Gibbon calls decoded

The sweet, swooping song of a pair of Lar gibbons is what first intrigued Durham University primate researcher Esther Clarke. She set off for the Far East to study their elegant melodies, but then noticed something a little unusual in the quieter so-called "hoo hoo" calls that the gibbons were making, as she explained to Kat Arney...

Esther - Originally, I was interested in gibbon songs, which is what we know a lot about. They're very loud, long distance communication between groups. But while I was following them in the field, I noticed that they were making all these very quiet noises as well. Everyone seem to know about them as well. Everyone who had studied gibbons, but nobody knew what they meant and what they were for. And so, this gave me the idea to try to record some of these quiet calls and try to analyse them.

Kat - So, this is more like kind of gibbon chit-chat rather than shouting.

Esther - Exactly. This is the kind of talk amongst themselves, between close family members. It's not something you can hear from far away.

Kat - What sort of things do you think the gibbons are trying to communicate with these close range calls?

Esther - It's interesting because they make them all the time. So, I subdivided them into that 9 different contexts and I was able to analyse calls from 6 of those where I had a big enough sample size. These are things like meeting a predator for example, a ground predator like a leopard or a tiger, or an aerial predator like an eagle. Also, when they're feeding and when they're traveling.

Kat - So, let's have a listen to some of these calls here. Which one is your favourite first?

Esther - I have to say probably, the raptor is my favourite because actually, when I was in the field, the way I got them to make these calls was I had to show them fake predators. So, I made my own raptor model and I stuck it up in a tree and waited for the gibbons to see it. Initially, I thought it was a failure because they never have seemed to make any noise whenever they saw it. I thought, "They just don't believe it." All I heard were these tiny, tiny little "hoo-hoo" and I thought, "It's just totally ridiculous." and then I was lucky enough to see them with a real eagle owl. When I saw them, they made exactly the same noise. I realised that I had stumbled upon the actual raptor response.

Kat - So, let's have a quick listen to that....

How about another call when they notice something else?

Esther - Okay, so I can give you another example when they see a different type of predator. When they see a ground predator, a tiger or a leopard, the "hoos" are pretty much similar. They're louder and they come much more quickly. They're more intense and they're a higher frequency.

Kat - Let's have a listen to that one...

You mentioned that people have known about these kind of noises for a long time but never analysed them. I guess this is really a kind of the language that they're using to talk to each other.

Esther - I don't know if I'd use the word language, but it is definitely showing a complex form of communication. It's not just random noises that they're making, because we're finding in every contexts, they're consistently using the right as it were, type of "hoo". So, what's really interesting about it is that we typically think of the "hoo" as one type of call and this research has shown, it splits up to at least 6 and presumably many more different subtypes. And that means that their vocal repertoire is much bigger than we thought.

Kat - How did it feel to you when you first realised, "Wow! They're using the same call for the same thing, that means leopard"?

Esther - It was pretty exciting. When I showed them my model leopard for example, they got quite scared obviously because they really believe that it was a real leopard. That was a little disconcerting at first because they would be throwing branches at it and defecating at it and this sort of thing. But what's nice about it is when you're walking through forest and you hear gibbons in the distance and you can tell immediately just from listening to them what's going on with them. That's what I really liked.

Kat - So you can eavesdrop on the gibbons.

Esther - Yes, exactly.

Kat - Humans have a huge vocal repertoire, but obviously, we've evolved from ape-like ancestors as have the gibbons. When you think about human communication, you can hear, that's a man, that's a woman, you can hear different pitches in individual's voices. You have a different pitch, slightly different sounding voice to mine. Is that the same for the gibbons? Do they have their own personal tone of voice?

Esther - Absolutely, yes. So you can individually identify gibbons by their tone of voice. But also, what's interesting is that males are pitched higher than females and that's unusual among mammals because typically, males are lower pitched than females. And also typically, larger than females and that's generally why. In gibbons, there's no real difference in size between males and females at all. So, why it's quite puzzling that female voices are pitched so much lower.

Kat - Why is it that gibbons have such a limited range of vocal noises whereas humans can make all kinds of wonderful singing, shouting speech?

Esther - That's an interesting question. I think personally that gibbons have a lot more flexibility in their vocal tracts than we give them credit for. I think that the actual act of singing allows them to use more of their vocal tract potentially than other non-human primates. This is something that still needs to be tested, but we already know that the way that gibbons produce their song is similar to the way a human produces soprano song for example. I think that the more that we study them, the more we'll find that there is more flexibility. As much as language? No, I'm not sure about that, but definitely more than we know about right now.