Zoopharmacognosy is a behaviour in which non-human animals self-medicate by selecting and ingesting or topically applying plants, soils and insects with medicinal properties, to prevent or reduce the harmful effects of pathogens, toxins, and even other animals. The term derives from Greek words zoon ("animal"), pharmacon ("drug, medicine"), and gnosis ("knowledge").
An example of zoopharmacognosy occurs when dogs eat grass to induce vomiting. However, the behaviour is more diverse than this. Animals ingest or apply non-foods such as clay, charcoal and even toxic plants and invertebrates, apparently to prevent parasitic infestation or poisoning.
Whether animals truly self-medicate remains a somewhat controversial subject because early evidence is mostly circumstantial or anecdotal. However, more recent examinations have adopted an experimental, hypothesis-driven approach.
The methods by which animals self-medicate vary, but can be classified according to function as prophylactic (preventative, before infection or poisoning) or therapeutic (after infection, to combat the pathogen or poisoning). The behaviour is believed to have widespread adaptive significance.
History and etymology In 1978, Janzen suggested that vertebrate herbivores might benefit medicinally from the secondary metabolites in their plant food.
In 1987, the term "zoopharmacognosy" was coined, derived from the Greek roots zoo ("animal"), pharma ("drug"), and gnosy ("knowing"). The term gained popularity from academic works and in a book by Cindy Engel entitled Wild Health: How Animals Keep Themselves Well and What We Can Learn from Them.
Mechanisms The three reported methods of self-medication are deglutition, ingestion, and topical application. When using one of these methods while appearing well, an animal may be using self-medication as a prophylactic measure. When it is unwell, the animal could be using self-medication as a curative measure.
Ingested material may have pharmacological antiparasitic properties, such as phytochemicals decreasing the ability of worms to attach to the mucosal lining of the intestines or chemotaxis attracting worms into the folds of leaves. Additionally, many plants have trichomes, often presented as hooked and spiky hairs, that can attach to parasites and dislodge them from the intestines. Another possible mode of action is that the ingested material may initiate a purging response of the gastrointestinal tract by rapidly inducing diarrhoea. This substantially decreases gut transit time, causing worm expulsion and interruption in the life cycle of parasites. This, or a similar, mechanism could explain undigested materials in the faeces of various animals such as birds, carnivores and primates.
Some animals apply substances with medicinal properties to their skin in a process known as topical application, which is often used by animals to treat wounds or repel insects. In some cases, this is known as self-anointing. When plant leaves are chewed and then directly rubbed onto fur, compounds from said leaves are released for use. These compounds can often be analgesic or antiparasitic in nature. In regards to an insect repellant, the secondary metabolites traditionally used by plants to deter herbivores and insects from eating them can be used by animals as a protective measure. By interfering with neuroreceptors, these secondary metabolites can specifically act as olfactory cues for insects to avoid a certain source.
Examples
Apes Great apes often consume plants that have no nutritional values but which have beneficial effects on gut acidity or combat intestinal parasitic infection.
Chimpanzees, bonobos, and gorillas eat the fruits of Aframomum angustifolium. Laboratory assays of homogenized fruit and seed extracts show significant anti-microbial activity. Illustrating the medicinal knowledge of some species, apes have been observed selecting a particular part of a medicinal plant by taking off leaves and breaking the stem to suck out the juice.
Chimpanzees sometimes select bitter leaves for chewing. Parasite infection drops noticeably after chimpanzees chew leaves of pith (Vernonia amygdalina), which contain sesquiterpene lactones and steroid glucosides that are particularly effective against schistosoma, plasmodium and Leishmania. Specifically, these compounds can induce paralysis within the parasites and impair its ability to absorb nutrients, move, and reproduce. Chimpanzees do not consume bitter on a regular basis, but when they do, it is often in small amounts by individuals that appear ill. Jane Goodall witnessed chimpanzees eating particular bushes, apparently to make themselves vomit.
Wild chimpanzees sometimes seek leaves of the Aspilia plant. These contain thiarubrine-A, a chemical active against intestinal nematode parasites. Because this compound is quickly broken down by the stomach, chimpanzees will pick up the Aspilia leaves and, rather than chewing them, they roll them around in their mouths, sometimes for as long as 25 seconds. They then swallow the capsule-like leaves whole. Afterwards, the trichomes of the leaves can attach to any intestinal parasites, namely the nodular worm (Oesophagostomum stephanostomum) and tapeworm (Bertiella studeri), and allow the chimpanzee to physically expel the parasites. As many as 15 to 35 Aspilia leaves may be used in each bout of this behaviour, particularly in the rainy season when there is an abundance of many parasitic larvae that can cause an increased risk of infection.
