Can animals actively seek out a plant to heal a parasitic infection or to numb pain? For years scientists have presumed that such activity was restricted to animals capable of more complex cognitive abilities, like chimpanzees. However recently scientists have described incidents of fruit flies (1), caterpillars (2) and butterflies (3) “self medicating” to prevent or cure parasitic infections, suggesting that such behavior may be both innate as well as learned.
As early as 1978, D.H. Janzen pointed out that many animals either avoid specific plants either innately or by learning because of their effects on fitness, and he asked the question, “is it possible that animals seek out plant parts by way of writing their own prescriptions?” (4). He described stories of elephants eating a particular legume, colobus monkeys in Kibale forests with low numbers of parasites, and pigs eating pomegranate roots, presumably for anti-helminthic effects.
During a 1987 expedition, Michael Huffman, a primate researcher at Kyoto University, noticed a mother chimp that was lying ill on a bed of branches while he was out on a field study. Eventually the mother chimp climbed to the ground and deliberately walked to a shrub known to the local tribe as “Mjonso”. The chimpanzee tore off several branches, removed the bark, chewed on the inner portion of the branches and sucked the juice. This behavior was unusual. In years of observing chimpanzees, Huffman had never seen a chimp eat this particular plant. The tribe member accompanying Huffman on the exhibition told Huffman that this shrub was used by the tribe for medicine, for stomachaches, malarial fevers, and intestinal parasitic infections. Huffman quickly realized that he may have just witnessed an animal “self medicating”, and further studies in the field allowed him to observe other chimps using the same plant to cure themselves of ailments (5).
Evidence of self medication and medication of others among insect populations has been more recent and includes examples of prophylactic treatments (reviewed in 6). Kacsho and colleagues published a study in 2013 describing a behavioral immune response in which fruit flies prophylactically “treat” the next generation in response to the presence of female parasitic wasps (1). In the presence of female wasps (but not males or pupae) the fruit fly will preferentially lay eggs in an area of higher ethanol concentrations. Interestingly, this group was able to identify a couple of proteins involved in the process: one of which is a regulator of the alcohol dehydrogenase gene, the transcription factor Adf1. Adf1 function is required for long term memory as well. Flies expressing a mutant form of Adf1 that affects long term memory, “forgot” seeing female parasitic wasps when they were transferred to wasp-free cages, and did not lay eggs on ethanol-containing food, unlike wild type flies that were able to “remember” exposure to the female wasps. The authors conclude that a single protein may be involved in long term memory formation and regulating tolerance to ethanol, a compound which normally reduces fitness, but protects against parasitic wasp infections (1). These may be the first pieces of a behavioral immune response pathway.
Is there anything to be gleaned from learning about self-medication in chimpanzees and fruit flies?
As Janzen also observed in his 1978 report:
“The plant world is not colored green; it is colored morphine, caffeine, tannin, phenol, terpene…” (4)
Perhaps there is tremendous value in ganing an increased understanding of the our world by looking at “what is in a tree from the view point of a monkey, sloth or koala” (4). We can learn more about the many and varied compounds produced by plant species and their effects on parasites, protozoans, and microbes by observing how these “animal pharmacists” use them. We may how to more effectively manage livestock, since access to plants that contain antiparasitic compounds can improve livestock health (5, 7,8). We may also learn more about bee decline and colony health (9). And, since some of the more recent studies are describing the molecular mechanisms and cellular signaling underlying the behavioral immune responses that drive these self-medication activities, we may also learn more about how the genes and proteins within our own cells interact, allowing us to gain a better understand human biology.
- Balint, Z. et al. (2013) Fruit flies medicate offspring after seeing parasites. Science 339, 947–950.
- Singer, M.S., Mace, K.C. and Bernays, E.A. et al. (2009) Self-medication as adaptive plasticity: increased ingestion of plant toxins by parasitized caterpillars. PLOS One 4, e4796.
- Lefèvre, T. et al. (2010) Evidence for transgenerational medication in nature. Ecol. Lett. 13, 1485.
- Janzen, D.H. (1978) Complications in interpreting the chemical defenses of tress against tropical arboreal plant-eating vertebrates. IN: The Ecology of Arboreal Folivores Montgomerie, G.C., ed. (Smithsonian Institution Press, Washington, D.C.)
- Oosthoek, S. (2014). Wild medicine Student Science. https://student.societyforscience.org/wild-medicine.
- de Roode, J.C., Lefèvre, T. and Hunter, M.D. (2013) Self-Medication in Animals. Science 340, 150–151.
- Amit, M. et al. (2013) Self-medication with tannin-rich browse in goats infected with gastro-intestinal nematodes. Vet. Parasitol. 198, 305–11.
- Villalba J.J. and Provenza F.D. (2007) Self-medication and homeostatic behaviour in herbivores: learning about the benefits of nature’s pharmacy. Animal 9, 1360–70.
- Simone-Frinstrom, M.D. and Spivak, M. (2012) Increased resin collection after parasite challenge: a case of self medication in honey bees? PLOS One 7, e34601.
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