Self-Medicating Behavior in Non-Human Primates

medicate by Megan Funck

Throughout history, humans have turned to various herbs, plants, and natural elements found in the environment to prevent and treat illness, and many of our modern pharmaceutical medicines are derived from plant sources. Scientists and researchers are constantly searching for novel compounds in plants that have the potential to cure, treat, and prevent diseases, illnesses, and various disorders. According to Michael Huffman, from the Primate Research Institute at Kyoto University, it is likely that the origins of herbal medicines have their roots deep within the animal kingdom, and “our earliest hominid ancestors may have exhibited similarities in plant selection criteria with both extant apes and modern humans” (Huffman 2001). Zoopharmacognosy, or the study of self-medicative behavior in animals, can provide insights into our own species evolutionary past. In recent years, Primatologists have recorded a substantial amount of evidence supporting self-medicating behavior among non-human primates, including the ingestion of plants and pith seeds known for their medicinal properties, leaf swallowing, fur rubbing, and geophagy.
Primates engage in self-medicating behavior through the ingestion of plants known for their medicinal qualities. Many plants are high in secondary compounds, which hold little nutritional value and are generally toxic to some extent. African great apes have been observed ingesting plants high in secondary compounds, and more frequently during the rainy season. The rainy season is a period of time associated with a higher risk of parasitic/bacterial/fungal infections- due to the increased amount of moisture and humidity in the air, which is an ideal breeding ground for bacteria and parasites.(Huffman 1997). The plant Vernonia Amygdalina (common name: bitter leaf) is known by African indigenous groups as a treatment for malarial fever, dysentery, and other intestinal parasites (Ijeh 2011). The plant is high in alkaloids in its pith, flower, bark, and leaf. When infected individuals from two groups of chimpanzees and a group of local human inhabitants were observed after ingesting the pith, there was a noted recovery time of 20-24 hours in both chimpanzees and humans. (Huffman 1997). After primatologists witnessed this, a chemical analysis of V. Amygdalina was conducted. They found the presence of four different types of secondary compounds. After further testing, two of the secondary compounds (vernonioside B and vernodaline) were found to inhibit the movement of adult parasites and the egg-laying capacity of female parasites. (Huffman 1997). These findings were consistent with the decline in nodular worm EPG level 20 hours after an adult female chimpanzee at Mahale ingested V. Amygdalina pith. (Huffman 1997). The secondary compound vernodaline was found to be lethal at higher doses. Vernodaline is abundant in the leaves and the bark of V. Amygdaline, but found in smaller amounts in the pith, which can explain why chimpanzees, when consuming this particular plant, tend to avoid the leaves and bark in favor of the pith (which has higher levels of the other secondary compound, Vernonioside B). (Huffman 1997).
Leaf-swallowing has also been observed in a number of primate species. Leaf swallowing behavior involving thirty plant species has been documented in groups of chimpanzees, bonobos, and eastern lowland gorillas across Africa (Huffman 1997). There is evidence that supports the hypothesis that these leaves are ingested for medicinal benefit rather than nutritional. For instance, studies conducted by primatologists revealed that plants from which the leaves are derived for swallowing were utilized infrequently and were not a regular dietary inclusion. The leaves were purposefully folded, then swallowed whole, without chewing (leaves are found intact and undigested in feces samples).The leaves were also utilized during periods of increased risk of parasite infection (rainy season), by primates with observable illness (and showed parasite infection in feces). Primates that seemed healthy (and confirmed as infection- free in feces samples) showed little interest in the plants. There was also a positive change in the health of the primates after ingestion. (Fowler 2007). In one study of chimpanzee leaf-swallowing, in Gashaka Gumti National Park in Eastern Nigeria, Primatologists identified parasitic worms attached to the surfaces of leaves found in the chimpanzees feces. The plant from which the leaves came is known as “takamahi” by the local indigenous groups, which means “the one that sticks to you”. (Fowler 2007). It was originally thought that the positive benefits from leaf swallowing stemmed from the pharmacological makeup of the plant. However, after discovering that there are many different species of leaves swallowed, which are composed of varying chemical makeups, it seemed unlikely that all benefit is derived from the pharmacological properties. However, the different leaves ingested share similar physical characteristics: they are spiky and rough. This feature could be a factor in expelling parasites; the worms becoming attached to the rough surface of the leaf, and the bulky, undigested leaf matter serving as a “plug”, cleansing and dislodging parasites, as the leaves make their way through the digestive tract (Fowler 2007). However, the most commonly view held today is that the benefits of leaf swallowing are due to the fact that by taking in plant matter that does not digest well, a purgative response (diarrhea) is induced in the system, and the transit time of the material in the intestines is quickened, which in addition to causing the expulsion of parasites, can interrupt their life-cycle (Fowler 2007).
