Dogs put their tail between their legs when they are feeling insecure. In putting their tail between their legs they are cutting off the scent signals from their anal region. In doing this it helps to prevent a more dominant dog smelling their scent. It is the human equivalent of a person hiding their faces or presenting body language which indicates a very retiring disposition. The tail-down position modifies the dog’s scent-signalling.
People know the tail-between-legs body language as indicating that the dog is subordinate, insecure and is displaying an appeasement body language due to low status. The opposite is the tail-up position, a sign of dominance and high status. In a single dog household the tail-down position has no importance. However, in a social group of dogs were status and rank are significant it is an important signal which protects the weaker dogs in a group from the stronger ones.
The anal glands carry a dog’s personal identifiers through scent. It appears that dog breeders over 10,000 years have removed, through selective breeding, a special pre-caudal gland on the tails of wolves, the wild ancestors of the domestic dog.
This special gland is positioned about 3 inches from the base of a wolf’s tail. It is surrounded by black-tipped stiffened hairs and the gland is a modified sebaceous gland which exudes a fatty secretion. It is, like an anal gland, concerned with scent-signalling. Its position on the outside of the tail is significant because it cannot be covered up by the tail. It acts like a “mimic anal zone”. When a wolf approaches to sniff another wolf’s rump it finds, in effect, two anal glands producing a different scent because the one above, on the tail, is positioned similarly to the one below the tail. This is a complex form of scent-signalling among wolves.
This special pre-caudal gland, as mentioned, no longer exists in domestic dogs and it remains a special difference between wolves and dogs but the reason why it was bred out is a mystery. Perhaps you can tell me why?
There is one other point to make. From a visual standpoint and from a distance, dogs can see the difference between a tail-up and tail-down position which provides a check to the observing dog as to whether there has been any change in status relations between the weaker and stronger dog.
In cats, tail-up is a visual signal indicating a friendly disposition and is sometimes accompanied by another friendly sign, the nose touch when meeting another cat. Therefore tail-up in cats has a similar purpose to that in dogs.
I am indebted to Dr Desmond Morris’s book “Illustrated Dogwatching” for the dog information.