Caterpillars make noise to threaten intruders

WASHINGTON -- When an outsider invades their territory, these animals stomp, sweep their heads back and forth and make loud tapping noises that declare: "This is my space! Back off!"

Sometimes they even butt heads with interlopers.

It sounds like bison or elephant seals or other feisty creatures protecting their homes, but scientists were surprised to see this same behavior in, of all things, caterpillars.

In territorial confrontations among a common type of moth caterpillar, the resident usually is able to chase off the stranger, although not always.

"Generally within a few minutes a decision has been made. It's fun to watch them," said Jayne Yack of Carlton University in Ottawa, Canada.

This is the first indication that caterpillars use sound vibrations to communicate, according to the research team led by Yack, a visiting researcher at Cornell University in Ithaca, N.Y.

In a different light

David Wagner, a moth and butterfly expert at the University of Connecticut, said the report "makes you marvel at the complexities of the natural world."

"When you think about caterpillars growling at each other it makes you regard them in an entirely different light," he said.

The researchers "convincingly demonstrate that these creatures are territorial," he said.

Yack said she was raising the caterpillars for other studies and began noticing that they made noise. She discovered that sounds produced by caterpillars were reported previously, but nobody had investigated a reason for the noise.

The caterpillars make sounds by scraping on leaves with special stiff "oars" on their rear ends and drumming and scraping with their jaws.

Her team conducted 53 experiments, using Drepana arcuata, the common hook-tip caterpillar, a moth widespread in the Northeast.

Caterpillars placed on a leaf set up housekeeping, nibbling the leaf edge and spinning a nest to protect themselves from weather and predators.

The arrival of another caterpillar prompted the resident to stop eating, back into its nest and begin scraping the leaf with its "oars."

It then drummed with the jaws, one to eight taps per oar scrape, and, finally, scraped with the jaws.

The sounds could be heard by humans 10 feet away.

'Acoustic duels'

Caterpillars don't have eardrums, Yack explained, but seem to sense the vibrations through the leaf.

The original leaf owner always started signaling first, speeding up as the outsider neared. In almost 90 percent of the cases, the intruder backed off fairly quickly.

Sometimes the intruder signaled back, resulting in what the team called "acoustic duels," that could last hours.

In a few encounters, especially if the outsider was bigger, the intruder was able to take over the nest.

"Close approaches occasionally resulted in head butting," the team reported. They did not see any cases of caterpillars biting one another, but sometimes an intruder would damage the resident's nest by biting the silk threads.