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Zombies April 13, 2011

Posted by Ezra Resnick in Evolution.
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The jungle can be a dangerous place.

Four new species of brain-manipulating fungi that turn ants into “zombies” have been discovered in the Brazilian rain forest.

These fungi control ant behavior with mind-altering chemicals, then kill them…

Once infected by spores, the worker ants, normally dedicated to serving the colony, leave the nest, find a small shrub and start climbing. The fungi directs all ants to the same kind of leaf: about 25 centimeters above the ground and at a precise angle to the sun (though the favored angle varies between fungi)…

Before dying, ants anchor themselves to the leaf, clamping their jaws on the edge or a vein on the underside. The fungi then takes over, turning the ant’s body into a spore-producing factory. It lives off the ant carcass, using it as a platform to launch spores, for up to a year.

Watch this video if you dare:

Such parasitic phenomena are not at all rare in nature. One interesting thing to notice is that the behavior of an infected host appears utterly baffling, until we understand that it is serving the parasite’s interests and not its own. Daniel Dennett opens his book Breaking the Spell: Religion as a Natural Phenomenon using zombie ants as an analogy:

You watch an ant in a meadow, laboriously climbing up a blade of grass, higher and higher until it falls, then climbs again, and again, like Sisyphus rolling his rock, always striving to reach the top. Why is the ant doing this? What benefit is it seeking for itself in this strenuous and unlikely activity? Wrong question, as it turns out. No biological benefit accrues to the ant. It is not trying to get a better view of the territory or seeking food or showing off to a potential mate, for instance. Its brain has been commandeered by a tiny parasite, a lancet fluke (Dicrocelium dendriticum), that needs to get itself into the stomach of a sheep or a cow in order to complete its reproductive cycle. This little brain worm is driving the ant into position to benefit its progeny, not the ant’s. This is not an isolated phenomenon. Similarly manipulative parasites infect fish, and mice, among other species. These hitchhikers cause their hosts to behave in unlikely—even suicidal—ways, all for the benefit of the guest, not the host.

Does anything like this ever happen with human beings? Yes indeed. We often find human beings setting aside their personal interests, their health, their chances to have children, and devoting their entire lives to furthering the interests of an idea that has lodged in their brains.

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