Why do Pugs and Shih-Tzus have curly tails if they are not spitz type dogs?

They were bred that way..
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Well all I know is that a lot of small dogs have curly tails. I know that sounds crazy but it’s true!
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It’s just a gene that is carried buried in the genetic material of most dogs. Dogs were bred from wolves - this involves elimiating 50% of the DNA with each successive generation… Dogs are just a version of the wolf with lots of lost (and unrecoverable without breeding them back to wolves) DNA.

So, any breed can have a curly tail if they have the genes for it and they haven’t been completely bred out - it doesn’t have to be a spitz type dog.

- Yeah, "My Unpredictable Irresistable Mutts" - even Yorkshire Terriers have curled tails when left undocked
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All of the below breeds have curled tails. Only a few are Spitz.

Chinese Shar-pei,
Pugs,
Alaskan Malamute,
sometimes defected Huskies have them,
I think Toy Poodles do, though I haven’t seen many without a docked tail,
Pomeranian,
Afghan Hounds have a ringed-tail,
Norwegian Elkhound,
Samoyed,
Tibetan Mastiff,
Javanese,
Japanese Chin,
Maltese,
Papillion,
Pekingese,
Shih Tzu,
American Eskimo Dog,
Bichon Frise,
Chow Chow,
Finish Spitz,
Keeshond,
Lhasa Apso,
Lowchen,
Shiba Inu,
Tibetan Spaniel,
Tibetan Terrier
Puli,
Norwegian Buhund.
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They were just bred like this
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Many of those breeds posted above do not have curly tails, their tails are straight and they simply carry them over arched over their backs when alert and in motion. They are perfectly capable of straightening their tails when they want to whereas true curly tailed breeds such as Pugs, Basenji, Bulldogs and so on can not.

Shih Tzus do not have curly tails, they are straight & are carried arched over the back as the hair drapes down.
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Sesshomaru, I haven’t had any problems with my spitz.
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my 6 month old shih tzu tail is always straight not even if he is alert it curls over to hi back…and i like to see when other dogs tail are curl over..what can i do to get his tail looking like the standard shih tzu….i think its cute…