Dogs sniff other dogs butts.  They communicate in such a fashion, however odd it may seem to us.

One day in the park, before I had a chance to stop him, my dog stuck his cold wet nose up the back of a womans shorts.  Yikes !!

She rose up straight on the balls of her feet, and shouted, *whoooooo* !!

Fortunately, she was a dog lover.  I profusely apologized, and she smiled broadly and said, *Thats okay, I have dogs too*.