(indris calling) - This noise was no bird call.

I had never heard anything like it before.

It must be the voice of the indris.

(indris calling) Using my new equipment, I made the first ever audio recording of the indri.

(indris calling) But could we also capture them on camera as well?

The song was so loud that it seemed impossible that the animals could be more than 20 or 30 yards away.

But where were they?

(indris calling) Until now, no one had even managed to photograph a living one, let alone film it.

Infuriatingly, the bush was so thick that I could see no sign of them whatever.

(indris calling) So the question was how could we get close enough to get a clear view of them without frightening them?

Well, I thought, what about doing it the other way round and trying to persuade them to get closer to us by playing their calls?

(indris calling) And they did exactly what I hoped they would do.

(indris calling) They called in return, came down close to us, stared at us, still calling.

(indris calling) I was thrilled.

We had recorded their song and filmed them singing.

But why had this trick worked?

(indris calling) Well, because they thought that the song I was playing meant a competitor was close by, and their response was to sing.

(indris calling) And this suggested one thing.

There are such things as battle songs.

(indris calling) Songs that say "Get out, this is my territory."

(indris calling)