Fur rubbing also provides evidence of self-medication in primates. Fur rubbing is commonly seen in primates, especially wild and captive capuchin monkeys (Huffman 1997). This is where plants are rubbed onto fur (in some cases, chewed first and mixed with saliva). Capuchins studied in Costa Rica were seen using five different plants for fur rubbing, engaging in both solitary and group rubbing (Huffman 1997). It is possible that the benefits of fur rubbing are the repelling of insects and ectoparasites, and skin treatment for bacterial infections (Huffman 1997). All plants used in fur rubbing have a pungent quality to them, and the plants contain various secondary compounds which have known benefits of: insecticidal, antiseptic, fungistatic, anti-inflammatory, and anesthetic properties.(Huffman 1997). Captive-born capuchins, when presented with onions and oranges, began fur rubbing, utilizing the resources each time. (Meunier 2007). There may be some non-medical benefits to fur rubbing, such as conditioning of the fur, group scenting, and the facilitation of social grooming (Huffman 1997). Capuchins have also been observed to frequently rub their fur with millipedes (this behavior is called “anointing”). Chemical analyses was completed on the millipedes, and it was found that they contained two different compounds called “benzoquinones”, which are potent insect repellants. (Valderrama 2000). Primatologists found that during a capuchin study, out of 87 bouts of self-anointing with millipedes, 40 of them were done in group settings. If one capuchin found a millipede and began self-anointing, others would approach, and attempt to take the millepede from its hand. If that didn’t work, they would then rub their tails over the fur of the self-anointing capuchin, and bring the tail back to their own body and rub it on their fur (Valderrama 2000). This has led Primatologists to believe that the sole purpose of this behavior is to acquire the insect repellant secretions of the millipedes. (Valderrama 2000).
Geophagy is another evidence of self-medicating behavior among non-human primates. Geophagy is the ingestion of soil-like substances, such as clay or chalk. Although providing no carbohydrates, proteins, or fats, these substances are very rich in minerals. In non-human primates, geophagy is beneficial for absorbing toxins that are acquired through a diet which is high in incidental secondary compounds (Huffman 1997). Clay soils ingested by primates were found to contain components which are similar to Kaopectate (medicine used to treat gastrointestinal distress). (Huffman 1997). Some primates, such as Red Colobus Monkeys, have been seen eating charcoal from leftover fires, which serves to counteract the heavy toxin-load that is present in their diet of leaves. An important question to consider is: what are the origins of self-medicating behavior? How do primates learn which plants and natural resources are effective in treating illness? Is it an innate, instinctual behavior, or learned socially? And if learned socially, how was the behavior initially started? The results of an experimental study of leaf swallowing in captive chimpanzees, conducted in 1997 at Kyoto University ’s Primate Research Institute, addressed this issue. A group of healthy chimpanzees were introduced to a leaf similar to the ones found and ingested in the wild in 38 different trial sessions. There was one interesting case of a male and female chimpanzee who were both born into captivity (eight chimpanzees in the group were wild- caught and brought into captivity by the age of one). When introduced to the leaves, they both folded and swallowed them whole, without hesitation. They had never previously observed any other chimpanzees exhibiting this behavior. There were six chimpanzees who rejected the leaves during their first trial session, but after two of them closely observed the leaf swallowing of one female, they followed suit, exhibiting the same behavior. (Hirata 2004). Two in the group chewed and swallowed the leaves, instead of folding and swallowing them whole. Four of the six who rejected the leaves continued to avoid the plant. The study concluded that “individuals born to wild chimpanzee mothers were no more likely