Perhaps unsurprisingly, these cryptic languages can also be a weapon of war. Meyer says that the indigenous Berber populations (also known as the Amazigh) in the Atlas Mountains used whistles to pass messages during their resistance against the French. The Australian army, meanwhile, recruited Wam speakers from Papua New Guinea to whistle messages across the radio so that they could confound Japanese eavesdroppers.

And let’s not forget that whistled speech is often used for less prosaic purposes, such as religion, romance and poetry – as the Hmong show so beautifully. Ancient Chinese texts record people whistling Taoist verses – a practice that was thought to send them into a kind of meditative reverie. Meyer has found that Southern China is still a hot spot for many diverse whistling communities among its ethnic minorities, including the Hmong and the Akha.

Clearly, whistled languages are not just the stuff of legend – but a vibrant method of communication for millions of people living today. Yet for the uninitiated, it may seem impossible to imagine the ways that the rising and falling tones could convey meaning.

Meyer has found that they typically rely on one of two strategies – both of which use changes in pitch create a kind of stripped-down skeleton of the spoken language. It all depends on whether normal, everyday speech is “tonal”. In some countries, particularly in Asia, the pitch of a single syllable in a word can change its meaning. As a result, the whistles follow the melodies that are inherent in any spoken sentence. But other languages – such as Spanish or Turkish – are not naturally tonal. In these cases, the whistles instead mimic the changes in resonance that come with different vowel sounds, while the consonants can be discerned by how abruptly the whistles jump and slide from note to note.