Published online 10 January 2008 | Nature | doi:10.1038/news.2008.427

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Why do chimps eat dirt?

Apes might eat soil to activate anti-malarial plants.

Dirty-mouthed chimps might munch soil for their health.Jean-Michel Krief

Chimpanzees in Uganda have been spotted eating dirt along with fistfuls of leaves. This might help to increase the plants' anti-malarial properties, say researchers.

Many animals, including humans, are known to deliberately eat soil, a practice called geophagy. Though the animals and people might not be aware of it, the main reason for this is that munching on dirt can have health benefits. Soil contains scarce minerals, such as iron, and can counter diarrhea, absorb toxins, and facilitate digestion. Eating earth can also reduce hunger pangs during famine.

Now, it seems that soil might also boost the pharmaceutical properties of foods.

Strange recipe

Sabrina Krief, a veterinarian at the National Museum of Natural History in Paris, noticed that chimpanzees in Kibale National Park often ate soil shortly before or after eating the leaves of Trichilia rubescens.

After finding that the leaves contained novel anti-malarial compounds, the researchers suggested that the apes were self-medicating1.

The team collected 14 soil samples similar to those eaten by chimpanzees, along with T. rubescens leaves from the same area. They then replicated chewing and digestion using a pestle and mortar along with acid and heat treatments.

They tested the soil, leaves, and soil-leaf combination against drug-resistant strains of the malarial parasite Plasmodium falciparum. Only the soil-leaf mix had significant anti-malarial activity, they report in Naturwissenschaften2.

Active ingredient

The team also looked at the samples using high-performance liquid chromatography (HPLC). This showed that mixing the leaves with clay reduced the amount of biologically available leaf compounds, presumably because they were bonded to soil particles.

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Krief thinks that the anti-malarial compounds stick to the clay less than other chemicals in the leaf, and so become concentrated in the soil-leaf mixture. But her team didn’t isolate the anti-malarial compounds in their test, so they cannot confirm this.

Pharmacist Michael Wink of the University of Heidelberg in Germany suggests another mechanism. It could be that reacting with the soil activates the anti-malarial compounds, he says: “For me, that could be more plausible.”

Further work is needed, he says, to identify and quantify the compounds at work, to determine what’s going on. 

  • References

    1. Krief, S., Martin, M-T., Grellier, P., Kasenene, J. & Sévenet, T. Antimicrob. Agents Chemother. 48, 3196-3199 (2004) | Article | PubMed | ChemPort |
    2. Krief, S., Klein, N. & Fröhlich, F. Naturwissenschaften DOI 10.1007/s00114-007-0333-0 (2008)
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