Abstract
The sand fractions of weathered regolith (subsoil) sediments from the flanks of Visoke Volcano in the Virunga Mountains, mined, pulverized, and eaten by mountain gorillas, were analyzed by scanning electron microscope (SEM) and energy-despersive spectrometry (EDS) to determine the stimulus of geophagic behavior. The samples show a mix of weathered and volcanic minerals consisting primarily of quartz, apatite, analbite, sanidine, amphibole, ilmenite, and magnetite. The ratio of weathered to fresh grains is approximately 6:1 for the coarse sands (2 mm – 250 µm) and 13:1 for the fine fractions (63 – 250 µm) with the fresh grains very extensively fractured and abraded after grinding and rubbing by gorilla hand. In many cases the core of individual grains was exposed beneath a surface cover extensively etched by chemical weathering and often coated with Fe and Al plus clay minerals. Ground quartz grains, for example, showed surface fractures and abrasion features equivalent to microtextures produced by transport in continental glaciers. Grinding of earth materials produces a high percentage of angular fragments of fine sand size that may or may not play some role in curing intestinal ailments or countering dietary deficiencies. In particular, the grinding of earth materials shows the immense power of the gorilla hand as a pulverizing force making coarse material finer and presumably assisting in the ingesting process. The number of halloysite clay minerals present in the samples might assist in countering the effects of diarrhea.
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Mahaney, W.C. Scanning electron microscopy of earth mined and eaten by mountain gorillas in the Virunga Mountains, Rwanda. Primates 34, 311–319 (1993). https://doi.org/10.1007/BF02382626
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DOI: https://doi.org/10.1007/BF02382626