Self-medication among animals is sufficiently well studied to have a term of its own: zoopharmacognosy. Until now, such behaviour has mainly been observed among higher primates. Chimpanzees with intestinal parasites may seek out and eat shoots that are not part of their normal diet – indeed, that are toxic and make them look like they had just bitten deep in a slice of lemon. Sometimes, the same plants are used by local humans in traditional medicine.
Cool, right? But hardly rigorous. What we would really like to see in order to have good proof for the concept is an example of an animal eating a foodstuff whose medical properties can be proven, and furthermore only eating it in times of illness, and preferably suffering negative effects if they eat it when they are healthy.
Such a study has been published for the first time today, with a very surprising animal species as the protagonist: the woolly bear caterpillar. This is the larval stage of a moth of the Arctiidae family, which is apparently well known for its habits of communicating by ultrasonic sound waves, generated from a specialised organ.
Michael Singer and his co-authors have infected woolly bear caterpillars with an intestinal parasite and supplemented their diet with plants that contain substances called pyrrolizidine alkaloids (PA:s), but that are very nutrient-poor.
They found that infected larvae fared better if they ate the PA-containing plant-stuff, whereas uninfected larvae fared worse. Fewer parasites survived to reach maturity if they had access to PA. As a group, the caterpillars did not eat significantly more of the PA-food when they were infected, but there was a suggestive trend where the larvae with a greater infection load ate more PA.
In this case, the behaviour is clearly not socially learned, as it may be in the apes, but somehow coded into the neurological hardware of the caterpillar. Or rather, a subset of the caterpillars. If the parasites ceased to exist, the subpopulation most likely to eat PA-containing plants would be at a disadvantage since PA is toxic. Here is another instance of evolutionary dynamics creating a phenotype that is only advantageous in the face of an external threat – just like sickle-cell anemia in humans, which confers a measure of protection against malaria, and many other genetic diseases.
A 2005 Nature paper by the same authors showed that the peripheral taste nerve cells actually change their signalling in infected caterpillars and fire off more signals in response to PA. They speculate that the immunological response to infection is the direct cause of this change, which doesn’t even need to involve the neural cluster that resembles the caterpillar’s brain. From the perspective of information processing, this paper is a small triumph for those who try to find complex behaviours in relatively simple biological organisms.
Singer, M., Mace, K., & Bernays, E. (2009). Self-Medication as Adaptive Plasticity: Increased Ingestion of Plant Toxins by Parasitized Caterpillars PLoS ONE, 4 (3) DOI: 10.1371/journal.pone.0004796