You’ll sometimes hear people lowering their voices to make themselves sound tougher or more commanding. We’re not the only ones – it seems that our close relatives, the orang-utans, pull the same trick, and they use tools to do it. Madeleine Hardus from the University of Utrecht has found preliminary evidence that young orang-utans use leaves for deception, in order to make lower-pitched calls that seem to come from a much larger animal.
While many animals are accomplished tool-users, most use their utensils to find food. A few populations of orang-utans, living in Borneo, are the only animals known to use tools to change the nature of their calls, much like humans use loudspeakers or microphones.
Their tools are leaves, and they’re used in the context of a specific call known as the kiss-squeak. It’s made by orang-utans when they are disturbed by predators like humans, tigers or snakes, or even by rivals of their own species. You can do it yourself: purse your lips together and suck air in sharply to produce a squeaky kissing sound. All orang-utans do it and because these apes are largely solitary, the calls are unlikely to be alarms – they’re probably deterrents instead.
Hardus spent over two years recording over 1,000 kiss-squeaks from 23 wild orang-utans and found that the pitch of the squeaks reflect the size of the animal. Calls made by adult females have a higher maximum frequency than the large elder males, while those made by immature youngsters are the highest of all. This means that the calls could potentially provide information to a canny listener about the size and power of the caller.
To some extent, orang-utans can fake a deeper call by placing a hand in front of their lips, but leaves are the greatest equaliser of all. By stripping a bundle of leaves from a twig and holding them in front of their mouths, even a small adolescent can drop the pitch of its call to below the level of a large unaided male.