Chemical analysis of soils of Kowloon (Hong Kong) eaten by hybrid macaques

Geophagy, the deliberate act of eating soil, has been observed in various primate species, although the reasons for this behavior are poorly understood. The present study supports the general hypothesis that the physical and/or chemical nature of the soil is a likely reason for geophagy in hybrid macaque monkeys of Kowloon, Hong Kong. Samples were taken from areas where macaques were seen to be eating soil and from areas where there was no evidence of geophagy. Samples that were refused by the macaques were also obtained. Soils were analyzed for the following chemical and physical properties: soil pH, cation exchange capacity, organically bound and inorganic crystalline and poorly crystalline Fe and Al, organic carbon, acid extractable elements, particle size, and soil mineralogy. The study supports a hypothesis that the monkeys are using visual cues to choose preferentially soils for ingestion. More frequently they chose fine-textured soils that were higher in Fe and Al oxides. We speculate that these properties are sought to aid in digestive processes.