Alligators (Alligator mississippiensis) are the most vocal non-avian reptiles, producing a variety of hisses, grunts and growls. Vocalizations are not limited to alligators; crocodiles, gharials and caimans are vocal as well. While most alligators direct most of their vocalizations at other alligators, hissing is a territorial or defensive behavior, used to repel conspecifics or dissuade perceived predators. Alligators expel air through their mouths when hissing, and often do so while adopting an aggressive posture.
Everyone is Doing It
Hissing is the most primitive vocalization that alligators produce; representatives from virtually all reptile lineages -- including turtles, snakes, lizards and birds -- hiss when confronted by a predator. Alligators can hiss while in the water or land, with their mouths open or closed. Sometimes, alligators will accompany hisses with exaggerated postures, jaw snapping or aggressive charges.
Alligators Play Marco Polo
Herpetologists have not studied the mechanisms by which alligators and other crocodilians generate their vocalizations in detail -- most studies have concentrated on the roles the vocalizations play in intraspecific communication. According to a 1977 study by biologists Leslie D. Garrick and Jeffrey W. Lang, vocal communication is more important to the breeding behavior of alligators than it is to that of other crocodilians. In contrast to the other crocodilians that inhabit wide-open habitats and form large colonies, alligators inhabit visually obstructed habitats -- marshes and swamps as opposed to large rivers and estuaries -- and do not form dense colonies. In such habitats, vocal communication is highly effective.
The Long and Short of It
Biologist Tobias Riede and colleagues from the University of Utah and Ritsumeikan University Department of Micro System Technology investigated how juvenile alligators produce sound. Publishing their results in a 2011 issue of “Journal of Experimental Biology,” the anatomical findings were surprising: alligators are unable to lengthen or shorten their vocal cords. This should prevent them from changing the frequency of their vocalizations, yet they manage to change the pitch of their vocalizations by changing the air pressure inside the trachea and flexing their vocal cords.
The "How" of the Hiss
Alligators have several anatomical features involved in sound production. In addition to their lungs, where sound production begins, and their mouths, where the sound ultimately emerges, alligators involve their bronchial tubes, larynxes and glottises to manipulate and transport air from the lungs to the mouth. The larynx contains the vocal cords and is used to change the sounds of grunts and bellows. The glottis is the muscular opening of the trachea and lies in the mouth, behind the palatal valve -- a structure that closes the back of their throats when their mouths are open underwater. Alligators can close or open their glottises to prevent water from entering the lungs or to allow pressure to build inside their lungs before vocalizing. To hiss, alligators expel air very forcefully through the glottis; by changing the shape of the glottis the alligator can change the sound's characteristics.
- Journal of Experimental Biology: Subglottal Pressure and Fundamental Frequency Control in Contact Calls of Juvenile Alligator Mississippiensis
- Brazos Bend State Park: Alligator Information
- The American Naturalist: Responses of Captive Alligators to Auditory Stimulation
- Integrative and Comparative Biology: Social Signals and Behaviors of Adult Alligators and Crocodiles