Caterpillars make noise to fend off intruders, researchers discover

September 28, 2001

CHAMPAIGN, Ill. -- Caterpillars defend their homes by drumming up vibrations with their mandibles to drive intruders away, scientists say. At times, the nest-owner and intruder engage in duels that create a symphony of drum-like sounds.

These sounds made by caterpillars - in this case the common hook-tip moth (Drepana arcuata) - were not new to scientists. As a specific behavioral activity, however, "this was unheard of," said Patrick J. Weatherhead, a professor of animal biology at the University of Illinois. "When this was called to my attention, it appeared to be similar to a vertebrate model of territoriality. We didn't anticipate that such a behavior occurred in organisms such as these."

The research appeared Sept. 25 in the Proceedings of the National Academy of Science. (Three audio-video clips are at Co-authors were Jayne E. Yack and Myron L. Smith, biologists at Carleton University in Ottawa, Canada, and Weatherhead.

Yack, the lead author, initiated the study after investigating a tapping sound coming from where she was raising larvae in her home late one night. She had found that the sound was coming from two caterpillars on the same leaf, raising a question as to the purpose of the sound.

In the laboratory, the researchers conducted 53 trials in which they placed a second caterpillar onto an already occupied leaf where a nest of silk had been constructed by the resident to protect it while it ate. The intruding caterpillar approached the resident, who stopped feeding, backed into its nest and began signaling by dragging or striking its mandibles against the leaf. Three distinct signals of engagement ensued. These interactions were recorded and the involved body parts were identified.

Resident caterpillars turned away the intruder in 87 percent of the tests, usually within one to five minutes. Intruders that were larger than the residents won 7.5 percent of the time. Three times, the intruder and resident both stayed; the intruder simply built a new nest on the same leaf.

In 39 percent of the experiments, both the resident and intruder produced the drumming sound, creating what the researchers called acoustic duels. These duels occurred in extended confrontations that frequently ended with intruders damaging the residents' nests by biting through the silk strands.

In follow-up tests, displaced residents, when returned to the leaf where their nests had been taken over, took on the role of invader, with the new occupant defending the nest as its own.

"We found that the caterpillars only do this communication when they are in the nest," Weatherhead said. The interaction, he added, suggests they are sizing up each other, and that the sounds may serve to attract predators, in which case the invader would be more vulnerable.
Canada's Natural Sciences and Engineering Research Council and the University of Illinois funded the research. The experiments were conducted in Canada.

University of Illinois at Urbana-Champaign

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