Orang-utans use leaves to lie about their size

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.

The leaves don't make the calls any louder, just lower, and Hardus suggests that their purpose is to deceive listeners into thinking that they are confronting a much larger animal. It's the audio equivalent of a pufferfish blowing itself up or a cat bristling its fur. The fact that orang-utans live in dense tree-tops, where sound carries but vision is often obscured, makes that explanation more plausible.

Humans are often on the receiving end of kiss-squeaks and Hardus found that only orang-utans who were unaccustomed to our presence used leaves when calling to us. Those that were used to naked, two-legged apes mainly made unaided kiss-squeaks.  This fits with the idea of leaves as tools for lying, for it seems that orang-utans resort to them more often in circumstances where they were more threatened.

So far, Hardus's team have only uncovered circumstantial evidence that the orang-utans are using tools to make deceptive calls. It's the audience reaction that's missing so far - they will need to understand which predators are worthy of leaf-boosted kiss-squeaks, and how they react to them. Does it actually matter if the orang-utans portray a false impression of their size?

It's unlikely that these questions will be answered by observing actual confrontations, given that both orang-utans and the animals that hunt them are rare. Instead, the best way forward will probably be to play the recorded calls to potential predators to see how they respond.

In the meantime, Hardus thinks that the technique is a local cultural innovation, for all wild orang-utans make kiss-squeaks but only some groups use leaves as well. It could be that orang-utans are particularly suited to developing unique vocal cultures, for last year, Hardus's colleagues reported a case of a captive orang-utan called Bonnie, who spontaneously learned to whistle after watching a human.

Reference: Proc Roy Soc B doi:10.1098/rspb.2009.1027

Images from Aaron Logan and Tbachner

More on orang-utans:

• Orang-utan study suggests that upright walking may have started in the trees
• Scientists tickle apes to reveal evolutionary origins of human laughter
• A burst of DNA duplication in the ancestor of humans, chimps and gorillas