Just as looks can be deceiving, so, too, can sounds mislead. With this idea in mind, Harvard biologist and lecturer in psychology W. Tecumseh Fitch has hit on a provocative new explanation for a long-known physiological oddity found in certain birds: some avian species have windpipes from two to 20 times longer than expected relative to their size. In these birds, the trachea typically loops and coils on its way from the lungs to the throat, rather than taking the shortest route. Fitch's theory not only explains tracheal elongation (TE) in birds, but suggests how physiological adaptations necessary for speech might have evolved in humans.
Fitch compared 60 birds with TE to six species with average-sized tracheae. What he found was initially puzzling. Elongated tracheae appear in both migratory and sedentary species, in birds large and small, sometimes in both sexes of a species and in others just in males. In one species, TE appeared exclusively in females. Only when Fitch looked at the habitats of birds with TE did a pattern emerge. He found that they typically nest in dense vegetation, where visibility is poor. In this kind of environment, vocalizations may provide the only clue about the physical size of the caller. Fitch further discovered that in species where only one sex exhibits TE, it is the same sex that defends the nesting territory. He proposes, therefore, that tracheal elongation has evolved because it allows the birds to sound much bigger than they actually are, either to attract potential mates or to scare off competitors.
Critics of earlier attempts to explain TE have asked why, if the anomaly confers significant benefits, it is not seen in any mammals.
Fitch's theory explains this apparent discrepancy. Birds produce sound at the base of the tracheal tube, in the syrinx. But in mammals, because the vocal cords sit atop the tracheal tube in the larynx, the trachea is not part of the vocal tract and is therefore not subject to evolutionary pressures of natural selection for elongation.
That does not mean that the human vocal tract has been free of similar evolutionary pressures, according to Fitch. On the contrary, his comparative work with the vocalizations of birds and other animals has led to a new theory about the origin of human speech. Because many of the important human sound-producing organs are soft tissues, there is little fossil evidence to indicate when the structures that would allow speech might have arisen. Fitch's work supplies no answer to the question of when, but provides a tantalizing theory for why speech-enabling structures might have arisen in the first place.
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 formant is a kind of filter," he explains, "like the bass or treble knobs on a stereo, that emphasizes certain frequencies. A longer vocal tract tends to create a more baritone sound, like that made by a bigger bird or animal."
W. Tecumseh Fitch
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. "During roar vocalizations," he wrote in the July 2000 issue of Trends in Cognitive Science, "they pull the larynx far down in the neck....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. But the puzzling question has remained: What evolutionary pressures might have led to a lowered human larnyx in the first place?
Fitch suggests, according to the size-exaggeration hypothesis, that "the original selective advantage of laryngeal lowering was to exaggerate size and had nothing to do with speech." This hypothesis, he points out, is consistent with the fact that "a second descent of the larynx occurs at puberty in humans, but only in males," when the voice deepens. "This second descent," writes Fitch, "thus appears to be part of a suite of sexually selected male pubertal changes that enhance apparent size, including shoulder broadening and facial hair growth." 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.
