Paul Star, of Dunedin, asks :-
Why do dogs wag their tails when they are happy, while cats swish their tails when they are cross?
Craig Johnson, a veterinary neurophysiologist at Massey University's Institute of Veterinary, Animal and Biomedical Sciences, responded.
Your question covers the area of communication in the animal kingdom. Whenever animals interact with each other they use languages that have developed over a long period of time. Animals (and people) use a variety of sounds, smells, touches and visual cues as communications. It is important that signals used to communicate can be clearly spotted by other members of the group and that all members of the group interpret the communication correctly. Because many animals do not have expressive faces, their visual signals are limited to other parts of the body. The tail is a very flexible and expressive part of both dogs and cats and is used for communication in both species.
Wild dogs are pack animals. They live within an established hierarchy and it is important that each member of the group knows everyone's place in the group. The tail is used alongside other behaviour such as muzzle holding, play bowing, rolling over etc. to indicate dominance and submission. These signals are used to bind the pack together so that they function as one unit with one leader when out hunting. Hierarchy is also important for getting food first and mating.
Wild cats live as solitary individuals within a territory. They do not need to work in a team, and most of their visual signalling is aimed at keeping others away. When a cat's territory is invaded, it will perform various warning signs with the aim of getting the intruder to back off without a fight. The tail swish is one of these aggressive warnings that trouble is coming.