The C.H.I.M.P.P. Group
Chemo-ethology of Hominoid Interactions with Medicinal Plants and Parasites

THE MEDICINAL USE OF PLANTS BY CHIMPANZEES IN THE WILD

Michael A. Huffman, Primate Research Institute, Kyoto University, Inuyama, Aichi 484 JAPAN


fax: from overseas 81 568 63-0085 No portion of this text may be cited without the explicit permission of the author.
Copyright 1996 M.A. Huffman

Throughout the world, legends and folk tales give many animals god-like or supernatural qualities and powers. Among the Navajo in the south western United States, it is said that the bear, a highly revered animal spirit in their culture, gave them the plant Ligusticum porteri to use as medicine. Indeed, North American brown bears and Kodiak bears are known to dig up the root of this plant, chew on it and then rub the juices all over face and fur (Grisanzio, 1992). The physiological activity of this plant is supported by the fact that it is noted to be particularly effective in the treatment of stomachaches and bacterial infections (Moore, 1979).
In Africa too, there are many cultures rich in the knowledge of local plants and animals. For example, Mohamedi S. Kalunde, a game officer in the Mahale Mountains National Park, western Tanzania along Lake Tanganyika is versed in the traditional use of medicinal plants by his people, the WaTongwe. Mohamedi was taught by his late grandfather Kalunde, a traditional healer. When Mohamedi was a boy, Kalunde told him several stories about how he acquired new medicines for humans by watching the behavior of sick animals. One such story describes a young porcupine that Kalunde had taken in after its mother was caught and killed in a snare. Shortly after being taken in, the young porcupine became sick; suffering from diarrhea and lethargy. He wandered off from the village and Kalunde followed him. The porcupine dug up the root of a plant the WaTongwe call 'mulengelele'. Noting that the baby porcupine recovered from its illness, Kalunde decided to collected some of these roots and try them out on people in his village whom had fallen ill. From Mohamedi's description of the properties of this plants, the roots appear to be extremely toxic. Now mulengelele is one of the plants of choice for the treatment of parasites amongst traditionally living WaTongwe.
As far as I know, medicinal plant use behavior has never been reported in porcupines. While these stories may prove to be just an interesting teaching devise to pass down important information about medicines to the next generation, the fact that animals may have something to teach us about the medicinal uses of plants cannot be ignored.
The production of toxins, drugs and other compounds, called secondary metabolites by the chemists who study them, is considered to be an evolutionary adaptation to help plants fight off predation from insects and herbivores. These compounds therefore greatly influence what plants animals can select as food (see Glander, 1982) and animal ecologists have focused a great deal of their research on understanding how animals can cope with such compounds present in the diet. An interesting break from this traditional view came in 1978 with a paper written by Daniel Janzen, an ecologist at the University of Pennsylvania.
Janzen was the first to suggest that the incidental ingestion of plants containing toxic compounds may help to combat parasite related disease. Based on a number of anecdotal accounts of medicinal plant use in animals, he argued that if animals can learn to avoid toxic substances that are harmful, they should also be able to learn to eat things that can make them better. While it is not yet known specifically just how animals would 'learn' about medicinal plants, in theory its makes perfect sense that they should. Pathogens and parasites can cause a variety of diseases, affecting the overall behavior and reproductive fitness of an individual.
With this type of selective pressure, coevolution between host and parasite is thought to have brought about a number of mechanisms by which the host can limit parasite infection and the parasite can overcome them (Toft et al., 1991). Some animals, a prime example being humans, may take advantage of such plant herbivore defense mechanisms and put these toxic secondary compounds to use against parasites and other disease causing organisms.
The most convincing and detailed evidence for the use of medicinal plants in animals thus far comes from research on our closest living relative, the chimpanzee. Chimpanzees are susceptible to a wide range of parasite species that also infect humans. Thus far, evidence of possible use of plants as an antiparasitic adaptation come from investigations of two types of medicinal plant use, whole leaf-swallowing and bitter pith chewing (Huffman & Wrangham, 1994; Rodriguez & Wrangham, 1993).

