Scientists at work: studying the clay-eating chimps of Budongo

I was walking in Budongo forest one day, years ago, when my Ugandan friend Geresomu pointed out a patch of grey soil on the forest floor. “What’s that?” I asked. “The chimps sometimes eat it,” he replied. I promptly forgot about it – it was of no interest at that time. Now, it’s the central focus of my research. So what’s happened in the meantime?

Back in the late 1980s, baby chimps from the Budongo Forest were being taken by poachers to Entebbe airport and illegally smuggled to Dubai and other places. I decided to set up a field station to protect the chimps, and spent years looking for funds to do so. In 1990, I founded the Budongo Conservation Field Station, and we hired a team of assistants to help us find and track the chimps. Geresomu was one of our first field assistants, and he’s still with us today. He knows more about chimps than any Westerner.

The holes in the raffia palms

In the early 90s, Geresomu showed me a raffia palm tree, growing in a swampy area of the forest, which had a carefully made hole in the trunk. He told me that the hole had been made by chimps. “They put in their hands and pull out the dead wood and chew it – then they spit out the wood, but they swallow the juice,” he said. “Why would they do that?” I asked. No one knew.

I lived with that question for years, then one day I read a paper about gorillas, which said that they ate dead wood for its sodium content. I live near Brighton, and the Dean of Science at Brighton University – Andrew Lloyd – runs a lab that measures mineral elements. He offered to analyse the raffia wood. On my next visit to Budongo, I collected some samples and sent them to him. They turned out to be rich in sodium.

Nom.Anne Schel, Author provided

Sodium is an essential mineral for all mammals, and many find it easily in their diet. But chimps mainly eat fruit and leaves, which contain little or no sodium. So, they need to supplement their diets. Besides the sodium, there was a cocktail of other minerals present in the raffia samples: iron, manganese, magnesium, aluminium, and others. It seems the dead raffia palms suck water from the swamp forest, which then evaporates, leaving the minerals behind in the decaying pith. We published this discovery in PLoS ONE, in 2009.

The threat from tobacco

Just at the time we published our findings, we found to our dismay that the raffia trees were disappearing fast. Their leaves were being cut down and used by local tobacco farmers to make raffia string, which they used to hang up their leaves in the smoking sheds. The tobacco trade was booming, and soon the last raffia trees were gone. The dead ones that remained were used so much by chimps that soon, all the pith was taken out and there was nothing left.

But the chimps turned out to be ingenious. That low level of clay eating Geresomu had told me about so many years ago suddenly became more important. At any one time we have a number of research students at Budongo doing PhDs and Masters degrees. One by one, they reported finding chimps feeding on clay, or using leaf sponges to remove clay-rich water from the waterholes under the trees. They would congregate around these trees, pull the leaves off surrounding undergrowth and chew them a little. Then, they would use the leaves as sponges to soak up the clay-water, chew and suck the leaves in their mouths, and dip them in the water again. This was seen and filmed by Cat Hobaiter, Brittany Fallon and Caroline Mullins of St Andrews University, Anne-Marijke Schel of Utrecht University, and Noemie Lamon of Neuchatel University.

Brittany Fallon records chimps eating clay - delicious and nutritious.

I was in the UK and the students wrote me emails describing this new behaviour, and collecting information about it and photographing and videoing it. The field assistants also recorded it when they came across it in the course of their normal work, so their records went into the archives too. Naturally, I could not wait to get out to Budongo myself and see the clay pits, the waterholes, and the clay-eating and drinking behaviour itself. Meanwhile, all I could do was speculate that this might be a new way for the chimps to get sodium and all the other minerals, which they used to source from the raffia palms.

Mineral supplements and detox

In due course, I visited Budongo and came back armed with a set of samples of clay and clay-water, as well as some control samples of forest soil and rainwater for comparison. We ran them in Andrew Lloyd’s Brighton laboratory. They were mineral rich, yes, but disappointingly low in sodium. We discovered that the grey clay was kaolinite – a particularly rich source of aluminium, which people also use to treat upset stomachs. The samples also contained high quantities of iron and all the other minerals that we had found in the raffia pith.

So we concluded that the clay was being eaten by our chimps as a mineral supplement, as well as for its digestive properties. And interestingly, when the clay-water was sponged up using leaves, it contained more minerals than when it was taken without the leaves. So the leaves and the clay were interacting with each other.

Just how the chimps knew to use the clay as a mineral supplement is still a mystery. Probably, it’s a habit passed from adult to juvenile: we see the young ones watching as the adults do it, and then doing it themselves. Or perhaps it may be that, back in evolutionary time, the chimps who took the supplements lived longer, healthier lives than those that didn’t, and so the habit got passed on, either through learning or as an inherited tendency. Such questions are hard to answer. But sometimes even the smallest clue – like that patch of grey soil Geresomu showed me all those years ago – can hold the key to a fascinating story.

Vernon Reynolds receives funding from the Mohamed bin Zayed Species Conservation Fund.