Crab-eating macaque 27% animal matter, 15% flowers and other plant matter, and 14% food provided by humans. This species exhibits particularly low tolerance for swallowing seeds. Despite their inability to digest seeds, many primates of similar size swallow large seeds, up to , and simply defecate them whole. The crab-eating macaque, though, spits seeds out if they are larger than . This decision to spit seeds is thought to be adaptive; it avoids filling the monkey’s stomach with wasteful bulky seeds that cannot be used for energy. It also can help the plants by distributing seeds to new areas: Crab-eating macaques eat
Crab-eating macaque durians such as "Durio graveolens" and "D. zibethinus", and are a major seed disperor for the latter species. Although the species is ecologically well-adapted and poses no threat to population stability of prey species in its native range, in areas where the crab-eating macaque is not native, it can pose a substantial threat to biodiversity. Some believe the crab-eating macaque is responsible for the extinction of forest birds by threatening critical breeding areas as well as eating the eggs and chicks of endangered forest birds. The crab-eating macaque can become a synanthrope, living off human resources. They are known to