Whole leaf-swallowing
The puzzling aspects of leaf swallowing as a feeding behavior in the chimpanzees at Gombe National Park and Mahale Mountains National Park in western Tanzania was first reported jointly in 1983 by Richard Wrangham of Harvard University and Toshisada Nishida of Kyoto University, Kyoto, Japan (Wrangham and Nishida, 1983). They noted that the rough surfaced leaves of Aspilia mossambicensis, A. pluriseta, and A. rudis are selected one at a time and placed into the mouth, whereupon they are not chewed but swallowed whole. The leaves are then defecated intact, neatly folded accordian style into two or three lengthwise sections, but having undergone no other visible signs of damage or digestion. In 1985, thiarubrine A, a potent antifungal and antibiotic agent, was isolated in the laboratory of Eloy Rodriguez at the University of California, Irvine, from the leaves of A. mossambicensis. Rodriguez's results later led Wrangham and Jane Goodall, the pioneer chimpanzee researcher at Gombe, to hypothesis in 1989 that the three Aspilia species swallowed whole at Mahale and Gombe might be ingested by chimpanzees to treat intestinal parasites. However, field studies had not been conducted to verify the presence of parasite infection in individuals observed ingesting Aspilia leaves.
From the fall of 1989 I began collecting chimpanzee fecal samples at Mahale along with detailed observations on as many individuals as possible. This parasitological study has continued during each successive field season; fecal samples mostly being analyzed by Shunji Gotoh, a parasitologist in the Laboratory Primate Center of Kyoto University's Primate Research Institute, Inuyama.
Species from three genera of nematode, Strongyloides, Trichuris, and Oesophagostomum one genus of trematode, Dicrocelium, and four genera of protozoa, Entamoeba, Endoliximax, Iodamoeba, and Giardia were identified. In addition, Schistosoma and Entamoeba infections were detected in yellow baboon populations sharing the area. In the local human population at Mahale, malaria (Plasmodium falciparum) infections are common. Because these parasites are known to be transmissible to chimpanzees, they are considered ecologically important and thus potentially threatening species to chimpanzees.
In the 1989-90 and 1991-1992 study periods, a significant increase in the number of parasite species infections per individual from the dry to rainy season was recognized. More importantly, in these periods and again in the 1993-1994 study period, it was Oe. stephanostomum that infected the greatest number of chimpanzees in the rainy season and was most commonly associated with illness at the time of medicinal plant use. This rainy season peak in Oe. stephanostomum infection, but not that of other parasite species, is closely correlated with a strong tendency for chimpanzees also to most frequently swallow leaves and ingest the bitter pith of V. amygdalina.
The chemical hypothesis for leaf-swallowing was expanded further in 1993 by Rodriguez and Wrangham (1993) who proposed that 100 ml of thiarubrine A per leaf (Rodriguez et al., 1985 report 5 mg present per leaf) of A. mossambicensis could kill all of a chimpanzee's intestinal nematodes in one dose (Rodriguez and Wrangham, 1993). This hypothesis is confounded by the fact that other scientists have not been able to duplicate Rodriquez's laboratory results. The strong antifungal activity of thiarubrine A was earlier isolated from the roots of a native North American Compositae, Chaenactis douglasii by Neil Towers and co-workers at the University of British Columbia (Towers et. al., 1985). Towers, Peter Constable Jon Page, and others, from the University of British Columbia collected A. mossambicensis from Mahale and other parts of East Africa but were unable to find any traces of the compound in their leaf samples. Similarly, a lack of any significant biological activity from A. mossambicensis leaf samples from Mahale analyzed by Ohigashi et al. (1991) raises further doubts about a significant biological property in Aspilia spp. leaves. Field studies had never been conducted to verify the presence of thiarubrine A in leaf samples collected from Aspilia spp. at the time they were selected by chimpanzees.
Since 1985, another 16 species have been observed and identified to be ingested in the same way, not only at Mahale and Gombe, but at five other study sites across equatorial Africa. This behavior is not limited to just the chimpanzee (Pan troglodytes schweinfurthii, P.t.verus), but has also now been observed in the bonobo (syn. pygmy chimpanzee, P. paniscus) and the eastern lowland gorilla (Gorilla gorilla graueri) (Huffman, 1993; Matsuzawa and Yamakoshi, 1995; Reynolds, personal communication). Given this wide diversity of plants from so many taxinomically divers plants now known to be swallowed whole, a hypothesis based on the proposed chemical activity of one species falls short of adequately providing a uniting theory for the mechanism and function of leaf-swallowing behavior.
I reasoned that the ideal study should examine plant specimens collected concurrently with observations of their use for the presence or absence of bio-active compounds as well as other characteristics of the plant that may be exploited by chimpanzees. In 1993, I designed and carried out such a study to correlate the behavioral and health related aspects of leaf-swallowing with the chemical and physical properties of the plants being utilized at Mahale.
