Chimpanzees in the forests of Uganda are boosting the minerals in their diet by eating clay, according to a recent study.

Researchers at the University of Oxford observed the chimps using leaves like clay "sponges," dipping the leaves in the clay water and putting them in their the liquid out with their tongues. They were also seen using their fingers to extract lumps of clay directly from the ground before eating it.

Researchers believe this change in diet may be partly due to the widespread destruction of raffia palm trees that chimps relied on for their minerals in the past. However, the main reason seems to be the chimps have recently started to boost the minerals in their diet by eating the clay which also helps them "detox" and digest their food.

"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. Instead the wide range of minerals present in their diet suggests that clay is eaten as a general mineral supplement," Vernon Reynolds, who led the study, said in a statement.

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. 

For the study, researchers analyzed the clay and termite soils. They found that both 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 researchers believe that the clay provides an important way for chimpanzees to neutralize these.

It is common practice for local women in Uganda's Budongo forest 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.

The findings are detailed in the journal PLOS One.