Water-Plant and Soil Consumption by Guereza Monkeys (Colobus guereza): A Relationship with Minerals and Toxins in the Diet?

Abstract

Black-and-white colobus monkeys, Colobus guereza, were frequently observed coming to the ground in a swamp and eating water plants at a site in western Uganda. Normally, guereza intergroup relationships were antagonistic, but many groups shared the swamp pools. Analysis of water-plant samples indicated high levels of sodium, iron, manganese, and zinc compared with other items of the guereza's leaf-dominated diet. There was also evidence of clay consumption by guerezas at a stream bank in the same area. Analysis of the clay showed it to contain considerably more magnesium, iron, and copper than neighboring soils. A comparison of estimated mineral intakes and requirements suggests that the dry-land diet of the guereza population may be deficient in sodium, and perhaps barely sufficient in copper, manganese, and zinc. Water-plant consumption may remedy mineral deficiencies, but clay may be consumed for other reasons: to adsorb plant toxins or to adjust the pH of the forestomach. AN HERBIVOROUS DIET can be deficient in essential minerals. Underwood (1971) lists many instances of deficiency disease in domestic grazing animals caused by low mineral levels in pasture. Wild mammalian herbivores (including ungulates, elephants, and primates) are often observed to, ingest soil, and reports of this behavior frequently propose or imply that its function is the acquisition of minerals. In several cases the sodium content of the soil is suggested to have particular significance (Stockstad et al. 1953; Schaller 1963; Dalke et al. 1965; Weir 1969, 1973; Weeks and Kirkpatrick 1976). Although sodium is one of the major constituents of animal bodies (Pike and Brown 1975), the aboveground parts of land plants rarely contain much of this element (Sutcliffe 1962). Soil consumption is not the only way in which herbivores may remedy mineral deficiencies. Botkin et al. (1973) suggest that water-plant consumption by moose on Isle Royale, Lake Superior, makes up for sodium deficiency in the animals' dry-land diet. The diet of colobine monkeys is composed largely or wholly of plant material, including considerable quantities of tree leaves and leaf buds (Yoshiba 1967, Ripley 1970, Hladik and Hladik 1972, Horwich 1972, Clutton-Brock 1975, Struhsaker and Oates 1975), and there are numerous reports of soil ingestion by colobines (McCann 1934, Pitman 1954, Rahm and Christiaensen 1960, Gee 1961, Poirier 1970, Ripley 1970, Clutton-Brock 1972, Hladik and Hladik 1972). Most of these reports have not been accompanied by any detailed analysis of the soil consumed, or by a discussion of the behavior's significance. Hladik and Gueguen (1974) have, however, analyzed termite-nest clay consumed by Presbytis species in Sri Lanka (and by chimpanzees in Gabon). After comparisons with some plant materials eaten by the animals, Hladik and Gueguen conclude that the clays may have been ingested for the tactile sensation they produce in the mouth or because of a physico-chemical action they perform in the gut, rather than for their mineral content. During field studies at a site in western Uganda, I observed black-and-white colobus monkeys, or guerezas (Colobus guereza Riippell), behaving unusually on the ground. The animals frequently ate plants growing in pools of water, and there was evidence that they consumed clay from small caves. To investigate whether by these means the monkeys might be obtaining minerals deficient in their diet, analyses were performed on samples of the water plants and clay and, for comparison, on samples of dry-land items of diet and soil collected away from the caves. This paper presents details of this gLereza behavior and the results of the plant and soil analyses. The guereza's likely mineral requirements are reviewed, and the extent to which these would be met by the normal diet is assessed. In this light, the water-plant and clay ingestion is discussed.