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Zoopharmacognosy is a behaviour in which non-human animals apparently self-medicate by selecting and ingesting or topically applying plants, soils, insects, and psychoactive drugs to prevent or reduce the harmful effects of pathogens and toxins.^[1]^[2] The term derives from Greek roots zoo ("animal"), pharma ("drug"), and gnosy ("knowing").

A well-known example of zoopharmacognosy occurs when dogs eat grass to induce vomiting. However, the behaviour is more diverse than this. Animals ingest non-foods such as clay, charcoal and even toxic plants, apparently to prevent parasitic infestation or poisoning.^[3] Self-medication in wild animals remains a controversial subject because evidence is mostly circumstantial or anecdotal,^[4] however, there are many purported examples.^[5] 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.^[6]

History and etymology

In 1978, Janzen suggested that vertebrate herbivores might benefit medicinally from the secondary metabolites in their plant food.^[7]

In 1993, the term "zoopharmacognosy" was coined, derived from the Greek roots zoo ("animal"), pharma ("drug"), and gnosy ("knowing").^[8] The term gained popularity from academic works^[4] and in a book by Cindy Engel entitled Wild Health: How Animals Keep Themselves Well and What We Can Learn from Them.^[9]

Methods of self-medication

Some animals ingest or apply the substance when they appear to be well, suggesting the behaviour is preventative or prophylactic. In other cases, animals ingest or apply the substance when unwell, suggesting the behaviour is therapeutic or curative. There are three methods of self-medication, namely, ingestion, absorption, or topical application.

Ingestion

Many examples of zoopharmacognosy involve an animal ingesting a substance with (potential) medicinal properties.

Mammals

Silky sifaka (Propithecus candidus) eating earth (geophagy)

Great apes often consume plants that have no nutritional values but which have beneficial effects on gut acidity or combat intestinal parasitic infection.^[1]

Chimpanzees sometimes select bitter leaves for chewing. Parasite infection drops noticeably after chimpanzees chew leaves of pith (Vernonia amyddalina), which have anti-parasitic activity against schistosoma, plasmodium and Leishmania. Chimpanzees don't consume this plant on a regular basis, but when they do eat it, it is often in small amounts by individuals that appear ill.^[11] Jane Goodall witnessed chimpanzees eating particular bushes, apparently to make themselves vomit.^{[citation needed]} There are reports that chimpanzees swallow whole leaves of particular rough-leaved plants such as Aneilema aequinoctiale; these remove parasitic worms from their intestines.^[12]

Chimpanzees, bonobos, and gorillas eat the fruits of Aframomum angustifolium. Laboratory assays of homogenized fruit and seed extracts show significant anti-microbial activity.^[5] 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.^[13]

Anubis baboons (Papio anubis) and hamadryas baboons (Papio hamadryas) in Ethiopia use fruits and leaves of Balanites aegyptiaca to control schistosomiasis.^[14] Its fruits contain diosgenin, a hormone precursor that presumably hinders the development of schistosomes.^[4]

African elephants (Loxodonta africana) apparently self-medicate to induce birth by chewing on the leaves of a particular tree from the family Boraginaceae; Kenyan women brew a tea from this tree to induce childbirth.^[15]

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;^[16] the resin contains triterpenes α - and β-amyrin, the eudesmane derivative β-selinene, and the sesquiterpene lactone 8β-hydroxyasterolide.^[5]

Domestic cats and dogs often select and ingest plant material, apparently to induce vomiting.^[17]

Ruminants appear to learn to self-medicate against gastrointestinal parasites by increasing consumption of plant secondary compounds with antiparasitic actions. This selective feeding improves health and fitness.^[10] Cattle eat clay-rich termite mound soil, which deactivates ingested pathogens or fruit toxins.^[1]

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.^[18]

Standard laboratory cages prevent mice from performing several natural behaviours for which they are highly motivated. As a consequence, laboratory mice sometimes develop abnormal behaviours indicative of emotional disorders such as depression and anxiety. To improve welfare, these cages are sometimes enriched with items such as nesting material, shelters and running wheels. Sherwin and Olsson^[19] tested whether such enrichment influenced the consumption of Midazolam, a drug widely used to treat anxiety in humans. Mice in standard cages, standard cages but with unpredictable husbandry, or enriched cages, were given a choice of drinking either non-drugged water or a solution of the Midazolam. Mice in the standard and unpredictable cages drank a greater proportion of the anxiolytic solution than mice from enriched cages, presumably because they had been experiencing greater anxiety. Early studies indicated that autoimmune (MRL/lpr) mice readily consume solutions with cyclophosphamide, an immunosuppressive drug that prevents inflammatory damage to internal organs. However, further studies provided contradictory evidence.^[1]

Several animals eat earth, a behavior known as geophagy. Tapirs, forest elephants, colobus monkeys, mountain gorillas and chimpanzees seek out and eat clay, which absorbs intestinal bacteria and their toxins and alleviates stomach upset and diarrhoea.^[20]

Birds

Many parrot species in the Americas, Africa, and Papua New Guinea consume kaolin or clay, which both releases minerals and absorbs toxic compounds from the gut.^[21] Great bustards eat blister beetles of the genus Meloe to decrease parasite load in the digestive system;^[22] cantharidin, the toxic compound in blister beetles, can kill a great bustard if too many beetles are ingested.^[23]

Invertebrates

Woolly bear caterpillars (Grammia incorrupta) are sometimes lethally endoparasitised by tachinid flies. The caterpillars ingest plant toxins called pyrrolizidine alkaloids, which improve the survival of by conferring resistance against the flies. Crucially, parasitised caterpillars are more likely than non-parasitised caterpillars to specifically ingest large amounts of pyrrolizidine alkaloids, and excessive ingestion of these toxins reduces the survival of non-parasitised caterpillars. These three findings are all consistent with the adaptive plasticity theory.^[7]

The tobacco hornworm ingests nicotine which reduces colony growth and toxicity of Bacillus thuringiensis, leading to increased survival of the hornworm.^[24]

Absorption

Wild chimpanzees sometimes seek whole leaves of the Aspilia plant. These contain thiarubrine-A, a chemical active against intestinal nematode parasites, however, it is quickly broken-down by the stomach. The chimpanzees pick 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. As many as 15 to 35 Aspilia leaves may be used in each bout of this behaviour, particularly in the rainy season when there are many parasitic larvae leading to an increased risk of infection.^[24]

Topical application

Some animals apply substances with medicinal properties to their skin. Again, this can be prophylactic or curative. In some cases, this is known as self-anointing.