Crocodilians do not have vocal cords, but they can hiss, grunt, cough, growl, and bellow to convey an incredibly wide range of meanings. The sounds that crocodilians make vary by species, age, size, sex, and context. Individualized differences in tones, intensities, and calling patterns can also occur, just as individual humans have distinct voices and speech mannerisms.
The purposes that crocodilian sounds serve include threats, distress signals, hatching calls, contact calls, and courtship bellows. Some species can communicate more than twenty different messages by vocal sounds.
A recent study of Nile crocodile (Crocodylus niloticus) hatchlings showed that a baby actually begins calling while still in the egg and continues immediately after coming out. The calls do not seem to identify the hatchling as an individual, but they do influence the behavior of other nearby juveniles and elicit maternal care from the mother.
During the first several days after hatching, the baby Nile crocodiles gradually change the sounds of their calls. Scientists theorize that the new sounds give the mother information about the age and size of each hatchling so that she can customize her care for each individual.
A similar study of American alligator (Alligator mississippiensis) hatchlings yielded much the same result. The alligator babies begin making prehatching calls just before emerging from the eggs. The posthatching calls attract the parent and trigger such parental care activities as opening the nest, picking up the hatchling, and carrying it to water. The calls also cause other hatchlings to stay nearby, thus creating a pod.
Among mature crocodilians, threat calls are particularly fascinating. Some alligators, for example, threaten in at least three vocally discernable stages.
At the first inkling of a potential aggressor, the alligator will emit a brief, low-intensity, sneezelike "cough." While coughing, the alligator will snap its head sideways toward the potential aggressor.
If the aggressor does not leave, the alligator moves to the next threat level, which is a low-intensity hiss lasting three or four seconds. While hissing, the alligator reinforces the seriousness of the threat by exposing its teeth, inflating its body, standing up, twitching its tail, and angling its back and head toward the aggressor. It may also lunge toward the aggressor in a bluff or real biting attack.
If stage two does not remove the aggressor, the alligator goes to stage three, dramatically raising the intensity of the hiss, while continuing the visual threats as well. At this stage, the likelihood of a real biting attack also increases. Alternatively, the alligator may decide to flee, usually into water.
The male alligator can produce a sound so powerful that its vibrations make nearby water "dance." When preparing to bellow, the male inflates his body, raises his head and tail out of the water, puffs out his throat, and, while his mouth is still closed, begins to vibrate air. Just before he bellows, the infrasonic (below human hearing) signals he emits become so strong that they vibrate nearby water (producing a "water dance"), ground, and other objects. Through water, the signals travel great distances, attracting females and asserting dominance over other males.
Alligators bellow not only to other alligators but also in response to thunder and sonic booms.
(To hear examples of croc talk, go to Adam Britton's "Crocodile Talk" webpage listed below.)
"American Alligator." http://www.corkscrew.audubon.org/Wildlife/Alligators.html#Communication (accessed April 15, 2009)
Britton, Adam. "Crocodile Talk." Crocodilian Biology Database. Florida Museum of Natural History. http://www.flmnh.ufl.edu/cnhc/croccomm.html (accessed April 15, 2009)
Vergne, Amãlie L., et al. "Parent-Offspring Communication in the Nile Crocodile." SpringerLink. http://www.springerlink.com/content/15p113+51771128n/ (accessed April 15, 2009)
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