If you are ever lucky enough to visit the foothills of the Himalayas, you may hear a remarkable duet ringing through the forest. To the untrained ear, it might sound like musicians warming up a strange instrument. In reality, the enchanting melody is the sound of two lovers talking in a secret, whistled language.

Joining just a handful of other communities, the Hmong people can speak in whistles. The sounds normally allow farmers to chat across their fields and hunters to call to each in their forest. But their language is perhaps most beautifully expressed during a now rarely-performed act of courtship, when boys wander through the nearby villages at nightfall, whistling their favourite poems between the houses. If a girl responds, the couple then start a flirty dialogue.

The practice not only highlights <a href="http://www.bbc.com/future/story/20140606-why-we-must-save-dying-languages">humanity’s amazing linguistic diversity</a>; it may also help us to understand the limits of human communication. In most languages, whistles are used for little more than calling attention; they seem too simple to carry much meaning. But Meyer has now identified more than 70 groups across the world who can use whistles to express themselves with all the flexibility of normal speech.

These mysterious languages demonstrate the brain’s astonishing capacity to decode information from new signals – with insights that are causing some neuroscientists to rethink the fundamental organisation of the brain. The research may even shed light on the emergence of language itself. According to one hypothesis, our first words may have sounded something like the Hmong’s courtship songs.

Meyer’s interest in whistled languages began with <a href="https://www.nature.com/scientificamerican/journal/v196/n4/pdf/scientificamerican0457-111.pdf">a 40-year-old Scientific American article</a> about Silbo Gomero – a form of whistled Spanish ‘spoken’ on one of the Canary Islands. The trilled sounds allow shepherds to communicate across deep ravines, and they are apparently so close to the local birdsong that blackbirds have been known to learn and mimic the human dialogues. You can hear a clip above of someone whistling 'En todo el mundo hay hombres que hablan silbando', which translates as 'Around the World, there are humans who whistle their language'. (Clip courtesy of Julien Meyer and Laure Dentel.)

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 <a href="https://books.google.co.uk/books?id=XzzABgAAQBAJ&lpg=PA68&ots=8Cag8k3EuL&dq=whistled%20speech%20wam%20julien%20meyer&pg=PA53#v=snippet&q=wam%20&f=false">whistle messages across the radio</a> 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 <a href="https://www.jstor.org/stable/610375?seq=1#page_scan_tab_contents">to send them into a kind of meditative reverie</a>. 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.