Nearly fifty years ago, a few brave primatologists working in Gombe, Tanzania, were forming a radical idea. They had seen the local chimpanzees behaving in such a strange way that they were wondering whether the apes were self-medicating. This would be an outrageous claim; one which could make them a laughing stock among their peers if they weren’t careful. They had to make sure they knew exactly what was going on before publishing their observations.

Primatologist Richard Wrangham, then a student working with Jane Goodall, noticed the chimps were getting up early and searching far and wide for particular leaves. As they climbed from tree to tree, they would gently take a leaf between their lips while still attached, place it on their tongue for a few seconds and make some kind of assessment to decide whether to pick it. Most leaves were rejected and left where they hung but those leaves which passed the tongue-test were picked and carefully folded, concertina fashion, first in one direction, then the other—a complicated procedure! The folded leaf was then carefully swallowed whole without chewing and individuals might swallow anywhere from one to 56 leaves in one bout. It was all very odd.

In the US, at the time, there was a trend to explore potential new pharmaceutical drugs by testing all the plants which our close relatives ate in the wild. One laboratory set off at full steam to isolate the active chemical constituent of the chimps favourite, Aspilia leaves [see photo]. People made big financial investments in the hope of a new wonder cure for… something—anything but although several interesting and important new compounds were discovered, none seemed to have any medicinal value.

Back in Gombe, field workers had continued to study this leaf-swallowing behaviour. They now realised that chimps were ‘swallowing’ leaves from many different species of plants, containing a wide range of chemicals with no obvious common denominator. Crucially, though, field workers had noticed the leaves still folded on the forest floor in the same state as they went in, apparently having passed through the gut undigested. Richard Wrangham did what all excellent scientists do and put some in his mouth. They were disgusting—akin to swallowing folded up sandpaper! Why would chimpanzees do this to themselves?

Years later, Michael Huffman of Kyoto university, stumbled across a lucky find—fresh chimp poo! The steaming deposit was packed with warm folded leaves. Inside, were nodule worms alive and wriggling. Excitedly, Huffman reassessed all the leaf species swallowed by chimps and realised they were ALL bristly, hairy, or otherwise rough in texture. This was the common denominator. Under the microscope they had tiny barbs, and combined with expert folding, they were perfect for scraping out a bunch of wriggly writhing gut parasites.

Leaf swallowing behaviour occurs more commonly after the beginning of the rainy season when worm infestation is greatest, and it was quickly established that worm infestation was present when the chimps were leaf swallowing. Huffman’s concluded that chimps use these rough hairy leaves act as a scour, scraping through the gut any loose nodule or tapeworms.

Within a few years it was established that chimpanzees, bonobos, and gorillas throughout Africa swallow at least 34 different species of bristly hairy leaves in this way and that they seem to do it in response to the discomfort of parasite infestation.

Since this discovery, scientists have looked again at other animal species which regularly swallow indigestible materials.

At certain times in an animal’s life, such as migration and hibernation, parasites are more of a health hazard than usual. If animals go into hibernation with a gut full of parasites, the parasites will eat the animal’s carefully stored food reserves while it sleeps. When food is going to be in short supply, you do not want to share it with freeloaders.

Biologist, Barrie Gilbert studied wild Alaskan brown bears for over six years. He noticed that the bears changed their diet a hew months leading up to hibernation to eat highly fibrous sharp-edged sedge and, at the same time, he found large dung masses full of long tapeworms. Canada snow geese similarly clear themselves out before long migrations by eating indigestible grasses. At this time of year, you can find large boluses of undigested grass full of tapeworms.

*Photo Right by Michael Huffman, chimpanzee tongue-testing a leaf to see if it is suitable for leaf swallowing. Photo Left, my dog eating grass.*

Ancestors of domesticated cats and dogs do this a lot. Wolves and wild tigers occasionally eat grass, and grassy scats have been found full of worms. Although theories abound as to why dogs and cats eat grass, gastrointestinal malaise (tummy ache) seems to be key. It does not matter if grass acts as an emetic (inducing vomiting) or as a purgative scour; both clear a problem out the gut and either end will do. It is likely that grass-eating by domestic dogs and cats is a remnant strategy--still an urge but not a fully formed effective strategy such as would be found in the context in which it evolved.

There is a common assumption that all this consumption of indigestible materials is a form of pica—abnormal eating behaviour but studies of wild animals suggest it is, in fact, a normal attempt at self-medication. Of course, in captivity or in the process of domestication, the animal may not have access to appropriate indigestible materials (your old socks may be all they have available) and they may also have other health problems too. I should point out that eating indigestible materials is not the same as earth-eating; that is another form of self-medication altogether. What we are talking about, here, is the consumption of stuff that won’t break down; that will pass through the gut like a mechanical scour.

Physical scours are safer than chemical anthelmintics (which, by necessity, must be toxic enough to harm parasites) which can make your pet feel pretty sick. But perhaps more importantly, these mechanical scours do not trigger the development of chemical resistance, as happens with our veterinary anti-worming medications. If you have animals in your care, it is worth bearing in mind that sometimes animal self-medication is more mechanics than pharmacy.

‘Wild Health: how animals keep themselves well and what we might learn from them,’ now re-released in paperback, £15 via Amazon.

Fancy a Bit of Rough? Some animal self-medication is just about having a good scrape through!