Soil-Eating by Red Leaf Monkeys (Presbytis rubicunda) in Sabah, Northern Borneo

Abstract

A group of Presbytis rubicunda (red leaf) monkeys was observed for 13 months in the lowland dipterocarp forest at Sepilok, Sabah, northern Borneo. The main items in the animals' annual diet were young leaves, seeds, whole fruits, and flowers. Large, dry seeds made up over 85 percent of the diet in 2 months, but young leaves, fruits, and flowers were generally eaten when large seeds were unavailable. Soil eating was observed on nine occasions. The soil was always collected by breaking lumps off termitaria and never from the forest floor. Analytical comparison of termite-mound and forest-floor soils showed that both had similar clay contents, but the termitaria soil was of higher pH and had higher levels of the main cationic nutrients and lower levels of labile aluminium. The geophagy was observed at times when different items predominated in the diet. It was noted when the dietary items were highly digestible and contained few tannins, indicating that geophagy need not be primarily for tannin absorption, although it might have helped to absorb toxins. The main benefits are more likely to be the alleviation of digestive disorders, such as forestomach acidosis, and the supplementation of mineral nutrient uptake in a generally oligotrophic environment. There is no reason why geophagy should have a single function; it may serve different functions at different times. SOIL-EATING HAS BEEN RECORDED in many groups of mammalian herbivores, including colobine monkeys, such as Colobus species in Africa (Clutton-Brock 1972, Oates 1978, Marsh 1978) and Presbytis species in South Asia (McCann 1934, Poirier 1970, Ripley 1970, Hladik & Hladik 1972) and South-east Asia (Bennett 1983, Davies 1984). A particular aspect that has been noted for the colobine subfamily is the selection of soil material from termite mounds rather than general topsoil (e.g., Hladik & Hladik 1972, Marsh 1978, Davies 1984). In a study of the mineral content of termite-mound soils eaten by Presbytis senex and P. entellus, Hladik and Guegen (1974) concluded that the clay-rich soil from the termite mounds was probably eaten for its physio-chemical effects rather than to offset mineral deficiencies in the diet. In a discussion of why C. guereza ate clay-rich soil from a stream bank at Kibale, Uganda Oates (1978) noted the the high copper content of the soil might be significant, but suggested that the clay could have been ingested to absorb toxins in the food or to buffer the pH of the forestomach. In this paper observations on the diet of a group of red leaf monkeys (Presbytis rubicunda) from Sabah, northern Borneo, are summarised and the composition of termite mounds is compared with that of surrounding top-soil in an effort to determine possible reasons why termite-mound soils are selected. STUDY ANIMAL AND STUDY SITE Presbytis rubicunda is endemic to Borneo and the adjacent Karimata Islands (Medway 1970) and lives at population densities of about two groups/sq km (Payne & Davies 1981). Animals tend to live in groups of 3-12 individuals and are almost wholly arboreal and herbivorous (Davies 1984, Supriatna et al. 1986). The group described in this paper occupied a home range of about 70 ha on the northern boundary of the Sepilok Virgin Jungle Reserve