Fitch says that the particular acoustic phenomenon that makes birds with long tracheae sound bigger is neither pitch nor loudness, but something called formant dispersion. A longer vocal tract tends to create a more baritone sound, like that made by a bigger bird or animal. Red and fallow deer are the only known mammals able to lower their larynx at will. By lengthening the tube through which their vocalizations must travel, they effectively create a sound filter, like the bass controls on a stereo, that may allow them to sound larger than they actually are. What birds achieve through tracheal elongation, mammals do through laryngeal lowering, which makes their vocal tracts longer.
Fitch cites male red and fallow deer as an example.
This maneuver lowers the formants, and presumably increases the impressiveness of these roars, which serve to intimidate rivals and impress females. In humans, the larynx dropped lower in the throat at some point during the six million years of evolution since homo sapiens diverged from chimpanzees, says Fitch. Many scientists agree that this laryngeal descent was a physiological prerequisite for human speech. In other words, selection for the size exaggeration that Fitch first studied in birds may have provided a pre-adaptation for the evolution of human speech. Research New England Books Etc.