Deep in the rainforests of Sumatra and Borneo you might be surprised to hear a familiar sound: kissing. But when solitary orangutans make the noise it is no romantic gesture. ‘Orangutans make these ‘kiss-squeak’ alarm calls if humans and dangerous animals come near’, explains Bart de Boers from the Vrije Universiteit Brussel, Belgium. And when Madaleine Hardus from the Pongo Foundation and Adriano Lameira and Serge Wich from the University of Amsterdam, The Netherlands, first discovered the threatening sound, they noticed that some animals cupped their hands around their muzzles, noticeably lowering the pitch that they produced. Were the apes trying to sound bigger and more threatening than they really were? Chatting to Lameira, de Boer realised that he might be able to help the biologists find out whether the animals were really modifying their alarm calls to create a false impression. ‘I said, “this sounds like a really interesting thing to model because then you will know for sure whether using the hand really helps to exaggerate size” ’, recalls de Boer.
But first he had to build two mathematical models – one to understand the sound production mechanism and the other to simulate the frequency signature – to learn more about how the apes deepen the alarm sound. Describing the first model, de Boer explains that he simulates the lips as a thin tube opening through a disc that represents the face. Then, he places a second disc a short distance above the face disc, to simulate the ape's hand. Deriving a set of equations that described how sound travels through the lips and then bounces back and forth in the cavity between the orangutan's face and hand, de Boer realised that by channelling the sound through the hand and face cavity – like the sound box of an instrument – the ape was effectively lengthening the pipe structure to lower the sound's pitch and make it deeper.
But simply lowering the pitch of a note is not enough to make it sound as if it is produced by a larger structure. The overtones that accompany the fundamental frequency and make the sound richer have to be lower too. If the orangutans were really going to pull off the feat and pass themselves off as being larger, they would have to generate more low overtones in their kiss-squeak calls. To test this, de Boer built the second more sophisticated model to calculate the frequency spectrum of calls and was very impressed to see that the lower overtones were amplified in the simulation when the hand was in place.
But how well did de Boer's calculations compare with Lameira and Hardus's hard-won recordings? ‘The problem is that the orangutan is not sitting still on a branch making its noises’ chuckles de Boer. ‘There are cicadas singing in the background, rustling leaves – all kinds of horrible stuff going on’, he adds. However, after painstakingly removing the background cacophony, de Boer was amazed to see how well the simulations agreed with the real thing. Not only did the mouth-cupping orangutans sound deeper, but they had managed to amplify the deeper overtones that make them sound larger.
And de Boer is excited because this might be the first hint that an animal can learn to modify sound, which is an essential tool for language acquisition. ‘Orangutans may be aware that they can influence their call and it changes the reaction of the predator, and this a simple form of learning, which is a very important first step in language’, he says.