Why does a frightened dog put its tail between its legs?

Everyone knows the meaning of this tail posture, but why has it evolved its particular role in the body language of dogs? Why should a tail-down position be linked with fear, insecurity, subordination, appeasement and displays of low status, while a tail-up position is the sign of dominance and high status?

The answer lies not in the tail itself, but in what is beneath it. By lowering the tail and then curling it tightly between the hind legs, the cringing dog is effectively cutting off the scent signals from its anal region.

When two high-ranking dogs meet, they proudly raise their tails high to expose their anal zones to close examination. Since the anal glands carry personal scents that identify the individual dogs, the tail-between-the-legs action is the canine equivalent of insecure human beings hiding their faces.

For a single pet dog living with a human family this display is of no great importance, but wherever there is a social grouping of dogs in which relative status and rank order are important, it is a vital signal that protects the weaker members of the group from the stronger.
Inevitably, it is particularly important in wolf society in the wild. A subordinate wolf can be observed lowering its tail as it approaches a dominant individual, tucking the tail tightly between its back legs as it passes close to the ‘top wolf’ and then raising it again as it moves on out of range.

There is an intriguing difference between domestic dogs and their wild ancestors in connection with these tail displays. On the tails of all wolves (but no dogs) there is a special pre-caudal gland, which can be observed as a dark spot about three inches from the base of the tail.
Surrounded by black-tipped stiffened hairs, this small skin gland is made up of a group of modified sebaceous glands which exude a fatty secretion. Like the anal glands, it is solely concerned with scent-signalling and its positioning on the outside of the tail is significant. Being placed where it is, it provides a scent-sniffing location that acts like amimic anal zone.

If a wolf approaches a companion to sniff its rump, it will find one kind of scent gland if the tail is up (the anal gland) and another, in the same position, if the tail is down (the pre-caudal tail gland). This means that the scent-signalling of the wolf is more complex than that of the domestic dog.

Why the dog has abandoned this tail-gland signal is not at all clear. All of the other changes that have taken place during the development of the dog from the wolf have been deliberately selected by human dog-breeders to improve this or that quality in their animals, ending up with the many distincfive breeds we have today.
But the function of the tail gland of the wolf has only been discussed in very recent times, so it is hard to see how it could have become the focus of breeding trends in previous centuries. Yet it must have been eliminated at a very early stage because loss appears to be complete in all dog breeds.

It is the one difference between wolves and dogs that remains a complete mystery at the present time.

A final point about the tail-up and tail-down displays of dogs and wolves: although the primary function is undoubtedly the modification of scent-signals, a secondary, visual message has also become important. It is possible for any animal watching from a distance, as an encounter takes place, to see at a glance which of the two ‘performers’ is dominant and which subordinate, simply by their silhouettes. A brief look is all that is needed to check whether there has been any change in status relations and whether a weaker animal is perhaps, at last, starting to challenge a stronger one.

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