The Sumatran orangutan (Pongo abelii) is a critically endangered great ape found exclusively in the tropical rainforests of northern Sumatra, Indonesia. As one of the world’s largest arboreal mammals, its survival is intrinsically linked to the health of its forest habitat and the diversity of its food sources. Their diet is highly specialized, reflecting a deep adaptation to the unpredictable fruiting cycles of the rainforest ecosystem. Understanding their diet provides a direct look into their complex relationship with the environment and the ecological role they fulfill.
The Primary Role of Fruit
Sumatran orangutans are primarily frugivores; fruit constitutes the largest portion of their caloric intake when available in abundance. During high-fruiting periods, their diet can consist of up to 90% fruit, a greater reliance than their Bornean cousins exhibit. They favor soft, sugary fruits that provide high energy, such as wild durians, lychees, jackfruit, and breadfruit.
A particularly important food source is the fig, derived from various Ficus species, which fruits more reliably than many other tropical tree species. Figs act as a dietary staple that supports the orangutan population even when other preferred fruits are scarce. They typically ingest the entire fruit and then pass the seeds intact in their feces.
This seed dispersal process has earned them the name “gardeners of the forest.” They help regenerate the rainforest by moving seeds far from the parent plant, ensuring that tree species with larger seeds are distributed across the landscape. The health and diversity of the Sumatran rainforest are directly dependent on the orangutans’ feeding habits.
Supplementary and Seasonal Foods
When the high-calorie fruit season ends, Sumatran orangutans shift to a “fallback diet” composed of less nutritious, more fibrous items. This seasonal adaptation sustains them through unpredictable periods of fruit scarcity. The largest component of this fallback diet is young leaves and shoots, which are easier to digest than older, tougher foliage.
They also consume tree bark, scraping off the inner cambium layer, which is rich in minerals and starches. Studies have noted a preference for the bark of trees like Streblus elongatus or Acacia mangium during lean times. Other plant matter, such as flowers, nectar, and the inner shoots of certain vines, supplements their diet when fruit is unavailable.
Beyond plant matter, the diet includes animal-based items, particularly invertebrates like ants, termites, and crickets, which supply necessary protein and fats. They use sticks to extract termites from holes, demonstrating ingenuity in accessing these resources. On rare occasions, they consume bird eggs and small vertebrates, such as slow lorises, especially when fruit availability is lowest.
Foraging Strategies and Water Consumption
Sumatran orangutans spend nearly all their time foraging and feeding high in the tree canopy. They dedicate up to 60% of their day to searching for and consuming food. Their large size requires a foraging strategy that maximizes energy intake while minimizing travel, often leading them to spend extended periods in a single large fruiting tree.
Intelligence is evident in their selective eating, as they identify ripe fruit and avoid items with defensive toxins. They demonstrate sophisticated techniques for consuming challenging foods. For example, they use simple tools like sticks to open the difficult, hair-covered Neesia fruit or to probe for insects.
Hydration needs are met without descending to open water sources on the forest floor, which can be risky. Orangutans obtain most of their water from the high moisture content of the fruits and leaves they eat. They also drink water that collects in tree hollows, leaf axils, or from dew gathered on leaves. During heavy rainfall, they have been observed drinking water that runs down their own arms.