We all admire animals that can move with ease between air and water, but how do these creatures cope with the challenge of interpreting sounds transmitted through different media? Sound is a key sense in many animals, yet relatively little is known about how hearing sensitivity differs between environments. D. Higgs and his colleagues at the University of Maryland wondered how sensitive alligators are to sounds in water and air, and discovered that alligators hear well in both environments.

The ultimate response to all sound, whether detected in air or water, is driven by the stimulation of sensory hair cells in the inner ear, and creatures that are sensitive to airborne sounds process the sensory signals in the brain stem region of the brain. But how the sound is transmitted to the inner ear depends on the animal's situation. For example, humans are acutely sensitive to airborne sounds, but when submerged, noises that are carried through the water are deadened as they are transmitted to the inner ear through the skull.

However, alligators are perfectly happy lounging on riverbanks or skulking in water. Higgs explains that the alligator's ear and brain structures are perfectly adapted for hearing airborne sounds. But they spend significant amounts of time immersed in murky waters. Does this mean that their senses are equally sharp in both environments?

The scientists tested the hearing of eight young alligators, above and below the surface. They played the reptiles a series of tone pips that ranged in pitch from 100 to 8000 Hz, and tested the animal's sensitivity at each pitch by increasing the volume. The team measured the animal's responses to the sounds they heard by recording the neurological signals from the inner ear through electrodes placed beneath the reptile's skin.

Not surprisingly, the alligator's hearing in air was as good as most air-adapted species, including their close relatives, birds. But how well did the animal's respond to water-borne sounds? Amazingly the alligator's hearing under water was as good as that of goldfish, who are real hearing specialists amongst fish! Although the reptiles heard over a greater range in air than in water, peak sensitivities were around 800 Hz in both environments — these peaks correspond quite well to the range of “chirping” sounds made by hatchling alligators. Since these animals have no obvious specialisation for channelling underwater sound, it's likely that they hear underwater by conducting sound information to their ears through their skull bones. Whilst this possibility remains untested, Higgs' study successfully shows that alligators have managed to overcome the problems associated with hearing in two different media. This success may be worth remembering if you ever holiday in the Everglades!

Higgs, D. M., Brittan-Powell, E. F., Soares, D., Souza, M. J., Carr, C. E., Dooling, R. J. and Popper, A. N. (
2002
). Amphibious auditory responses of the American alligator (Alligator mississippiensis).
J. Comp. Physiol. A
188
,
217
-223.