One of the ways most animals communicate is with sound, they may neigh, bark, purr or quack; we speak. These auditory messages convey meaning for the species concerned. We understand words, horses understand neighs, dogs understand barks and so on.
Quacks don?t correspond with words, neighs mean little to cats, but what is universal is body language. Animals can understand the body language of their species and of others. They usually recognize an aggressive or submissive posture.
This picture shows a seriously submissive small dog. She is trying to make herself very small. Her body is hunched, her head is down and she is looking at the floor. The young cat is striding purposely across the floor with a look of determination on her eyes. She is not going to be sidetracked by the dog. From this picture, it is very likely that the cat has previously demonstrated and confirmed her rank with the dog. The duck just seems to be looking on and assessing the situation.
Remember these two words: aggressive - to be ready to fight. submissive - to try to avoid a fight, to cower
The above article can be originaly found in http://poniesonline.co.uk/animalLanguage.htm