to perform the behavior than captive-reared group mates” (Hirata 2004:113), and that “the acquisition of leaf swallowing behavior is based in part on a propensity to fold and swallow rough leaves, but that the acquisition and spread of leaf swallowing within a group is likely to be socially influenced.” (Hirata 2004:114). They also conclude that the results of the study supports that leaf swallowing is not an innate response to a parasite infection, since none of the chimpanzees that ingested the leaves had any kind of parasite infection. (Hirata 2004:117) Rather, they theorize that the rough texture of the leaves is what causes primates to swallow them whole, and that leaf swallowing “may have originated in the feeding context of a novel plant item”, and “the self-medicative function of leaf swallowing behavior must be a secondary adaptation of a feeding response brought about by leaf texture.”(Hirata 2004:117). However, this theory doesn’t explain why in the wild, primates only utilize these plants occasionally, and more frequently during the rainy seasons (high risk period of infection), which supports that these plants are purposefully ingested for their medicinal qualities.
It is still crucial to consider the way in which other forms of self-medication, such as bitter pith chewing, may have originated.. In bitter pith chewing, it is not just the species of plants that matters, but which specific part of the plant is ingested. The pith supplies the most medicinal benefit, while the other parts of the plant are sometimes too toxic too consume comfortably. Primates have learned to only consume the pith, the most beneficial part, and discard the rest of the plant. How do they know this? Michael Huffman hypothesizes that the specific selection of plants may be based on chemosensory criteria, and the similarity of the plants consumed between neighboring groups of primates may represent cultural traditions- information of behavior being exchanged between groups via female group transfer (Huffman 1997) He also asserts that “given the high degree of conservatism in chimpanzee feeding habits, random sampling of novel food items, especially when ill, does not seem likely to occur very frequently. If so, traditions must have started rarely, perhaps as a result of ill, hungry chimpanzees trying new foods during periods of extreme scarcity, recovering their health, and associating their improved health with the new item.” (Huffman 1997:192).
By observing and researching specific, non-nutritive plants that non-human primates (and other animals) consume and use topically, it is possible to discover novel medical benefits, and apply them towards modern medicine. The similarity between medically beneficial plants chosen by great apes to use, and the same use of these plants by humans to treat illnesses, is exciting evidence for the evolution of medicine. (Huffman 2001). Huffman said “humans have looked to wild and domestic animals for sources of herbal remedies since prehistoric times. Both folklore and living examples provide accounts of how medicinal plants were obtained by observing the behavior of animals” (Huffman 2001:651). By researching and discovering the self-medicating behavior and criteria among non-human primates (bitter pith chewing, leaf swallowing, fur rubbing, and geophagy), we can learn not only about our own evolutionary past, but discover exciting new possibilities applicable in the field of medicine, today.

Fowler, Andrew. 2007.“Leaf Swallowing in Nigerian Chimpanzees: Evidence for Assumed Self-Medication”. Primates (2007) 48:73-76
Hirata, Satoshi. Huffman, Michael. 2004. “An Experimental Study of Leaf Swallowing in Captive Chimpanzees: Insights into the Origin of a Self-Medicative Behavior and the Role of Social Learning”. Primates. 2004. 45:113-118
Huffman, Michael. 1997.. “Current Evidence for Self-Medication in Primates: A Multidisciplinary Perspective.” Yearbook of Physical Anthropology 40:171-200. 1997.
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Valderrama, Zilda. “Seasonal Anointment with Millipedes in a Wild Primate: A Chemical Defense Against Insects?” Journal of Chemical Ecology. Volume 26. Number 12. 2000.
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