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1 Steffan Robertson and Kirsten (UK) Ltd, Cardiff CF1 3BX, UK
2 Department of Zoology, The Natural History Museum, London SW7 5BD, UK
3 The African Ele-fund, PO Box 308, Bristol BS99 7LQ, UK
Herbivores such as the African elephant receive most of their nutrient uptake through digested vegetation or water. When these nutrient sources do not fulfil dietary requirements, eating and digestion of soil and rock can be a common phenomenon. In the Mount Elgon National Park on the Kenya-Uganda border, elephants have taken this activity one step further. Deposits of calcium-sodium-rich alkaline rocks show evidence of quarrying by elephants on the surface, but most of the activity takes place underground in caves. The Na-Ca-Mg-rich rocks are leached by groundwater which reacts with animal excreta in the humid environment of the cave floor to form a series of secondary carbonate, sulphate, halide, nitrate and phosphate minerals by evaporation. Additionally some salts are precipitated on the cave walls by direct evaporation of the cave waters. Inside the caves, elephants tusk and ingest the salt-enriched rock fragments. In an area like Mount Elgon, where supergene processes leach chemical elements essential for dietary requirements from the surface ecosystem, the secondary salts in the caves are potentially an important mineral supplement for wildlife nutrition. Although elephants are the principal exploiters, other wildlife species and humans (for livestock) also utilize the cave salts and surface diggings.
