Opera singers and dry air don't get along. In fact, the best professional singers require humid settings to help them achieve the right pitch. “When your vocal cords are really dry, they're a little less elastic,” says Caleb Everett, an anthropological linguist at the University of Miami. As a result, singers experience tiny variations in pitch, called jitter, as well as wavering volume—both of which contribute to rougher refrains.

If the amount of moisture in the air influences musical pitch, Everett wondered, has that translated into the development of fewer tonal languages in arid locations? Tonal languages, such as Mandarin Chinese and Cherokee, rely on variations in pitch to differentiate meaning: the same syllable spoken at a higher pitch can specify a different word if spoken at a lower pitch or in a rising or falling tone.

In a survey of more than 3,700 languages, Everett and his collaborators found that those with complex tones do indeed occur less frequently in dry areas than they do in humid ones, even after accounting for the clustering of related languages. For instance, more than half of the hundreds of languages spoken in tropical sub-Saharan locations feature complex tones, whereas none of the two dozen languages in the Sahara do. Overall, only one in 30 complex tonal languages flourished in dry areas; one in three nontonal languages cropped up in those same regions. The results appeared in February in the Proceedings of the National Academy of Sciences USA.

Those conclusions run counter to a traditional linguistic view that the structure of language is independent of its environment, says Robert Ladd, a linguistics researcher at the University of Edinburgh. To bolster the Miami group's findings, researchers in the field will need to prove that tonal languages require a precise control of pitch.

Along those lines, Everett and his team will next measure experimentally how well people voice complex tones in arid air. Although the evolution of tonal languages over the course of centuries cannot be observed, witnessing the physiological effect under controlled conditions could really make the hypothesis sing.