Ants sense each other's whispers with their antennae, researchers propose in the Journal of the Acoustical Society of America1. This insight into ant 'hearing' could help scientists use sound to protect crops against ant infestations.

Although ants communicate mainly by exuding chemical signals -- 'smells' -- they also exchange sounds that are barely audible to the human ear2. They produce them by rubbing one part of their bodies against another, an activity called 'stridulation'.

Because ants seem to be entirely unresponsive to normal noise, it has been assumed that they detect, not the sound waves in air, but the vibrations that stridulation creates and transmits through the ground.

Not so, say acoustics expert Robert Hickling of the University of Mississippi and ant biologist Richard Brown of Mississippi State University. They have studied stridulation in the black imported fire ant Solenopsis richteri, and their findings challenge conventional thinking on ant conversation.

Ants stridulate just as skiffle musicians play their washboards: by rubbing a set of ridges attached to the posterior abdomen. Hickling and Brown find that the insects can thus create a range of different sounds that convey different messages such as 'attack!' or 'watch out!' or 'I'm distressed!'. (Ant talk is invariably exclamatory.)

One of the reasons that previous researchers believed that ants sense stridulation through the ground rather than the air is that ants trapped under subsided soil can stridulate to bring others to the rescue. But soil is full of pockets of air, Hickling and Brown point out, so the sound could quite easily escape without being transmitted through vibrating soil grains.

The researchers also note that many insects detect sound waves in air by using hairlike sense organs called trichoid sensilla, rather like the sound sensors in our ears. It seems likely that ants do the same, as their antennae are covered with such hairs. Moreover, it would be hard for an ant to adapt its stridulation to transmission through the wide range of materials it encounters -- leaves, wood, soil, animal tissue -- rather than simply using the air in every case.

But if ants do indeed send sound signals through the air, why do they apparently not hear much louder sounds? Hickling and Brown suggest that they don't sense propagating sound waves, but instead the 'near-field' sound pressure that radiates very close to a sound source. In this region the air is being moved directly by the vibrating source, rather than by the pressure of the adjacent peaks of a moving sound wave.

In the near field, which extends perhaps ten centimetres or so from a stridulating ant, the speed of the sound impulse falls rapidly from a high value very close to the source to the lower value that is characteristic of normal sound waves in air. Hickling and Brown say that this sharp change in sound speed might be easier for another ant's trichoid sensilla to detect than is a normal sound wave moving at constant speed, which needs more sensitive aparatus.

They propose that an ant might in fact not be detecting the pressure of the moving air at all (as our ears do), but instead the difference in sound speed at each of its two antennae. This difference is appreciable in the near field, even for antenna tips spaced just a few millimetres apart; but it falls rapidly to zero beyond the near-field region. Sensing sound in this way would give ants excellent hearing for sound sources close to them (such as other ants), while making them deaf to more distant sounds, however loud.

The researchers point out that the imported fire ant, so called because it invaded the USA from South America, causes widespread damage to animal and plant life. They hope that, by learning how these insects communicate, it might become possible to control them by non-chemical methods.