Abstract
Geophagy (soil-eating) is one of the well-known behaviours in many primate species, but the factors influencing this behaviour have been less known. In the captive environment of Tama Zoological Park, 2 female Borneo orangutans (Pongo pygmaeus) showed geophagic behaviour that was restricted to a particular site in the naturalistic outdoor enclosure. We compared the properties of the soil at this site with those of soils from 7 other different sites in the enclosure to determine the differences between the soils. To this end, we examined the landform, vegetation type, the physical and chemical characteristics of the soils at these sites. The enclosure was situated on the hillside of secondary woodland comprising Fagaceae sp. with a gently sloping ridge on the east side and valley bottoms on the west side. The site at which the animals exhibited geophagic behaviour was located at the lowest area of the valley bottoms. We found that this area was thinly covered by a herbaceous layer with Gramineae sp., and most of ground surface was bare. The soil eaten by orangutans had a low density and was highly friable, soft, and wet. Chemical analysis revealed that the soil in the enclosure had a high Ca content (70-80%) and that soils at some points in the enclosure, including the soil at the site of geophagic behaviour, had high Fe and Mg contents. The site of geophagic behaviour was located at the bottom of the valley; therefore, soil ingredients may have accumulated easily in this soil. However, we could not find any definitive chemical factors to explain the geophagic behaviour of orangutans. One possible explanation is that since the site was bare with highly friable, soft, and wet soil, the orangutans would have been able to easily eat the soil from that site.
