The other week when I went to have a look at my plump sausage (Douglas).
and arrived back at my mums house, her Sheltie obviously smelt the pups and sniffed with his teeth chattering. Why would he do this? 

It seems to be their way of pushing more scent into their nasal cavities.
:)

"tasting" the scent

Monsterman does this whenever I have been in contact with any other dogs outside his harem :P

It's definitely to do with tasting and inhaling their scent I think ;)

I remember our cats used to do this sometimes when they were watching all the birds in the garden.