American alligators are chatty reptiles. They start out their lives chirping for their mother’s help as they push themselves out of their eggs, and, as they grow up, the knobbly archosaurs communicate with a suite of hisses, rumbles, and bellows.
But how do alligators make such sounds? Anatomists have known that alligators and other crocodylians vocalize through their larynx for over a century and a half, but the acoustic abilities of the reptiles have not been as extensively-studied as those of birds and mammals. A new study by Tobias Riede and colleagues is helping to remedy that, including the discovery that a hitherto-unappreciated muscle helps create the crocodylian chorus.
The alligator vocal apparatus isn’t all that different from ours. The reptiles have a larynx and multilayered membranes called vocal folds – better known as vocal chords – that alter airflow as they dilate and vibrate. But to get those parts into the right positions to make sound, alligators rely on muscles.
A muscle called the glottal adductor does some of the work. Depending on which part of the muscle contracts, either the top or the bottom of the vocal folds close. Anatomists have known about this for quite a while. But Riede and coauthors also found that alligators have another important muscle involved in the way other reptiles vocalize, but was thought to be unimportant to crocodylians.
The muscle’s name is a bit of a tongue-twister – cricoarytenoideus. It originates on the first ring of cartilage in the larynx and extends to two other cartilaginous anchors – called the basihyoid and arytenoid, respectively – and the vocal folds. What the muscle does depends on how it contracts. When the rear of the muscle contracts, the whole larynx is pulled back and the vocal folds are held tenser. When front of the muscle contracts, the vocal folds open wide.
How retracting the larynx contributes to an alligator’s vocal repetoire isn’t entirely clear yet. Studying soft tissues while in-use by a toothy owner is quite difficult. All the same, the cricoarytenoideus and other aspects of the alligator’s larynx shows that they have a great deal of vocal control that’s comparable to what’s seen in mammals and birds. And through such comparisons, biologists may be able to give paleontologists a better idea of what the vocal anatomy of long-extinct creatures was like. We may never be able to reconstruct a tyrannosaur’s roar, but, thanks to its living avian and crocodylian relatives, we may be able to narrow down the range of sounds such charismatic, prehistoric creatures were capable of creating.
Riede, T., Li, Z., Tokuda, I., Farmer, C. 2015. Functional morphology of the Alligator mississippiensis larynx and implications for vocal productionFunctional morphology of the Alligator mississippiensis larynx and implications for vocal productionFunctional morphology of the Alligator mississippiensis larynx and implications for vocal production. The Journal of Experimental Biology. doi: 10.1242/jeb.117101