From the novel results of comprehensive geological–hydrobiogeochemical studies conducted on Olkhon Island (Lake Baikal), it follows that the phenomenon of geophagy among wild and domestic ungulates on the island is caused by the deficiency of light subgroup rare earth elements (LREE) in plant food and natural waters, while not denying the possible role of other factors. This deficiency reflects the low LREE specificity of the most abundant metamorphic terrigenous carbonate rocks on the island, as well as their overlying eluvium–deluvium and soils. The resulting specificity of the landscapes may be the cause of the LREE imbalance in the nervous and hormonal systems of the body, which makes the animals regularly and periodically consume clay (kaolinite) products from weathering, enriched with the necessary nutritional elements. The identified cause of geophagy on Olkhon Island is likely to extend to a significant part of geophagy cases among animals, including humans, especially in the equatorial zone of the Earth.
