Why does the Hawaiian language flow melodically from vowel to vowel, whereas Georgian is peppered with consonants? It may have something to do with the climate and terrain where those languages developed, a new study of more than 600 languages from around the world suggests.

Previous research has shown that some other species’ vocalizations are shaped by their environment. Birds such as the song sparrow, for example, sing at higher pitches in cities, where lower frequency notes would be drowned out by urban noise. And birds living in forested areas tend to sing at lower frequencies than birds living in open spaces, suggesting different species and populations may optimize their vocalizations to travel through branches and other obstacles that deflect high-frequency sounds. The phenomenon—called “acoustic adaptation”—“is seen in species after species,” of birds, bats, and other animals, says Caleb Everett, an anthropological linguist at the University of Miami in Coral Gables, Florida, who was not involved in the new work.

How much, if any, acoustic adaptation occurs in human languages is unclear, says Ian Maddieson, a linguist at the University of California, Berkeley. To explore that question, Maddieson and colleague Christophe Coupé, of the French National Center for Scientific Research’s Laboratoire Dynamique du Langage, combined data on 633 languages worldwide with ecological and climatic information on the regions where those languages developed, excluding internationally spoken languages—such as English, Mandarin Chinese, and Spanish—that are no longer restricted to the geographic regions where they emerged.

A subtle, but clear pattern emerged: Languages in hotter, more forested regions such as the tropics tended to be “sonorous,” employing lower frequency sounds and using fewer distinct consonants, whereas languages in colder, drier, more mountainous places were consonant-heavy, the team reported today at the 170th Meeting of the Acoustical Society of America (ASA) in Jacksonville, Florida. Taken together, these ecological variables accounted for about one-fourth of the variation in how “consonant-heavy” a language is, Maddieson says. One possible explanation for why vowel-rich languages appear more frequently in the tropics is that they travel farther than languages dominated by rapid-fire, high-frequency consonants, which lose their fidelity in humid, forested environments, he says. Heat and humidity interrupt sound, as do solid tree branches and leaves, he adds.

In the study, Maddieson and Coupé simply looked at the number of vowels, consonants, and consonants per syllable for each language. Next, they plan to use data taken directly from spoken recordings to examine “how these elements are actually put together in a continuous flow of speech,” Maddieson says.

The data lend credence to an older, much smaller study of 70 languages, which found a similar pattern, and are “very much in line” with studies of acoustic adaptation in other species, Everett says. Although the findings remain purely correlational, and not based on any experimental evidence, he notes, the notion that ecological factors such as tree cover could affect the sounds a language develops is “a totally reasonable idea.”

*Correction, 6 November, 2:35 p.m.: A previous version of this article referred to Hungarian as a particularly consonant-heavy language. At the request of the study author this has been amended to Georgian.