Because the Towers' laboratory in Vancouver has all the required facilities for the analysis and Page was active in the Aspilia - thiarubrine A problem, I approached them for their assistance in this project. Another colleague I approached was Michael Sukhdeo and his graduate students at Rutgers University in New Brunswick, New Jersey, who specialize in parasite behavior in the host.
As fate would have it, there was a terrible epidemic at Mahale in 1993 that took the lives of 17 chimpanzees. In general, all the chimpanzees appeared to be stressed, and many suffered from weight loss, malaise, and diarrhea. Later analysis of fecal samples systematically collected from target individuals over the study showed signs of ill health and medicinal plant use with multiple parasite infections of Oe. stephanostomum, Trichuris trichiura, and Strongyloides fulleborni. Oe. stephanostomum was subsequently shown to be a major parasite influencing and affected by medicinal plant use.
A total of 79 samples of A. mossambicensis including leaf, twig and root specimens were collected from 15 different plants in the park. Most where collected immediately after use by chimpanzees. Another 6 samples of Lippia plicata and Hibiscus aponeurus were also collected. Later analyzed by Page using HPLC (high- pressure liquid chromatography), thiarubrine A could not be found in the leaves of Aspilia or any other species. It was present, however in only small amounts, in the roots of A. mossambicensis.
Twenty-seven cases of leaf-swallowing, using A. mossambicensis, L. plicata, H. aponeurus, Trema orientalis and Aneilema aequinoctiale were obtained, 13 by either direct observation or from whole leaves in the feces of known chimpanzees. The other evidence came from plant remains found after chimpanzees had left the area. The swallowing of H. aponeurus and of A. aequinoctiale were observed here for the first time at Mahale, but have been observed to be swallowed at Gombe, Kibale, and most recently at Budongo (table 1; Huffman and Wrangham, 1994; V. Reynold, personal communication).
On two separate days, small groups of 18 and 12 individuals were followed as they approached patches of A. mossambicensis, H. aponeurus and V. amygdalina. In these two separate groups, eight individuals swallowed leaves, chewed bitter pith, or both. Leaf-swallowers that could be observed closely that day showed signs of ill health, such as malaise and diarrhea. Later analysis of the feces collected from these individuals on the day they were observed leaf-swallowing or the following day, revealed that they were indeed suffering from single or multiple parasite infections. As in previous years, parasite infections of Oe. stephanostomum were most common. This was the first verification of sickness in chimpanzees observed leaf-swallowing.
In the six cases of leaf-swallowing for which I could collect samples for parasitological analysis over the following 1-13 days afterwards, there was no evidence for a suppression of adult worm egg-laying activity. In most cases, parasite egg counts in the feces remained stable for all three parasite species detected. This was different from what we had documented for the bitter pith chewing of V. amygdalina and suggests that perhaps a different mechanism for parasite control might be involved.
What I found was something quite surprising! The adult Oe. stephanostomum are about 2 to 3 cm long and thus easy to spot, but rarely appear in the feces. They cannot survive on their own outside of the host and will die. During this particular study, adult Oe. stephanostomum were found in only 9 out of 245 fecal samples examined. Although little is known of the biology of these parasites in the host, the worms morphology suggest that they remain lightly attached to the mucosa where they presumably feed on epithelial cells and bacteria of the host. They reproduce by laying eggs which are evacuated in the feces and must develop into infective stage larvae in the soil before they can enter another host. Infective nematode larvae do not survive well in dry, hot weather, and may be infective for longer periods when the external environment is cool and damp. This seasonally influenced reproductive behavior partially explains why Oe. stephanostomum infections show up most frequently in chimpanzees during the rainy season at Mahale.
In six of the nine feces containing adult worms, the whole undigested leaves of A. mossambicensis, T. orientalis or A. aequinoctiale were also found. As many as 18 or 21 worms were expelled with 21 to 56 leaves in a single feces. On three occasion, I was able to closely examine the leaves and worms in these feces back at camp. The worms remained moving and alive in the dung for up to at least four days. By the time these fecal specimens could be examined, some worms were moving freely within the liquid feces, but others were caught, alone or in pairs, in the compartments created between the folds of the leaves. In one case, two Oe. stephanostomum worms were actually firmly stuck to the surface of the leaf as if attached by Velcro TM. Only after three tugs with tweezers did they come completely off.
