Leaves of two plant species eaten by Macaca fascicularis in Bukit Timah Nature Reserve, Singapore, were collected and colour-tested. Leaves matching those eaten by M. fascicularis were examined by energy-dispersive X-ray micro-analysis. The leaves of Streblus elongatus (Moraceae) and Gluta wallichii (Anacardiaceae), together forming 19.6% of the leaf diet of the macaques, contained silica. In G. wallichii, this in the base of hairs that project from the underside of the leaf, whereas S. elongatus leaves have short sharp siliceous trichomes which are densely packed on the undersurface of leaf veins. We predict from an indentation analysis that chewing on the latter species could cause dental microwear at low occlusal forces. The leaves are reportedly common in the diet of three other primate species in peninsular Malaysia and the finding could have general significance for studies of dental wear.
The seeds in fruits consumed by primates may be chewed and digested, swallowed and defecated intact, or separated from the flesh and spat out. We show by a combination of close field observations and experiments with caged animals, that long-tailed macaques (Macaca fascicularis) have a remarkably low threshold of 3-4 mm for swallowing seeds and also that wild macaques rarely break them. The seeds of 69% of the ripe fruit species eaten are spat out intact or cleaned outside the mouth and dropped. Seed-spitting significantly reduces the swallowed food bulk and may lessen the risk of releasing seed toxins during mastication. However, it requires that even small fruits are processed in the mouth one or a few at a time. We suggest that fruit storage in the cheek pouches of cercopithecine monkeys allows them to spit seeds individually without excessively slowing fruit intake while feeding on patchily distributed fruit. In contrast, Apes and New World monkeys apparently swallow and defecate most ripe seeds in their diet and colobine monkeys break and digest them, detoxifying seed defenses by bacterial fermentation.