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. 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 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 etymologyEdit
In 1993, 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.
The anti-parasitic effect of zoopharmacognosy could occur by at least two mechanisms. First, the ingested material may have pharmacological antiparasitic properties such that phytochemicals decrease the ability of worms to attach to the mucosal lining of the intestines, or chemotaxis attracts worms into the folds of leaves. Many ingested plants during purported zoopharmacognosy have a consistent physical property, e.g., the rough surface of the leaves sports many hooked and spiky hairs. So, parasites may became attached to the bristly surface or the coarse structure may function as a rasping plug, dislodging parasites from the intestines. The second possible mode of action is the material may initiate a purging response of the gastrointestinal tract by rapidly inducing diarrhea. This substantially decreases gut transit time, causes worm expulsion and interrupts the life cycle of parasites. This, or a similar, mechanism could explain undigested grass in the faeces of various animals such as birds, carnivores and primates.
Methods of self-medicationEdit
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.
Many examples of zoopharmacognosy involve an animal ingesting a substance with (potential) medicinal properties.
Ants infected with Beauveria bassiana, a fungus, selectively consume harmful substances (reactive oxygen species, ROS) upon exposure to a fungal pathogen, yet avoid these in the absence of infection.
Great apes often consume plants that have no nutritional values but which have beneficial effects on gut acidity or combat intestinal parasitic infection.
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. Jane Goodall witnessed chimpanzees eating particular bushes, apparently to make themselves vomit. There are reports that chimpanzees swallow whole leaves of particular rough-leaved plants such as Aneilema aequinoctiale; these remove parasitic worms from their intestines.
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.
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.
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.
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.
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, apparently to induce vomiting.
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.
A study on domestic sheep (Ovis aries) has provided clear experimental proof of self-medication via individual learning. Lambs in a treatment group were allowed to consume foods and toxins (grain, tannins, oxalic acid) that lead to malaise (negative internal states) and then allowed to eat a substance known to alleviate each malaise (sodium bentonite, polyethylene glycol and dicalcium phosphate, respectively). Control lambs ate the same foods and medicines, but this was disassociated temporally so they did not recuperate from the illness. After the conditioning, lambs were fed grain or food with tannins or oxalates and then allowed to choose the three medicines. The treatment animals preferred to eat the specific compound known to rectify the state of malaise induced by the food previously ingested. However, control animals did not change their pattern of use of the medicines, irrespective of the food consumed before the choice. Other ruminants learn to self-medicate against gastrointestinal parasites by increasing consumption of plant secondary compounds with antiparasitic actions.
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 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.
Many animals eat soil or clay, a behaviour known as geophagy. Clay is the primary ingredient of kaolin. It has been proposed that for primates, which can logically be extended to other animals, there are four hypotheses relating to geophagy in alleviating gastrointestinal disorders or upsets:
- soils adsorb toxins such as phenolics and secondary metabolites
- soil ingestion has an antacid action and adjusts the gut pH
- soils act as an antidiarrhoeal agent
- soils counteract the effects of endoparasites.
Furthermore, two hypotheses pertain to geophagy in supplementing minerals and/or elements:
- soils supplement nutrient-poor diets
- soils provide extra iron at high altitudes
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. Cattle eat clay-rich termite mound soil, which deactivates ingested pathogens or fruit toxins.
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. Great bustards eat blister beetles of the genus Meloe to decrease parasite load in the digestive system; cantharidin, the toxic compound in blister beetles, can kill a great bustard if too many beetles are ingested. Great bustards may eat toxic blister beetles of the genus Meloe to increase the sexual arousal of males.
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.
Absorption and adsorptionEdit
The swallowing of whole leaves by apes without chewing has been observed for over 40 plant species.
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.
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.
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.
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.
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.
More than 200 species of song birds wipe ants perform anting. Birds either grasp ants in their bill and wipe them vigorously along the spine of each feather down to the base, or sometimes roll in ant hills twisting and turning so the ants crawl through their feathers. Birds most commonly use ants that spray formic acid. In laboratory tests, this acid is harmful to feather lice. Its vapour alone can kill them.
Some birds select nesting material rich in anti-microbial agents that may protect themselves and their young from harmful infestations or infections. European starlings (Sturnus vulgaris) preferentially select and line their nests with wild carrot (Daucus carota); chicks from nests lined with this have greater levels of haemoglobin compared to those from nests which are not, although there is no difference in the weight or feather development of the chicks. Laboratory studies show that wild carrot substantially reduces the emergence of the instars of mites. House sparrows (Passer domesticus) have been observed to line their nests with materials from the neem tree (Azadirachta indica) but change to quinine-rich leaves of the Krishnachua tree (Caesalpinia pulcherrima) during an outbreak of malaria; quinine controls the symptoms of malaria.
Zoopharmacognosy is not always exhibited in a way that benefits the individual. Sometimes the target of the medication is the group or the colony.
Wood ants (Formica paralugubris) often incorporate large quantities of solidified conifer resin into their nests. Laboratory studies have shown this resin inhibits the growth of bacteria and fungi in a context mimicking natural conditions. The ants show a strong preference for resin over twigs and stones, which are building materials commonly available in their environment. There is seasonal variation in the foraging of ants: the preference for resin over twigs is more pronounced in spring than in summer, whereas in autumn the ants collect twigs and resin at equal rates. The relative collection rate of resin versus stones does not depend on infection with the entomopathogenic fungus Metarhizium anisopliae in laboratory conditions, indicating the resin collection is prophylactic rather than therapeutic.
Honey bees also incorporate plant-produced resins into their nest architecture, which can reduce chronic elevation of an individual bee's immune response. When colonies of honey bees are challenged with the fungal parasite (Ascophaera apis), the bees increase their resin foraging. Additionally, colonies experimentally enriched with resin have decreased infection intensities of the fungus.
Zoopharmacognosy can be classified depending on the target of the medication. Some animals lay their eggs in such a way that their offspring are the target of the medication.
Adult monarch butterflies preferentially lay their eggs on toxic plants such as milkweed which reduce parasite growth and disease in their offspring caterpillars. This has been termed transgenerational therapeutic medication.
When fruit flies detect the presence of parasitoid wasps, they preferentially lay their eggs in high-ethanol food; this reduces infection risk in their offspring. This has been termed transgenerational prophylaxis.
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