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.