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Animal Mind Control
Examples of parasites that manipulate the behavior of their hosts are not hard to come by, but scientists have only recently begun to understand how they induce such dramatic changes.
A normally insatiable caterpillar suddenly stops eating. A quick look inside its body reveals the reason: dozens of little wasp larvae gnawing and secreting digestive enzymes to penetrate its body wall. They have been living inside the caterpillar for days—like little vampires, feeding on its “blood”—and are finally making their exodus to build their cocoons on its bright-green exterior.
In the caterpillar’s brain, a massive immune reaction is taking place—the invertebrate equivalent of a cytokine storm—and among the factors being released is an invertebrate neurohormone called octopamine. “It’s a very important compound for controlling behavior in insects,” says invertebrate behavioral physiologist Shelley Adamo of Dalhousie University in Halifax, Nova Scotia. “Octopamine levels go up, and that plays a role in shutting off feeding.”
But the parasitic larvae don’t stop there. They also inhibit the host’s ability to break down the substance. “Octopamine levels remain high for days, and this caterpillar never really eats again,” Adamo explains. “Basically, it starves to death.” This plays the important role of preventing the caterpillar from picking off the cocoons, one by one, and eating the metamorphosing larvae alive. Simply killing their host isn’t an option, Adamo says, because if the caterpillar dies, its body will become overrun with fungal pathogens—unwelcome visitors to a wasp nursery. Plus, non-eating caterpillars retain their defensive reflexes, which protect both them and the young wasps from arthropod predators. “They’ve turned their host from being a meal ticket [into] their bodyguard,” Adamo says.