Although better known for their chemical signals, ants also chirp. They scrape a tiny guitar pick-like appendage on their abdomens against grooved ridges on their posteriors, like a spoon against a washboard. Now researchers have discovered that the sounds allow one kind of ant to distinguish between workers and queens. Some caterpillars can mimic the queen's noises, the research also reveals, granting them food, care, and protection.
Researchers believed the chirps mainly functioned as alarm calls and were not part of normal communication. But the spacing between a queen's grooves is greater than the spacing between a worker's grooves, prompting entomologist Jeremy Thomas of the University of Oxford in the United Kingdom to wonder whether these sounds were a bigger part of ant communication.
Thomas and his team recorded Western European Myrmica schenki ant sounds produced by unstressed ant workers and queens. Acoustic analysis revealed that worker chirps differed from the chirps of queens, their sounds emitting at a lower frequency. When the researchers played recordings of queen ants on tiny speakers, workers became more attentive to the queens' sounds, gathering and sometimes standing guard around the speaker. They relaxed if they heard the sounds of other worker ants, white noise, or silence, the team reports today in Science . The findings show that, for ants, "sound may play a much more important role" than believed, Thomas says, and it will surely prompt others to look for similar behavior in other species.
The team also unearthed evidence that one of M. schenki's natural parasites--the Maculinea rebeli caterpillar--has evolved to use these sounds against it. The caterpillar mimics the ants' smells, allowing it to slip undetected into a colony. Once the caterpillar gets inside, it's cared for like a queen. But smell isn't the reason, Thomas discovered. (Dummy caterpillars coated with ant scent do not get the royal treatment and are even considered inferior to ant larva.) Thomas and colleagues found that sound mimicry was the key: When the team played the caterpillar recordings  on the tiny speakers, the worker ants responded as though they were in the presence of the queen . "It's the first time this type of mimicry has been looked for--or found--in any species of social parasite," says Thomas.
Spencer Behmer, an insect physiologist at Texas A&M University in College Station, says the findings jibe with studies showing that other insects such as jumping spiders and tree hoppers also use noise to communicate. As for the crafty caterpillars, Seán Brady, a curator and entomologist at the Smithsonian Institution's National Museum of Natural History in Washington, D.C., says that the study is "an important step forward in understanding how a bewildering variety of organisms are able to crack the defenses of ant colonies in order to inhabit their nests and steal their resources, in essence achieving free room and board."