Living on top of a mountain can alter the way language is spoken, scientists have discovered.
Languages containing "ejective" consonants - guttural bursts of sound not found in English - mainly exist among high-living communities, the research shows.
Experts think the reason might be that it takes less effort to produce ejectives in thinner mountain air.
Until recently most linguists believed environment mainly had an influence on vocabulary rather than sound.
Researchers in the US found that 87% of the languages with ejectives studied were located within 500 kilometres of a region of high elevation on all continents. As the height of the land increased, so did the likelihood of there being languages with ejectives.
"This is really strong evidence that geography does influence phonology - the sound system of languages," said lead scientist Dr Caleb Everett, from the University of Miami.
An area of high elevation is defined as having an altitude higher than 1,500 metres above sea level.
Most of the inhabitable high altitude areas of the world are found in six regions, including the North American Cordillera, the Andes and the Andean altiplano, the southern African plateau, the east African rift plateau, the Ethiopian highlands, the Caucasus range, the Javakheti plateau, and the Tibetan plateau.
The researchers, whose findings appear in the online journal Public Library of Science ONE, looked at the locations of around 600 representative languages, 92 of which had ejectives.
A computer program was used to match linguistic sounds to particular geographical locations. The results showed a strong correlation between elevation and ejective languages on, or near, five of the six major inhabited high altitude regions on Earth.