Nematocidal activity due to chemicals produced by these plants, as earlier proposed by Rodriguez, was ruled out in all of these cases because the worms were alive and moving when they were found in the feces and survived for days afterwards. Instead, the short flexible hairs, called trichomes (Figure 2: ESM of Itesa leaf surface), located all along the surface of the leaf and the compartments created by the accordian like folds of the leaves, traits common in all known cases of leaf-swallowing across Africa, appeared to be responsible for the expulsion of these parasites.
In 1994, Wrangham and I speculated that chimpanzees select species for their physical properties (Huffman and Wrangham, 1994), because among the plants species known to be swallowed whole, roughness has remained the one constant property. However, based on our findings from Mahale I was able to refine this idea. I have named this hypothesis the 'velcro effect', referring to the physical purging action of adult parasites by the leaves as they pass undigested through the chimpanzees intestinal tract (Huffman, et al., in press).
While it is possible that there are other functions associated with leaf-swallowing, our current hypothesis is that this behavioral adaptation is part of a self-medication strategy used by chimpanzees against gastrointestinal parasites (Huffman et al.. in press). The details of the mechanism by which whole leaves might remove parasites is yet unclear, however, Sukhdeo and I speculate that large whole leaves entering the chimpanzees large intestine move closely to the mucosal lining where the probability of contact with Oe. stephanostomum is high. Because the worms are only lightly attached to the mucosa, they are easily pulled or scrapped off by the velcro-like action of the hairy leaves. With this scenario, the physical anti-parasitic action of the leaves would be improved by large size and may explain why the leaves are swallowed whole.
In the life cycle of these parasites, the infective larvae penetrate the wall of the mucosa, develop and moult twice to become adults, and the adults leave the tissue to migrate back to the lumen of the bowel. These parasites are called "nodular worms" because the larvae become encapsulated by excessive reactive inflammation, especially in previously sensitized hosts. This acute inflammatory reaction leads to diarrhea. Importantly, the adult worms in the lumen rarely cause harm to the host. Yet, leaf-swallowing removes the adult worms and not the larvae in the tissue. Why? The answer may lie in "premunition" - a state where the presence of a stable population of adults in the lumen inhibits the maturation of larvae within the tissue. It is only when adults are removed, say with drugs in humans, that the larvae will develop. Thus, in chimpanzees, removing the adults with leaves stimulates the maturation of the larvae which then migrate out of the tissue and it is the movement of the worms out of the tissue that alleviates the symptoms.
The purging of adult worms via a form of physical entrapment is a unique hitherto unknown anti parasite adaptation. I propose that this mechanism functions to control heavy strongyle infections and relieve their adverse symptoms. This approach may be considered an alternative behavioral adaptation for a controlled co-existence with strongyles under conditions in which reinfection is an inevitable fact of life.
Bitter leaf-chewing
In 1989, I published the first detailed account of bitter pith chewing (Huffman and Seifu, 1989). This study was the first study to document sickness at the time of ingestion of a known medicinal plant by a chimpanzee in the wild and to follow the individual through to apparent recovery. It was this work that we based our later investigations of leaf-swallowing on. In the first case study, a female was observed to meticulously remove the leaves and outer bark from several young shoots and chewed on the exposed pith, sucking out only the extremly bitter juice (figure 3: picture of male ingesting Vernonia & one of Gunter 's pictures of Vernonia?). Within 24 hrs, she had fully recovered from a lack of appetite, malaise, and constipation (Huffman and Seifu, 1989).
A similar incident was observed in December of 1991, and in this case an infection of the nematode worm Oe. stephanostomum dropped noticebly within 20 hours after the adult female chewed V. amygdalina pith (Huffman et al. 1993). In both cases, the rate of recovery (20-24 hrs) was comparable to that of WaTongwe who traditionally use this plant as a treatment for similar symptoms. Indeed, among many African peoples this plant is prescribed treatment for stomachaches, and a number of parasite infections including bilharzia, malaria and pinworm (Huffman et al., 1996).
As I began to formulate a hypothesis, it became apparent that the investigation could not possibly be done alone. As luck would have it, a team of natural plant product chemists at Kyoto University, headed by Koichi Koshimizu and Hajime Ohigashi, were working on the isolation, structural determination and activity testing of chemical compounds extracted from tropical African flora (Ohigashi et al., 1991). When I visited their laboratory in January of 1988 they were keen to help me with the analyses. They had come across V. amygdalina the previous year in Cameroon, west Africa and were interested in it because of the use of the leaves of its domesticated variety as a tonic food called 'Ndole'. Eaten with meat and plantain bananas, this bitter tasting dish, made less bitter by soaking and squeezing the leaves in water, is said to restore stamina. It is also plausible that its inclusion in the daily diet works as a prophylactic against parasites and or viruses. We agreed to collaborate and laboratory investigations began in early 1988.
