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Ants: No Longer the Strong Silent Types

I’ve just been reading a book on katydids, which are insects renowned for their ability to sing. What about ants? Are they the strong silent types?

It turns out that ants can make plenty of music. They can squeak, drum and rattle as well. There is nothing quiet about ants. As Dr. Francesca Barbero of the University of Turin says,  “Sound in information exchange within ant colonies has been greatly underestimated.”

Stridulation

pachycondyla-villosa-stridens2

Detail of the pars stridens (in yellow) on the forth abdominal tergite in a Pachycondyla villosa worker (Scanning Electron Micrograph, Roberto Keller/AMNH)

Ants with underground nests occasionally get buried when a tunnel collapses. Scientists have shown trapped ants of certain species can make sounds by rubbing sections of their rear section or gaster together. On one segment of the gaster there is a patch of tiny ridges like a file. On the petiole is a curved ridge called a scraper. The ant produces a squeaking sound when she rubs them together, which known as stridulation. You can produce a sound in a similar way by rubbing a craft stick across a comb. When other workers detect the sound, they rush to help dig out the trapped ant(s). (Be sure to visit Roberto Keller’s website for more SEM views of the apparatus).

Researchers have been able to record sounds from individual ants. The sounds are in the audible range for humans and can be heard distinctly when amplified. Listen to the sounds of a fire ant stridulating at Stridulation Sounds of Black Fire Ants by Dr. Robert Hickling.

Stridulation has other functions as well. Male and female harvester ants sing to one another as they take off on their mating flights. Mated females also stridulate to signal to pursuing males that they are no longer interested. In other species, foragers may stridulate when they find food to attract help, although they often release pheromones as well.

Leafcutter ants are known to stridulate while cutting pieces of leaf. It appears the noise the cutting ants make attracts other ants to come take the pieces of leaf to carry them back to the nest. Some ant scientists have suggested that the vibrations improve the ants’ ability to cut smoothly through the leaf. Leafcutter ants also produce sounds while building their nests in the soil.

Example video of Acromyrmex ant stridulating

Some other insect species associated with ants also stridulate to attract the ants’ attention. The caterpillars of the beautiful Imperial blue butterfly have “teeth” on their abdomen, which they scrape against a series of grooves to produce grunts and hisses. They can also make a drumming sound. All these different calls seem to be used for different situations, but not all the details are clear yet. What is known is when scientists glued up the noise-producing organs with shellac, the ants took longer to find the caterpillars and spent less time with them than with the ones who had not been silenced.

As I mentioned in the post about blue butterflies, Rebel’s large blue (Maculinea rebeli) larvae have recently been shown to mimic the sounds produced by the queen ants of their hosts to elicit food, care and even rescues, at the expense of the colony’s own offspring. Go to “Caterpillar noise tricks ants into service” article at Science News to actually hear the sounds the caterpillars and ants make.

Drumming
I studied carpenter ants, and whenever I opened a nest, I could hear the ants react. The workers strike their mandibles and gasters on the surface of the tunnels in their wooden nest to create a drumming sound. Ants deeper in the nest rush to the site of the disturbance and assist their sisters with defending the nest.

Rattling
The rattle ants of Australia also tap their gasters when they encounter an enemy intruder, such as a bird feasting on their nestmates. They live in leaf nests high in trees. The leaves may be slightly dry and the tapping produces an audible rattling sound that gives these ants their name.

Rattan ants of Asia live in thorny rattan vines. When an intruder gets too near their nest the ants hit their mandibles against the stem of the plant. Because they hit in a synchronized way, the sound pulses. (I was interested to learn that some katydids also drum or vibrate the plants, especially species that stay hidden deep in foliage. )

Where are the ants’ ears? No one knows for absolutely sure how it all works. Some ants have ways to detect surface vibrations in their legs. It is also likely that certain ants use hair-like sensors in the tips of their antennae. When the hairs are displaced the ants detect signals that are called nearfield. Those are signals from sources that are extremely close. The ants remain completely unaware of sounds produced by far objects, like us shouting at them for example.

In any case, as our human sound equipment becomes refined, I’m sure we will be hearing more from ants.

More sound recordings:

Take a look at how ants respond to a microphone and listen to a recoding by by Adriano Zanni.

Bug Bytes, the USDA Sound library of Richard Mankin, has a number of ant recordings.

References:

Barbero, F., J.A. Thomas, S. Bonelli, E. Balletto, and K. Schönrogge. 2009. Queen Ants Make Distinctive Sounds That Are Mimicked by a Butterfly Social Parasite. Science 323 (5915) 782.

Donato A. Grasso,  Marco Priano,  Gianni Pavan,  Alessandra Mori,  Francesco Le Moli. 2000. Stridulation in four species of Messor ants (Hymenoptera, Formicidae). Italian Journal of Zoology, Volume 67, Issue 3: 281 – 283

Judson, Olivia. 2009. Operator? Can You Put Me Through to Ant Nest 251? New York Times – gives an historical perspective of some of the earlier works on ant sound communication.

Markl, H. 1965. Stridulation in Leaf-Cutting Ants. Science 149 (3690), 1392-1393.

Tautz, J., F. Roces, B. Hölldobler. 1995. Use of a Sound-Based Vibratome by Leaf-Cutting Ants. Science 267 (5194), 84.

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