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CHIMP WATCHERS in East Africa have a theory about an odd piece of behaviour
that seems to narrow the gap between chimps and humans still further: the
animals may treat themselves when they feel sick with the same plants that
local people use for the same sort of illnesses.

Over the past decade, a number of reports in scientific journals have
hinted that chimpanzees seek out certain leaves and seeds for their physiological
or pharmacological effects. Although there is still no direct evidence that
a particular plant has a specific effect, three leading primatologists have
found the strongest circumstantial evidence so far that our primate cousins
treat themselves for a range of ailments.

Since the early 1970s, Richard Wrangham, professor of anthropology at
Harvard University, and Jane Goodall, the most famous of the chimp watchers,
have recorded in detail the behaviour of chimps as they foraged in the Gombe
Stream National Park in Tanzania. In the Mahale Mountains National Park,
south of Gombe, Toshisada Nishida, a zoologist at the University of Kyoto,
has made similar observations of chimps’ feeding behaviour. The scientists
recorded what the chimps ate every day, and at what time of day: at Gombe,
they catalogued 146 species in the diet; at Mahale, the chimps ate plants
belonging to 198 species.

The main focus of their studies is one genus of plant, Aspilia, which
chimps eat only rarely. There are several species of Aspilia throughout
Africa’s grasslands. The plants are tall but rather nondescript members
of the sunflower family. They caught Wrangham’s attention because he had
never seen any animal behaving as strangely as the chimps at Gombe did with
Aspilia.

For the past two decades, observers at Gombe watched about 50 animals,
ranging from two-year-old toddlers to balding elders, making special trips
some distance from their nests and regular foraging areas to feed on young
Aspilia leaves. Wrangham watched Gombe chimps feeding at dawn on two particular
species, Aspilia pluriseta and A. rudis, while Nishida’s chimps at Mahale
consumed another species, A. mossambicensis, at any time of day. The chimps
completely ignored two other species of Aspilia growing in the area. ‘Normally,
chimps stuff leaves – which serve as a protein supplement – into their mouths
as fast as they can pull them off the branches, and chew them rapidly,’
says Wrangham. ‘But the chimps at both reserves select Aspilia leaves more
slowly and carefully, closing their lips over a leaf – sometimes pausing
for a few seconds – and rejecting it without pulling it off the branch.
When one meets with approval, the primate rolls the sandpaper-like leaf
around in its mouth and then, in a thoroughly un-chimplike manner, swallows
it whole before selecting the next one – slowly.’ Chimps generally eat much
faster: the researchers timed the chimp’s consumption of leaves of a rather
tasty plant, Mellera lobulata, at an average of 44 a minute, compared with
only five of Aspilia.

To find out what these rarely eaten leaves were doing for the chimps,
the primatologists looked for clues in chimp dung. Whole Aspilia leaves
appeared in more than 400 samples, sometimes folded neatly in half. The
collection of dung dates back to 1964, when Goodall first began to collect
samples for a study of seeds. (Ironically, says Wrangham, Goodall was the
first to observe the phenomenon of the leaf ‘pill’, but did not know what
to make of it at the time.) Because these leaves were not chewed up, the
scientists doubted that the chimps were ingesting them either for food or
to increase their intake of fibre. But scanning the leaves under the microscope
at a later date revealed that the leaves were pockmarked with tiny ruptures,
enough to release significant amounts of chemicals as they passed through
the gut.

