I believe there have been previous threads on this one, but just for those interested in "sourcing" - or the slightly variant phrasing:

A Book of Vulgar Verse, by A Gentleman About Town, Checker Books, Toronto, 1981, ISBN 0-89009-411-X


WHY DOGS LEAVE A NICE FAT BONE
Anonymous

The dogs once held a meeting; they came from near and far.
Some came in automobiles; some came in a car.

But before inside the hall they were allowed to take a look,
They had to take their assholes off, and hang them on a hook.

They all walked in, one by one, mother, son and sire,
But no sooner were they seated than someone hollered, "Fire!"

Then they all rushed out in a bunch; they had no time to look,
So each one grabbed an asshole and ran it off the hook.

They got their assholes all mixed up, it made them awfully sore
To think they'd lost the asshole they'd always worn before.

And that's the reason why when you go down the street,
A dog will stop and swap a smell with every dog he meets.

And that's the reason why a dog will leave nice fat bone,
To smell another's asshole, in hopes to find his own.

A slightly longer(?) version parodies (the right description?) the Union Hall Fire, and my recollection is it was in SingOut! a couple of years back. The initial lines were extended to "explain" that the dogs were meeting for some purpose relating to organizing to improve some social condition - and it is a band of "drunken cats outside," who were excluded from the meeting, and who yelled "fire."

John