Whistling is the product of air being forced through a small hole made by your lips. A whistle is different from your voice because it is clear, and the frequency is narrow and high-pitched. The sound of a whistle can carry for more than 5 miles (8 kilometers) and it holds its form, while a shout can splinter into an echoey mess. Birds have discovered this and use it to communicate between treetops and mountainsides the world over.
So, although humanity has relied on spoken language for millennia to accomplish everyday, face-to-face communication, there are more than 70 groups around the world that engage in special whistled languages — millions of people speak them, although the advent of text messaging has certainly seen a dive in whistled language worldwide. They're most commonly found in mountainous regions where shepherds or farmers need to pass messages around without huffing and puffing up and down hills to do it, but whistles are also used to communicate through the impenetrable undergrowth of the Amazon rainforest and are useful to Inuit at sea as well.
Hunters can use whistling to speak to each other in a way that doesn't alarm their prey like voice-produced language might. Whistled languages have even been useful in battle among soldiers fighting on the same side.
"The most fascinating thing about whistled languages is their birdlike aspect which encodes the complexity of human languages while highlighting a tight relation between human language and the environment," says Julien Meyer, a researcher at the University of Grenoble in France and author of "Whistled Languages: A Worldwide Inquiry on Human Whistled Speech."
According to Meyer, when whistled speech is still present, it signals that traditional activities are still commonly practiced and therefore a relationship with the land is maintained.