‘In 1983, Toshisada Nishida and I reported that Aspilia might be a food
stimulant, perhaps giving the chimps the munchies,’ said Wrangham. But further
observations proved that the chimps did not eat more (or less) after swallowing
Aspilia leaves. The primatologists also thought that chimps might eat Aspilia
only at particular times of the year, depending on what else was available,
as is the case with many of their foods. But the dung samples showed that
chimps were as likely to eat Aspilia in January as in July. Other possibilities
were that Aspilia might be an intoxicant, a painkiller or a curative. Apart
from a few chimps vomiting or wrinkling their noses as if they had swallowed
a bitter-tasting pill, however, no behaviour seemed particularly significant.
(Wrangham even did a taste test – as chimps and humans both generally avoid
noxious tasting things. He could detect nothing unusual.) The scientists
were stumped: just what kind of chemical kick were the chimps getting? When
they discovered how widespread the medicinal use of Aspilia is among people,
the researchers began to think that the chimps might be dosing themselves
with some form of medication. The local Tongwe people make a tea out of
the leaves to treat wounds, burns and other external ailments and stomach
disorders, often caused by worms. Both chimps and humans use the same three
species, Aspilia mossambicensis, A. rudis, and A. pluriseta; some species
rejected by the chimps are not used by humans either. Both prefer leaves
to any other part of the plant. ‘It was at this point that chimps and people
appeared to converge,’ said Wrangham.

Although Aspilia, in particular the species preferred by the Mahale
chimps, is one of the most popular African remedies, says Wrangham, no one
had analysed the chemistry of the plants to find out what the active ingredients
might be. For that information, Wrangham turned to Eloy Rodriguez, a pharma
cognosist at the University of California, at Irvine, and an expert on this
family of plants (the Compositae).

Rodriguez’s findings were striking: the leaf pills contained high concentrations
of a potent antibiotic, called thiarubrine-A. This bright red chemical had
been found before in the roots of another plant in the same family, Chaenactis
douglasii. Native Canadians make a remedy for skin sores from these roots.
But chemists were unaware that thiarubrine is also present in the young
leaves of plants belonging to this genus. ‘I was struck by the wisdom of
the chimp and the African peoples, who figured out without benefit of a
high-tech analysis that only the young leaves contain the potent chemical,’
said Rodriguez. Moreover, chimps pick leaves from two of the species only
around dawn, suggesting that they know that the plant will be more effective
then. This is quite likely because the concentrations of secondary metabolites
in plants often follow a daily cycle.

Rodriguez also discovered that thiarubrine-A is a potent antifungal
and worming agent: low doses (five parts per million) of the chemical are
very effective against a variety of parasitic worms. The compound also has
antibacterial and antiviral properties, and it is more potent than the anti-cancer
drug vinblastine in in vitro toxicity tests. This is the first chimp food
known to contain high concentrations of a powerful bioactive drug, the primatologists
point out.

The artfulness of chimpanzees

Wrangham and Goodall suggest that the medicinal use of plants is yet
another sign of how much more intelligent chimps are than other primates.
Chimpanzees, who learn what to eat by watching and imitating their elders,
learn new behaviours quickly and retain that knowledge for future use, says
Wrangham. This distinguishes the chimpanzee from other primates at Gombe,
such as baboons, which do occasionally eat medicinal plants, but apparently
for no particular reason other than as food. Despite Aspilia’s hard bristly
leaves, baboons chew them as quickly as they do any other leaves, suggesting
that they are simply eating them in the same way as anything else and that
the baboons might be able to detoxify the chemicals in their gut. ‘It’s
striking that in 20 years of observing baboons, we only saw two eat Aspilia
and not in the chimp style,’ says Wrangham.

Other animals sometimes exploit the antiparasitic properties of certain
plants, but not for internal maladies. Starlings, for example, control parasitic
infestations of their nests by relining them with certain leaves. But no
other animal except the chimp has the capacity to link consumption of particular
plants with relief from sickness or pain, or seek different solutions to
a problem. Unlike the starling, the chimp is a rather discerning shopper
– selecting three species of Aspilia and at different times of the day when
the chemistry of the plant varies – apparently with the quality in mind.

Even more telling, Wrangham and other experts on primates have found
that the Tanzanian chimps use other medicinal plants, which they treat in
the same way as the Aspilia-leaf pill. One such plant is Lippea picata,
a woodland shrub. At Mahale, primatologists watched a female chimp swallow
the leaves of this plant and then go immediately to rest. The Tongwe people
make an infusion of crushed leaves of this species to treat stomachache.
Elsewhere in Africa, people use other species of Lippea to treat dysentery
and malaria. Analysis shows that Lippea contains some potent compounds,
called monoterpenes, which are probably active against a range of parasites.

