By 30 days about 50 percent of puppies wag their tails. At an average age of 49 days puppies are tail-wagging. They wag their tails when feeding from their mother (bitch). Puppies do not wag their tails when they are very young. Apparently the earliest observed tail wagging by a puppy was at 17-days-of-age.
The reason why puppies wag their tails when at their mother’s belly is because their minds are in conflict. At the age of two weeks puppies cuddle up together and there is no rivalry between them. At the age of six or seven weeks puppies engage in rough-and-tumble and there is some tension.
When they feed from their mother they come close together. They come close to their sibling puppies with whom there is some rivalry in chasing, nipping and perhaps bullying. This causes a slight amount of fear. But they have to be at their mother to feed because they are hungry.
Therefore they are in conflict between the need to be at a certain place and the fear of being there. This is expressed visually through their wagging tails.
This back-and-forth movement of the tail signifying mental conflict is typical in animal communications. It occurs when animals are pulled in different directions.
It occurs when cats are in conflict about whether to attack prey or hold back. If, for example, a cat lacks cover when stalking prey she may have to hold back. But at the same time she wants to attack. This causes a mental conflict. In turn this is expressed through wagging her tail.
It is said that the reason why animals in general and dogs in particular wag their tails when they are in emotional conflict is because it reflects the physical action of trying to maintain balance. When, for example, a cat like the snow leopard maintains balance on a rocky slope they use their tail. The tail moves from left to right. This same action is reflected in a decision-making process which is in balance and uncertain.
A lot of people still believe that when a dog wags his tail it is a sign of friendliness. This isn’t true. If a dog wags his tail at his human master he is feeling two emotions; emotional friendliness and excitement at seeing the pack leader set against a slight apprehension because humans are dominant over them and they are much larger.
In more dominant and confident dogs the tail is held in a more vertical position when it is wagged. More submissive dogs hold their tail lower when tail-wagging.
Tail-wagging is seen in a variety of situations where there is mental conflict such as when puppies are food-begging from adult animals and when they are adult in greetings between dogs after a separation.
My thanks to Dr Desmond Morris and his excellent book Illustrated Dog Watching