what to eat and how to process it. A field study of immature Bornean orangutans found that juveniles rarely foraged independently. Instead, they consistently followed their mothers’ food choices, and their diets were essentially identical to their mothers’ even when different mothers ate somewhat different things. This pattern points to vertical transmission: young orangutans learn their diet from their mother, not from the population at large.¹³

The learning process is active on both sides. Young orangutans beg food from their mothers, and by taking items directly from her they gain information about what is edible, how it tastes, and how nutritious it is. This is especially important for foods they cannot yet reach or process themselves.¹⁴ Immatures also watch their mothers closely during complex food processing tasks, and this selective observation leads to more goal-directed practice of those skills.

Research on Sumatran orangutans has added another layer. Immatures solicit food items that require multiple processing steps more frequently than easy-to-eat items, and they reduce these solicitations as they grow older and more skilled. Mothers adjust too: they are more tolerant of food begging when the item is difficult to process and during the developmental window when their offspring is learning that skill.¹⁵ Mothers, in other words, are not just passive models. They play a more active role in teaching feeding skills than researchers once assumed.

How Age and Sex Shape the Diet

Not all orangutans eat the same way. A large-scale study tracking over 2,300 full-day observations of 62 habituated orangutans in a Bornean peat swamp found clear differences in total energy and macronutrient intake across age and sex classes, even after accounting for body size. Travel distance, time spent feeding, and day-journey length also varied among these groups and shifted with fruit availability.¹⁶ Flanged adult males, the largest orangutans, face different energy demands than females caring for dependent offspring, and adolescents forage differently from both.

Females with infants face a particular nutritional challenge. They need enough calories and protein to sustain lactation while also spending time resting and carrying their young. Their feeding strategies tend to be more conservative, sticking close to reliable food sources and investing heavily in fallback foods when fruit declines. Males, especially large flanged males, can afford to range more widely and may access fruit trees that smaller individuals cannot easily reach.

Orangutans as Seed Dispersers

All that fruit eating has consequences for the forest. Orangutans swallow seeds whole, carry them in their gut as they travel through the canopy, and deposit them in their feces far from the parent tree. Research at Gunung Palung National Park in Borneo found that orangutans dispersed seeds from at least 24 plant genera. Seeds that passed through an orangutan’s gut germinated at significantly higher rates than uneaten control seeds for most of the species tested.¹⁷

Gut retention times of around 45 hours resulted in average dispersal distances of roughly 500 meters, with some seeds traveling over 1,300 meters from where they were eaten. At 60 hours of retention, average distances approached 600 meters.¹⁸ These distances matter for forest regeneration. Seed movement drives tree colonization, succession after disturbance, and recovery of degraded forest patches.¹⁹ Losing orangutans from a forest does not just mean losing a charismatic species. It means losing a dispersal engine that helps maintain the forest itself.

What Happens When the Forest Is Degraded

Logging, fire, and conversion to palm oil plantations have transformed vast stretches of orangutan habitat. When the forest is degraded, orangutans do not simply eat less. They eat differently. In one study of orangutans living in a logged and partially converted landscape, wild and cultivated fruits combined still made up the largest portion of feeding time, but bark consumption rose dramatically compared to orangutans in intact forests. Raided cultivated fruits and bark from planted trees contributed about 21% of total feeding time, while wild foods accounted for the remaining 79%.¹

This has practical implications for conservation. Orangutans in degraded forests sometimes raid crops, creating conflict with local farmers. Understanding that these animals turn to cultivated fruits not out of preference but because their natural food base has been hollowed out helps reframe the problem. It also highlights that preserving even small patches of diverse forest within agricultural landscapes can reduce raiding by maintaining wild fruit availability. The orangutan diet, in the end, is not just a catalogue of foods. It is a running record of the state of the forest around them.