By Rohini Rangarajan
Published: Friday 15 May 2009

GARY JAMESIF YOU see a sick dog eat grass, you will know what self-medication in animals is all about. Many animals tend to increase their intake of certain food items when they are unwell. African chimpanzees, when infected by parasites, have been observed to drink the bitter sap of the young leaves of the herb, Vernonia amygdalina, and get better in a few hours. Such behavioural patterns have even been incorporated in our medical practices. There are many such studies on animals turning to plants for selfmedication. A study for the first time reports this in insects. Michael Singer from Wesleyan University in Connecticut, US, and other researchers manipulated the foraging habits of woolly bear caterpillars (Grammia incorrupta), commonly found in North America, in response to lethal flies. The flies lay their eggs inside the caterpillars. Once the eggs hatch, the larvae eat their way out of the host. As a defence against these flies, the woolly bear caterpillars eat the bitter tasting leaves of plants like the common ragwort (Senecio sp.). The plants have alkaloids that kill the fly larvae. When the caterpillars were not provided with any other food, they ate these leaves in very low amounts. This intake automatically increased by 111 per cent when they were parasitized. The alkaloids greatly reduced mortality but the defence came at the cost of decreased growth efficiency suggesting a kind of fitness trade-off. Michael Huffman, associate professor, Primate Research Institute, Kyoto University, Japan, said, “When infested with parasitoids, geneti - cally triggered changes in taste preference apparently direct woolly bear caterpillars to prefer bitter leaves.” Such behaviour in insects challenges the view that self-medication is restricted to animals with advanced abilities of recognition. While insects have a simpler diet of one or two plants, higher animals, like chimpanzees, need to evolve further in recognizing the appropriate plants that should be ingested. The study was reported in the March issue of PLoS One.

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