Moving a gibbon directly from a typical captive diet to a wild forest diet can cause digestive issues, weight loss, and malnutrition. A gibbon’s gut microbiome and digestive system need time to adjust to processing high-fiber wild fruits and foliage, which include complex carbohydrates, and secondary plant compounds. Months before the planned release, we need to begin slowly replacing cultivated foods with endemic forest foods, ideally gathered from the target release site or a similar habitat. This includes wild fruits, young leaves, shoots, flowers, and wild figs (Ficus spp.), which function as a critical fallback food for most gibbon species. By the time the gibbons are moved to the forest, their daily intake should consist almost entirely of the natural flora they will encounter in the wild, ensuring their digestive tracts are prepared, they visually recognise these wild items as food and can process them efficiently.
When preparing an animal for release, we also need to consider that increased physical activity will have an increased energy requirement. The diet may need to be adjusted for that, though this adjustment should carefully balance providing enough energy without risking significant weight gain. Free-ranging wild animals nearly always have more muscle mass and less body fat than captive counterparts. A gibbon which may be considered to be in good body condition in captivity will likely gain more muscle as they become increasingly active in the wild. In additional gibbons have a naturally very athletic body, which should be our goal by the time of the release.
During this preparation phase, we also need to also ensure that gibbons are physically capable of acquiring water naturally. Many captive gibbons are used to drinking from bowls or bottles, and need time to adjust to more natural water sources. By the time the gibbons are moved to the forest, their habits should also mirror the natural conditions they will encounter in the wild.