Siamangs eat primarily fruit and leaves in roughly equal proportions, spending about 44% of their feeding time on fruit and 45% on leaves. The remaining time goes to flowers, insects, and occasional animal prey. This balance between fruit and foliage makes siamangs unusual among gibbons, which generally lean much more heavily toward fruit.
Fruit and Figs Form the Core Diet
Figs are the single most important food in a siamang’s diet. Various species of wild figs top the list of consumed plants across study sites in Sumatra and the Malay Peninsula. After figs, a tropical fruit tree called Dracontomelon dao ranks as the second most commonly eaten plant. Siamangs pick ripe and unripe fruits while moving through the canopy, often covering their entire home range over the course of a few days to revisit productive trees.
An adult siamang eats roughly 1.5 kilograms (about 3.3 pounds) of food per day. That’s a substantial amount for an animal that averages around 11 to 13 kilograms in body weight, meaning they consume about 13% of their body mass in food daily. Most of that bulk comes from water-rich fruits and tender young leaves.
Leaves Provide What Fruit Cannot
Unlike most other gibbons, siamangs include a large proportion of leafy matter in their diet. They strongly prefer immature leaves, which are softer, easier to digest, and higher in protein than mature foliage. Young leaves from climbing plants called lianas are especially important because lianas produce new growth more reliably and abundantly than trees do. This gives siamangs a steady protein source even when tree canopies aren’t flushing new leaves.
This heavy reliance on leaves is thought to be one reason siamangs can coexist with other gibbon species in the same forest. Where siamangs and lar gibbons share territory, the lar gibbons eat more fruit while siamangs fall back on foliage, reducing direct competition for the same food.
Flowers as a Fallback Food
Flowers play a bigger role in the siamang diet than casual descriptions suggest. At a research site in southern Sumatra, siamangs spent 12% of their total feeding time eating flowers over a two-year study period. In some months, flower consumption spiked above 40% of feeding time.
This spike wasn’t random. Siamangs ate the most flowers during months when non-fig fruit was scarce, and they simultaneously reduced their overall energy expenditure by resting more and traveling less. This pattern suggests flowers serve as a fallback food: not the preferred choice, but a reliable alternative when fruit runs low. One plant in particular, Hydnocarpus gracilis, was the third most commonly consumed species at the study site, and siamangs ate only its flowers, never its fruit or leaves. These flowers were available in most months, making them a dependable safety net.
Insects and Animal Prey
Siamangs are technically omnivores. Beyond plant foods, they eat insects, spiders, bird eggs, and occasionally small vertebrates like lizards or nestlings. These animal foods make up a small fraction of the overall diet, but they provide concentrated protein and fat that plant foods alone may not supply in sufficient quantities. Foraging for insects typically happens opportunistically as siamangs move through the canopy rather than as dedicated hunting trips.
How Siamangs Get Water
Siamangs live high in the forest canopy and rarely come to the ground, so they don’t drink from streams or rivers the way many animals do. Instead, they get most of their hydration from the water content in fruits and leaves. When they need additional water, siamangs drink from tree holes, small natural cavities in branches and trunks where rainwater collects. This behavior has been documented in several primate species and may help siamangs avoid predators on the forest floor or cope with seasonal dry spells when fruit is less abundant and juicy.
How the Diet Changes With the Seasons
Tropical rainforests don’t have the dramatic seasons of temperate climates, but fruit availability still fluctuates month to month. Siamangs handle these fluctuations by shifting their diet composition. When fruit is plentiful, they eat as much as they can. When fruit drops off, they increase their intake of young leaves and flowers. The availability of figs helps buffer against the worst shortages because different fig species fruit at different times of year, providing at least some ripe fruit in most months.
These dietary shifts come with behavioral changes too. During low-fruit periods, siamangs tend to travel shorter distances and spend more time resting, conserving energy to match the lower caloric value of their fallback foods.
What Young Siamangs Eat
Infant siamangs rely entirely on their mother’s milk for the first several months of life. Most wild infants don’t start eating solid food until around 6 months of age, though some begin handling food items earlier out of curiosity. Between 3 and 9 months, infants begin tasting plant foods, and by about 11 months, nursing drops off sharply. Weaning is generally complete by the end of the first year, after which young siamangs eat the same foods as adults, learning what to pick and where to find it by following their family group through the canopy.
During the second year of life, nursing contributes very little to nutrition even in infants that still occasionally suckle. By that point, the young siamang is functionally independent in terms of diet, though it will stay with its family group for several more years before dispersing.