Wednesday, 28th July. Semliki Time: 03:22pm

The reason I’m in Semliki is because I’m working on a project for my third year dissertation and a couple of you have asked to hear more about the project itself. I’m studying the ‘bark eating’ habits of the local Chimpanzee population, a project which is rather more interesting than it sounds. Bark eating is almost always completely disregarded in the literature and, in the few articles discussing it, the conclusion has been “it’s obviously a fallback food”. Interestingly, at Semiliki, while I’ve yet to examine the seasonality of bark eating, it seems that chimps eat bark both when food is scarce and, peculiarly, when food is plentiful. I’ve therefore constructed five hypotheses which I’m going to try and systematically disprove.

Number One – Ne’er a drop to drink.

Water, at Semliki, is less available that at other sites. The rainfall is significantly less than at Mahale, Budongo, Kibale and the like, and chimpanzees are forced, during the dry season, to dig wells in order to quench their thirst. Incidentally, they sometimes dig wells during the rainy season as well, but that’s Alex’s project. Why then, would chimpanzees turn to chewing on trees? Maybe they want to extract extra water from the bark to make up for the water which is lacking in other areas of their life.

This is supported by the fact that they chew the phloem and the cambium, the parts of the tree where the most liquid is found. Sadly, this hypothesis is disproved by almost anything else. While I’ve been here, steams have been flowing freely, though admittedly they’re a rather long way down. The one wad of freshly chewed bark I’ve managed to collect (my numerous other samples are all dry), however, had not been sucked dry and, conversely, actually contained more liquid than you’d expect a non-chewed piece of tree to, due to excess saliva. Last but not least, the other foods that the chimps have been eating while I’ve been here, such as ‘Saba’ fruit, look to have a far higher water content than tree bark. Of the two times I’ve observed bark-eating ‘in the flesh’, as it were, they have been between bouts of feeding on juicier food all together. Okay, one down, what’s next.

Number Two – Mr Masticator

Chimps in other sites have been seen to chew up large wads of leaves and/or pith at the same time as other foods such as meat or eggs. Nishida (ninteen-seventy-something…) proposed that this was either to retain the taste of the second food for longer, or to help ‘digest’ the food even further before its swallowed. Are you thinking what I’m thinking B1? I think I am B2!

Could it be that the chimps are chewing up bark along with some other food source in order to grind it up even more? Or maybe to extract and preserve the toothsome flavours? Well, the first collection of wads I found supported this hypothesis quite well. Almost all of them contained pieces of acacia leaf that had been mashed up. Furthermore, one of them contained two pieces of snail shell which had obviously been heavily ‘mashed up’ by the bark. Sadly, all further pieces of bark, except for one or two containing a very few leaves, have been free from detritus of any kind. On top of that, of the two times I’ve observed the chimps chewing bark, one of them was not directly preceded or followed by any other form of food eating and, the second, while combined with leaves, was ejected too quickly to have been used to mash up food or conserve flavour. In fact, the second incident of bark eating looked more like the bark and the leaves were being used more for the purposes of cleaning Saba fruit off teeth, which brings us on to…

Number Three – The Dental Hygiene Hypothesis.

Chimpanzees, if they could, would live almost solely on fruit. They eat leaves and pith during seasons when fruit is scarce, and they eat meat when they can get it (which is rarely), but the vast majority of chimp foods are fruit. Fruits are filled with two things – natural acids and copious sugar – both of which aren’t great for the teeth. But chimps can’t buy Colgate, so how do they get rid of all that plaque. Maybe the bark, which they chew into wads, is actually not being eaten, but is being used as a makeshift toothbrush.

Indeed the first piece of behavioural evidence I recorded seems to directly support this hypothesis. Fuller, a young adult male, after eating Saba fruit for several hours, leapt over to a cola tree, tore off strips of bark and shoved them into his mouth. The bark was then moved to the front of the mouth and seemingly rubbed around the front teeth. Leaves were then inserted into the mouth and chewed for a few seconds before Fuller spat everything out and started to pick leaves, pieces of bark and pieces of saba from his teeth and lips. Sadly, I’ve yet to find any other evidence to support the dental hygiene hypothesis. For one, very few of the trees we’ve discovered with stripped bark have had evidence of chimpanzee-eaten fruit nearby. For a second, Buzz, the other chimpanzee who has been observed to bark strip, had hardly touched any fruit all day. Thirdly, Fuller's bark chewing bout came directly between feeding bouts – why clean your teeth and then start feeding again? Sadly, because of the fact that the Semliki chimps are unhabituated and run away each time they see us, it will be very difficult to establish a correlation between feeding on sugary foods and bark chewing. From the ‘etho-archaeological evidence’, however, it looks like the chimps strip bark less often when feeding on saba, rather than more often.

Soon I hope to be able to find some method of testing fresh samples for plaque. After that, all I need is some more fresh samples (five to be statistically significant, more’s the pity) and I can knock this one out of the park for good.

Number Four – A taste of your own medicine

As one is quite often told, tropical rainforests contain a whole wealth of plants whose medical benefits remain untold. Secondary compounds found in treebark and used to fight off bacteria can, very often, have a positive effect when eaten by humans and might have mild antibiotic properties or contain other, so-called, ‘active ingredients’. Indeed, a number of interviews which I conducted earlier today have revealed that four out of the seven known trees that chimpanzees eat bark from are used in medicine. The phloem and cambium of two species of Acacia and Tamarindus are boiled with other herbs to create a cough medicine. Tamarindus fruit is juiced to create a liquid which ‘purified the blood’. The outer bark of Ficus Mucuso is also boiled with leaves by the Mkonjo people to cure “problems of the uterus”. Excellent.

The problem here is that even if these barks are used locally as medicine, this is no guarantee that they actually work. Furthermore, even if they do work, something I have no way of finding out without embarking on a full-on, double-blind, 50-subject bark-eating binge, there’s no way to tell if it's the bark or the other leaves and herbs which cure coughs and uteral complications. Finally, half of the darned plants are never eaten by the locals and, quite possibly, have no medical benefits whatsoever.

Hypothesis Number Five – Dinner

Fallback food. Maybe Nishida was right. Maybe it's a fallback food. Hopefully I can use some backdata to try and see if bark eating correlates to food scarcity. If it does, then there’s your boring answer!

Any other ideas? And questions? Anything you think I should be looking for? Give me an e-mail or a comment, do!