Self-medication in a caterpillar

ResearchBlogging.orgSelf-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.

A woolly bear caterpillar. Image from Wikipedia. NB: probably not the exact same species as in the study in question

A woolly bear caterpillar. Image from Wikipedia. NB: probably not the exact same species as in the study in question

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

UPDATE:
BOth Carl Zimmer at The Loom and Ed Yong at Not Exactly Rocket Science have written excellent posts on the same research.

5 Responses to Self-medication in a caterpillar

  1. zeitgeiber says:

    Thanks for the fascinating post! The problem of how evolution ever encoded a self-medicating behavior into a brain this tiny is fascinating (not to mention the mechanics of the genes that must encode for this behavior)… it truly boggles the mind.

  2. M.o.M. says:

    Interesteing!!
    (The chimpanzee researchers examined the stools of the chimpanzees in order to find out if the chimpanzees eat the unappetizing leaves mainly when infected by intestinal parasites, which they do, and also tested extracts from the plants on the parasites in vitro to see if the plants were in fast toxic to the parasites, which they were…)
    Really interesting that self-medication seems to have such a long evolutionary history! Now that it has been found in caterpillars (if the findings are correct) more examples will probably be found in the near future, and also maybe a pharmacognosy area in the brain which can become dysfunctional, leading to a new diagnosis: dyspharmacognosia, in which unsuitable remedies are used…

  3. evolvingideas says:

    Thanks!

    I tried to look up some of the original research on the chimps, but it’s not on PubMed. Now I tried again with Google Scholar, and found some interesting stuff. Kenneth E. Glander has written a good review (http://www.biology.duke.edu/upe302/pdf%20files/glander1994.pdf). He writes that the toxicity of plants to parasites was confirmed in vitro, but only at doses that could not be tolerated by animals in vivo. Michael A. Huffmann writes in a more recent review (http://fusion.sas.upenn.edu/caterpillar/files/related/Huffman_apemed2001.pdf) about attempts to measure the parasite load in self-medicating chimps, but it seems that only one single case has been thoroughly invesigated so far.

    Dyspharmacognosia is a fantastic idea! If you could cure it you would put the homeopaths out of their jobs in an instant…!

  4. watcat says:

    Hi this blog is great I will be recommending it to friends.

  5. M.o.M. says:

    I must have misremembered Huffmans study…

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