There analyses in Kyoto revealed the presence of sesquiterpene lactones, a group of compounds previously known for their significant bioactivity. Koshimizu's lab succeeded in isolating and identifying a total of 11 new constituents from a class of compounds called the steroid glucosides (Ohigashi et al., 1994). The antiparasitic activity of some of the sesquiterpene lactones isolated from V. amygdalina, and also found in Vernonia colorata , have been investigated in the laboratories of Guy Balansard and colleagues at the University of Marseilles (Gasquet et al., 1985). In Great Britain, a group represented by John D. Phillipson and Colin W. Wright at the University of London School of Pharmacy, and David C. Warhurst and Geophery C. Kirby at the London School of Hygiene and Tropical Medicine were using new methods to test the bioactivity of tropical African plants against parasites causing ameobic dysentery (Entamoeba hystolitica) and malaria (Plasmodium falciparum). In Japan, Masanori Kawanaka and Hiromu Sugiyama at Tokyo's National Institute of Health had developed a new method for testing the antischistosomal (Schistosoma japonicum) activity of plant compounds.
Koshimizu, Ohigashi, and I contacted these scientists to inquire about a joint investigation and by the middle of 1991 we had outlined the scope of our collaboration and testing began. The results of these in vivo tests of compounds found in the pith of V. amygdalina showed antiparasitic activity. The compounds linked most closely to Mahale chimpanzees' observed preference for the pith, vernonioside B1 and vernoniol B1, where shown to be effective in suppressing movement and egg-laying activity of Schistosoma japonicum. Further tests by our collaborators in Tokyo, Marseilles and London on vernodalin, a highly toxic sesquiterpene lactone, also showed significant activity against Schistosoma a well as Plasmodium and Leishmania (Ohigashi et al., 1994).
In October 1991, Koshimizu and his team joined me at Mahale to collect specimens of an additional 15 plant species with possible relevance to the investigation. It was while at Mahale that we conceived of The C.H.I.M.P.P. Group as a name for our growing group of collaborators (Chemo-ethology of the Hominoid Inteteractions of Medicinal Plants and Parasites).
An assay of the plant samples collected in November 1991 after use by an ill female chimpanzee confirmed that the two most bioactive constituents in this plant, vernodalin, and vernonioside B1 were present. The highly toxic compound, vernodalin, was found only in the leaves, not in the pith of the plant she used, which instead contained significant amounts of vernonioside B1. This same pattern was latter verified in analyses made on other V. amygdalina specimens collected at various locations in Mahale during different seasons. We believe that because chimpanzees rarely use anything but the pith, and discard the bark and leaves in the process, they have learned to differentiate between the plant parts that contain compounds harmful to them from those parts that contain compounds of benefit. Observations have shown that young chimpanzees will closely copy the processing techniques of both bitter pith-chewing and leaf-swallowing performed by their sick mothers or those nearby.
In general chimpanzees tend to be very conservative in their feeding habits. As part of growing up, they have to learn what and how to eat by watching the behavior of their elders. In this way, knowledge and experience of the group is passed down in the form of behavioral tradition, a process which allows many individuals to benefit from the experience of a few. In doing so, as in the case of medicinal plant use, the dangers of individual experimentation with poisonous plants is limited to a few. This resembles somewhat the way in which Mohamedi's grandfather learned from trial and error about mulengelele and the experience was passed on to his grandson. However, there is one big difference. Chimpanzees do not teach each other or encourage a sick individual to take medicine.
This new discipline has grown out of the scientific endeavor to understand more about the ways in which animals may be treating themselves with the use of plants. Due to the constraints and difficulties of systematic research of this kind on wild animal populations, our knowledge is still limited.
Regardless, there truly seems to be something to the age-old idea that animals have potentially important lessons to teach about medicinal plants. We still have much to learn, but time is running out to do so. An ever increasing number of plant and animal species are being threatened by the destruction of their habitat due to the encroachment of humans. We owe it to them and to ourselves to find a way to correct this before we humans are forced to swallow our own bitter medicine.

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Adult male Jilba chewing on bitter pith of Vernonia amygdalina. He had a heavy infection of nematodes.(La Recherche 1995)


Adolescent male Iwan swallowing Aspilia mossabicensisleaves.


Huffman, Seifu, and Luhembe in the field. Efforts are being made to teach young Tongwe about the use of medical plants.


Aspilia mossabicensis(Oliv.) Aneilema aequinoctiale(P.Beau.)

  M.A.Huffman


CHIMPP Publications - May, 1996
Some basic guidlines for research

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