Insect eats to fight what ails it
When the Arizona tiger moth caterpillar gets a bug, the medicine it needs suddenly tastes good.
When the caterpillar is infected with potentially lethal parasites, it develops a hankering for plants that contain a toxin that kills the parasites, scientists at Wesleyan University and the University of Arizona reported last week in the journal Nature.
“It’s like if you got sick and penicillin started to taste good,” said Elizabeth A. Bernays, an entomologist and professor of neurobiology at the University of Arizona.
Finding an organism that seeks health cures by changing its own physiology was a total surprise, the researchers said. Could similar types of chemically induced, self-medicating behavior happen in other species, such as humans?
“We don’t know because nobody has ever looked,” Bernays said.
It is clear the tiger moth caterpillars did not rely on their brain or other tiger moth caterpillars to figure out which exotic plants contain the parasite-killing chemicals, said Michael Singer, an assistant professor of biology at Wesleyan.
“Even among insects, they are not that bright,” said Singer, adding that they are also loners. “They are essentially a big gut. There isn’t a lot of room for brain power.”
Yet when they are attacked by parasites that infest the grasslands and savanna of southern Arizona, the caterpillars defend themselves quite effectively. The fuzzy black-and-orange creatures develop a bigger appetite for plants that contain parasite-killing chemicals called pyrrolizidine alkaloids.
The reason for the change in the caterpillars’ eating habits lies in their taste buds, or what scientists call the gustatory cells.
The researchers measured how rapidly receptors on the caterpillars’ taste buds fired in the presence of different foods. They found that receptors of infected caterpillars fired much more rapidly when exposed to the parasite-killing compounds than the receptors of noninfected caterpillars.
In nature, infected caterpillars get the munchies for Senecio longilobus, also known as threadleaf groundsel, a shrubby member of the sunflower family.
Studying the biological basis for feeding can be vexing. Scientists generally agree that most animals inherit a predisposition for nutritious foods. But Singer said plants that contain the poisonous alkaloids are not as nutritious as other plants in the tiger moth caterpillar’s habitat.
For many animals, medically advantageous eating behavior can be learned. Chimpanzees, for instance, seem to have a preference for the stems of certain types of plants that may provide some defenses against illness.
But Singer said he didn’t know whether chimps have the same sort of taste bud defense system as the caterpillars.
“Having found it in a caterpillar may make people look elsewhere,” Bernays said. “It may be one of those weird things that happen once in nature, or it may be more common than we know.”