Chimpanzees sometimes eat the leaves of the herbaceous Desmodium gangeticum. Undigested, non-chewed leaves were recovered in 4% of faecal samples of wild chimpanzees and clumps of sharp-edged grass leaves in 2%. The leaves have a rough surface or sharp-edges and the fact they were not chewed and excreted whole indicates they were not ingested for nutritional purposes. Furthermore, this leaf-swallowing was restricted to the rainy season when parasite re-infections are more common, and parasitic worms (Oesophagostomum stephanostomum) were found together with the leaves.
A 2022 study reported that chimpanzees crushed and applied insects to their own wounds and the wounds of other chimpanzees.
Bonobos sometimes swallow non-chewed stem-strips of Manniophyton fulvum. Despite the plant being abundantly available all year, M. fulvum is ingested only at specific times, in small amounts, and by a small proportion of bonobos in each group, demonstrating that it is indeed only utilized when the bonobos are unwell.
A male Sumatran orangutan known to researchers as Rakus "appeared to have used the plant intentionally" when he chewed up leaves of the "antibacterial, anti-inflammatory, anti-fungal, antioxidant, pain-killing and anticarcinogenic" vine Fibraurea tinctoria and applied the masticated plant material to an open wound on his face. According to primatologists who had been observing Rakus at a nature preserve, "Five days later the facial wound was closed, while within a few weeks it had healed, leaving only a small scar".
Monkeys Tamarins were observed swallowing the large seeds of the fruit they regularly ingest. Although they are consumed along with the rest of the fruit, these seeds have no nutritional value for the monkeys. Since tamarins are routinely infected by trematodes, cestodes, nematodes, and acanthocephalans, there is speculation that the deliberate swallowing of these large seeds can help dislodge the parasites from the monkey's body.
Tufted capuchins (Cebus apella) rub various parts of their body with carpenter ants (Camponotus rufipes) or allow the ants to crawl over them, in a behaviour called anting. The capuchins often combine anting with urinating into their hands and mixing the ants with the urine. A female capuchin monkey in captivity was observed using tools covered in a sugar-based syrup to groom her wounds and those of her infant.
A range of primates rub millipedes onto their fur and skin; millipedes contain benzoquinones, compounds known to be potently repellent to insects. As the millipede secretions are also psychoactive, the behavior may also be a form of recreational drug use in animals.
Callicebus oenanthes have been observed rubbing leaves of Piper aduncum on their furs and abdominal areas. Since these leaves contain insecticides like dillapiole and phenylpropanoids, it is speculated that this fur-rubbing is an indication of a preventative measure to ward off insects. Additionally, another species of titi monkeys, Plecturocebus cupreus, were seen rubbing their furs with the leaves of Psychotria, whose compounds have antiviral, antifungal, and analgesic properties.
Anubis baboons (Papio anubis) and hamadryas baboons (Papio hamadryas) in Ethiopia use fruits and leaves of Balanites aegyptiaca to control schistosomiasis. Its fruits contain diosgenin, a hormone precursor that presumably hinders the development of schistosomes.
Bears Similar to the wild chimpanzees, Alaskan brown bears will swallow whole Carex leaves in the springtime to ensure the complete expulsion of parasites during their hibernation. Specifically, as tapeworms thrive off previously digested nutrients in the gut, the rough Carex leaves will lacerate their scolices, facilitating the defecation process. The proactive swallowing of these leaves will ensure low levels of active parasites within a hibernating bear.
North American brown bears (Ursos arctos) make a paste of Osha roots (Ligusticum porteri) and saliva and rub it through their fur to repel insects or soothe bites. This plant, locally known as "bear root", contains 105 active compounds, such as coumarins that may repel insects when topically applied. Navajo Indians are said to have learned to use this root medicinally from the bear for treating stomach aches and infections.
Other Mammals
Indo-Pacific bottlenose dolphins have been observed to swim near and rub themselves against specific types of corals and sponges. A team of scientists followed up on this behavior and discovered metabolites with antibacterial, antioxidative, and hormonal activities in the corals and sponges, suggesting that they might be used by the dolphins to treat skin infections.
African elephants (Loxodonta africana) apparently self-medicate to induce labour by chewing on the leaves of a particular tree from the family Boraginaceae; Kenyan women brew a tea from this tree for the same purpose.
White-nosed coatis (Nasua narica) in Panama take the menthol-scented resin from freshly scraped bark of Trattinnickia aspera (Burseraceae) and vigorously rub it into their own fur or that of other coatis, possibly to kill ectoparasites such as fleas, ticks, and lice, as well as biting insects such as mosquitoes; the resin contains triterpenes α- and β-amyrin, the eudesmane derivative β-selinene, and the sesquiterpene lactone 8β-hydroxyasterolide.
Domestic cats and dogs often select and ingest plant material either to induce vomiting or for anti-parasitic purposes.
Indian wild boars selectively dig up and eat the roots of pigweed which humans use as an anthelmintic. Mexican folklore indicates that pigs eat pomegranate roots because they contain an alkaloid that is toxic to tapeworms.