Ants raise fungal crops, herd aphids, and wage war, all of which requires efficient communication. Entomologists know that ants relay information to one another with chemicals called pheromones. But the chemical grapevine may not be the preferred medium for urgent communication, says acoustical engineer Robert Hickling of the University of Mississippi. His experiments with fire ants suggest that the ant hot line for distress or danger may be acoustic.
It’s well known that ants generate faint sounds by rubbing a small appendage called a gaster up and down over ridges on their backs, and that other ants detect those vibrations with their sensitive bodies. (Ants have no ears.) But there has not been enough research into the role these signals play in ant behavior, says Bradleigh Vinson, an entomologist at Texas A&M; familiar with Hickling’s work. Bob’s behavioral observations have certainly stirred up interest, says Vinson, especially because his acoustic equipment is so sensitive.
Hickling has been studying ways to control fire ants--an aggressive South American species, accidentally introduced into this country, that has spread throughout the South. One day, out of sheer curiosity, he inserted a small microphone into a fire ant mound and listened. It wasn’t activity sounds that we heard--you know, ants running up and down, says Hickling. Instead we heard agitated stridulation-- scraping--like a general alarm. Immediately they attacked the probe.
Hickling placed some of the ants in a plastic box with acoustic sensors on the bottom to measure the frequency of their noises. At the same time, he monitored the ants with a video camera. At first, he says, they were pretty quiet. Then he put a caterpillar into their box. The ants put gaster to ridges for a long low-frequency signal, then set upon their prey. This attack call, says Hickling, was similar to the alarm that had sounded when the probe penetrated the mound.
Later, when one ant got an antenna caught between the wall and the lid of the box, Hickling says, I could actually see it moving its gaster up and down, and the sound matched the video image. Other ants quickly freed their nest-mate. Hickling concludes that the trapped ant had put out a distress signal and that the other ants had responded.
Although Hickling can’t be sure the ants in any of his experiments weren’t emitting pheromones as well, he points out that it would make sense for them to favor acoustic communication when time is of the essence. Because sound is faster than pheromone molecules diffusing through the air, he says, the ants may rely on it in urgent situations.