Is it an evolutionary leftover ?? I once read that they descended from reptiles ( long ago, no cites).
If they descended from reptiles, why do they still have tails ?? is it to keep the flies away ??
reagrds
indian
All mammals are descended from some really old reptiles, including us. And most mammals have tails. Humans don’t, but we do retain the largely useless tailbone.
Lots of mammals use tails for balance (cats, squirrels, certain monkeys, etc.) Some mammals have prehensile tails that they can grab stuff with. (Rats, capuchins, possums, etc. )
Many, including dogs, use theirs for communication. You know what your dog is thinking when they wag their tail or have it between their legs.
Cows use their tails to swat flies. Very useful because cows smell pretty bad.
Baby elephants hold onto their mothers’ tails with their trunks.
Long ago that may have been the prevailing view, but under most modern taxonomies that’s no longer considered the case. The old version of Reptilia was more or less a hodge-podge of critters that were not necessarily closely related.
Nowadays, mammals and reptiles are both descended from an ancestral group called Amniota (aka, amniotes - critters that produce self-contained eggs, which is what allowed them to venture forth from the seas onto land). Within this group, there are two major subdivisions: the Sauropsida (which is more or less the modern name for “reptilia”) and the Synapsida (the early members of which are what are/were commonly called “mammal-like reptiles”). Within Synapsida lies the clade of Mammalia (after passing through such forms as therapsids, cynodonts, and several others). Thus, under the current system mammals are no longer considered to be descended from reptiles, but rather some reptile-like critters (that is, the “mammal-like reptiles” were neither mammals nor reptiles, but shared some properties of each).
As for tails, most vertebrates have them, so the fact that mammals have them can be considered an “evolutionary leftover” of sorts. It’s what is called a “plesiomorphic character” for mammals in general - an ancestral trait (commonly defined as any part of the vertebral column which extends past the anus, thus allowing such animals as snakes to have them) common to most members of a group, but not unique to them, having originated much earlier than the group in question. In this case, the tail evolved amongst the early chordates, and the majority (if not all) chordate descendants have a tail.
So, the simple answer to the OP’s question is that dogs have tails primarily because their ancestors had them (and the same can be said for any mammal group). The tail has been much reduced in some vertebrate groups (e.g., adult frogs and humans) because there were evolutionary pressures to reduce the tail size. In those groups that have them, there were no such pressures - indeed, they can be remarkably versatile structures, so natural selection has often molded the tail into a variety of functions.
If dogs had no tails, they would have to wag their feet to express happiness. Every happy dog would fall down, from the foot-wagging. 
My dog has no tail, or at least no tail worthy of mention. She has to wag her whole butt…
All traits of all animals and people are the leftovers.
Evolutionary pressure does not create things for a purpose, but only eliminates things that bring early death.
The tail is actually simply the body, the undifferentiated part of what was a continuum in early creatures like worms.
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No, it also rewards things that increase reproductive fitness. If a monkey’s unusually prehensile tail lets him reach out farther and grab fruits his fellow monkeys can’t reach, and that lets him get bigger and stronger and attract more females ( or just bribe them with fruits ), he’ll probably pass on his more-prehensile tail to future monkeys. Success is just as good at driving evolution as failure.
Any speculation as to what those pressures would have been?
Well, there’s always pressure against tails or any other organ; they take energy and resources to build and maintain. If tails are useless to an animal, over time they’ll likely vanish for that reason alone, much less any other anti-tail pressure.
The only reason dogs have tails is that their ancestors had tails, and it didn’t keep them from reproducing.
Ancestors got some tail, did they?
And for communication, as mentioned. A dog that can more easily signal it’s emotions to dogs or humans is a dog with an advantage.
I’m not sure about that. From my understanding of evolution, if a beneficial mutation occurs (giraffes with large necks) then that strain reproduces better and carries on.
A distant ancestor who shed his tail wouldn’t provide enough of a benefit to reproduce more frequently and propogate that benefit/mutation.
Dogs have tails because otherwise the “tail wagging the dog” meme would be unable to be expressed, and so modern life (ie journalism) could not continue.
Why not? Even a slight benefit will win out over hundreds of generations.
My husky uses her prodigious tail to keep warm in the winter. She curls up into an apostrophe-shape, with her feet and nose tucked under her tail.
Well, would you sleep with a man with a tail?
But that doesn’t mean that the negative pressure produced by the metabolic burden of that neck ( or tail or whatever ) isn’t there; just that the advantages in reproductive fitness for that structure are larger. It’s the same principle as cavedweller’s eyes atropying, or most of a parasite’s body; unneeded body parts tend to vanish over time, since they take up resources for no benefit.
I have observed my dog underwater while snorkeling.When she is heading out to sea,her tail moves like a propeller.When she is traveling parallel to the shore,she uses it as a rudder.
Among frogs, it may have had to do with jumping ability; jumping farther necessitates reducing weight, and reducing the weight of the tail by reducing the tail itself probably outweighed any beneficial effects (such as potentially acting as a rudder, etc.). In addition, the pelvic girdle was strengthened and elongated to provide more jumping force
For humans, tail loss was likely plesiomorphic, as our closest relatives (e.g., chimps) lack tails as well. As for why other great apes may have lost their tales…couldn’t tell ya. I know little about human/ape evolution, so any speculation would likely be misinformed.
A “useless” trait would, if anything, become selectively neutral. Reduction would be more likely to occur in instances where the reduced trait is actively more beneficial than the previous, non-reduced state of that trait.
Again in the case of frogs, we see in the fossil record that the tail shortens over time as the legs and pelvic girdle increase in size, probably because of the increased emphasis in jumping as a form of locomotion. But, this is also a case where the tail is not, in fact, fully lost, as the tadpole form possesses a tail which is gradually lost during metamorphosis (with the webbed feet taking over the roles previously filled by the tail).
In fact, reduction frequently results from ontogenetic causes, rather than true “loss”; the genes to produce traits often remain, but they are typically not, or are only partially, expressed (the non- or partial-expression of those genes, again, being more beneficial than their full expression). Chickens still possess the genes to make teeth, whales still possess the genes to make rear legs, horses still possess the genes to make more than one toe per foot, etc. And humans still possess the genes for tails (.pdf).