In the given question, four statements are labelled as P, Q, R and S which when logically ordered form a coherent passage. Choose the option that represents the most logical order.

P: With dogs, for example, a wagging tail is not always the sign of a happy dog.

Q: Animals have sophisticated communication systems and we need to learn about these if we are to "hear" what they're saying.

R: You have to listen for pitch, speed, spacing and volume to work out what they are saying.

S: And a bark can indicate excitement, fear, surprise, frustration and more.