The same sick female chimp also consumed another plant, Vernonia amygdalina,
a woody bush with known medicinal value. People throughout tropical Africa
use the plant to treat parasitic infestations. Analysis of Vernonia revealed
antibiotic and antiviral agents, and compounds that seemed to boost the
immune system. This incident was the first in which the chimp’s recovery
seemed linked to her self medication. Two primatologists, Michael Huffman,
of Kyoto University, and Mohamedi Seifu, of the Mahale Mountains Wildlife
Research Centre, last year published a detailed study of the actions of
this female chimp.

According to Huffman, the chimp was lethargic and seemed to be suffering
from diarrhoea, a common symptom of parasitic infection. She bypassed succulent
stalks of Pennisetum purpureum that other chimps were eating and sought
out the bitter-tasting juice from the shoots of V. amygdalina. Instead of
eating the entire shoots, the chimp sucked out the juice and spat out the
rest. She then rested in her tree nest while her chimp companions stayed
nearby. Within 24 hours, the off-colour chimp was back in the swing of things.

‘Although we can’t prove conclusively that chimps self-medicate, this
Japanese report and others show that these primates were able to respond
to an immediate problem – in this case illness,’ says Wrangham. ‘There’s
no evidence that other animals, even baboons, do this.’ Following the same
line of reasoning, Huffman has begun to look at how heavily infested with
parasites the chimps at Mahale are. His early results from analyses of their
dung suggest that some of the chimps carry enough parasites to cause uncomfortable
symptoms. Such discomfort, he suggests, might prompt the chimps to seek
relief – by treating themselves with the appropriate plant.

Wrangham reports widespread use of medicinal plants by chimps in other
communities and reserves, outside Tanzania: he has identified several species
at the Kibale Reserve in western Uganda which chimps swallow whole rather
than chew, and which are also traditional African remedies. Rodriguez has
analysed many of the plants and found signs of pharmacological activity.
The leaves of a local fig tree, Ficus exasperata, for instance, contain
well known antibacterial and antifungal compounds, including phsolins, which
the Egyptians have used to treat skin diseases since ancient times. The
chimps eat these leaves as a part of their normal diet, but probably benefit
from their medicinal properties in the process. Rodriguez has also isolated
a small polypeptide from Rubia cordifolia, a common remedy in Uganda for
stomachaches, which is active against parasites and, according to researchers
in Japan, also has some effect on tumours.

In both Uganda and Mahale, chimps swallow, rather than chew, the very
bristly leaves of yet another plant, a scrambling herb called Commelina.
The herb contains tannins and a milky substance that African people use
as a general antibiotic, and as an antiviral, to treat fevers, as a treatment
for earaches and to stop bleeding. Rodriguez is analysing the leaves to
find out what active ingredients they contain.

The Americans are particularly interested in developing effective antifungal
treatments, which Western medicine lacks; the Japanese are more interested
in Vernonia’s ability to boost the immune system. If Aspilia turns out to
be effective against worms, says Rodriguez, it could be an alternative to
expensive, synthetic drugs and more appropriate for treating people and
livestock in the Third World.

Meanwhile, the primatologists have many other puzzles to solve, ranging
from how the chimp’s selection of medicinal plants evolved to why the female
chimps at Gombe eat Aspilia three times as often as males. Then there is
the question of whether these potent chemicals act alone or with other agents
in the plants. The task is formidable: Wrangham and Goodall report that
they have another 27 rarely eaten chimp foods to analyse. To that end, Rodriguez
is training Wrangham’s primatology students the techniques of plant chemistry
and vice versa.

‘What’s so amazing,’ concludes Rodriguez, ‘is that not only do we have
good evidence that chimps may self-medicate, but that they are showing us
these potential new drugs from their own jungle medicine cabinet.’

Cathy Sears is a freelance writer and contributing editor to American
Health.

Further reading Understanding Chimps, edited by Paul G. Heltne and Linda
A. Marquardt, Harvard University Press 1989. The Chimpanzees of Mahale Mountains,
edited by T. Nishida, Tokyo University Press 1990.