YOU can tell a lot about your dog's mood by observing his tail. Everyone knows that dogs wag their tails when they are happy - perhaps at the prospect of going out for a walk, or greeting you back home when you've been out.

When a dog is sure of itself and relaxed, it will hold its tail up, swinging it from side to side. If, however, it is nervous, it will keep its tail tucked down low, beneath its back. In this case it may still try to wag its tail in a gesture of appeasement, if it has been caught in the act of doing something wrong, but is usually sent away with its tail between its legs - quite literally.

A dog which is in an aggressive frame of mind will keep its tail up, but in this case the tail will be held still, apart from the tip which wavers slightly.

Young puppies will start to wag their tails at an early age, but it will take over a month for all the puppies in an average litter to start displaying this behaviour. If your dog is unfortunate enough to be involved in a fight, you will see it lower its tail to protect its exposed hindquarters and reduce the risk of injury.

Selective breeding has altered the ability of some breeds to communicate with their tails, particularly in the case of those breeds whose tails are naturally curled.

The controversial act of tail docking was originally practised on hunting dogs whose long tails were at risk of injury whilst clambering through thorns and dense undergrowth. Tail docking interferes with the dog's ability to express itself, and in some cases docking is carried out in such a way as to affect the tendons causing the stump of the tail to be permanently raised.

The justification for docking for hunting is virtually non existent these days and in July 1993 it became illegal for anyone other than a veterinary surgeon to dock a puppy's tail at any age - even soon after birth. This traditional right is now denied to dog breeders or anyone else not holding a degree in veterinary surgery. Anyone who docks tails is now liable to prosecution by the RSPCA on a cruelty charge.

Unlike the situation with cats, there is no naturally tail-less breed of dog. Dogs do not use their tails like cats - extending them out when walking on a narrow ledge for example, probably because dogs only climb very rarely. Their bodies are far better adapted to running than climbing. Their tails can help them to retain balance when turning at speed, and dogs with stout tails, such as labrador retrievers, use them effectively as rudders when swimming.

Finally, if your dog should ever lose its tail and you want it replacing, don't bring it to us because we're not licensed to retail dogs.