How does Bruce discover that the Motilone language is a tonal language?

Bruce discoveres that the Motilone language is a tonal language. He shouts trying to mimic the phrases of the men. One day he has a sore throat from yelling. At first he can barely talk, then later his voice is lower than usual. He asks Cobaydra for a banana, but the boy brings him an axe. This happened several times until Bruce pinched his nose and asked again in a high voice. This time the boy brings him a banana.