Wild chimpanzees of the Sonso community in the Budongo forest, Uganda, are increasingly eating clay to supplement the minerals in their diet, says a team of scientists led by Prof Vernon Reynolds of Oxford University, UK.
Adult female chimp Nambi using a leaf-sponge at waterhole, watched by 10-year-old juvenile Karo. Image credit: C. Hobaiter.
Prof Reynolds and his colleagues from the UK, the Netherlands, and Switzerland, observed the eastern chimpanzees (Pan troglodytes schweinfurthii) in the Budongo forest eating and drinking from clay pits and termite mounds.
The scientists observed the chimps using leaves like clay ‘sponges,’ dipping the leaves in the clay water and returning to their mouths where they squeezed the liquid out with their tongues. The chimps were also seen using their fingers to extract lumps of clay directly from the ground before eating it.
The team suggests this change in diet may be partly due to the widespread destruction of raffia palm trees (Raphia farinifera) that chimps relied on for their minerals in the past.
An analysis of the clay and termite soils shows they are very high in a range of minerals, but the clay was particularly high in aluminum – a feature of kaolinite clays eaten by a range of species, including humans, to aid digestion and detoxification.
The chimps’ diet, which consists mainly of fruits and leaves, is very high in tannins and the team believes that the clay provides an important way for chimpanzees to neutralize these.
It is common practice for local women in Budongo to drink or eat forest clay mixed with water for stomach problems and during pregnancy.
“Clays, such as kaolinite, also contain sodium, calcium, iron, magnesium and potassium, which the Budongo chimps seem to have discovered they can access with their leaf sponges,” the scientists explained.
Accessing the clay with the leaf sponges was also found to provide higher mineral concentrations than taking clay-water or clay directly from the ground.
“Before 2000 feeding on raffia palms was commonly observed among the Sonso chimps, but after 2005 it started to decline.”
“Meanwhile, after 2005, clay soil feeding in Budongo seems to have increased, possibly because of the scarcity of raffia-palm trees which are now used in the local tobacco industry with the leaf stems being used for tying and curing tobacco leaves. The decaying pith of these palm trees was previously an important source of minerals for the chimpanzees.”
“Raffia is a key source of sodium, but to our surprise the sodium content was very low in the diet so this does not appear to be the main reason for the new clay-bingeing,” said Prof Reynolds, led author on the study published in the journal PLoS ONE.
“Instead the wide range of minerals present in their diet suggests that clay is eaten as a general mineral supplement.”
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Reynolds V. et al. 2015. Mineral Acquisition from Clay by Budongo Forest Chimpanzees. PLoS ONE 10 (7): e0134075; doi: 10.1371/journal.pone.0134075
