Schroeder, Fred E. H. Say Cheese! : The Revolution in the Aesthetics of Smiles [pdf]. In The Journal of Popular Culture, Vol. 32, No. 2. (1998), pp. 103-145.

Have people always smiled? In a way, this relates to the question of whether animals smile.’ Cat lovers often ascribe the upward turn of cat’s mouths to smiles, but this is clearly fallacious, as the only change in the muscles of cats’ mouths are in aggressive and defensive snarling and hissing. Dogs are different, because when dogs are happy, with their tongues hanging out, they appear to smile. Dogs (that is, family companions) really do seem to have a sense of humor, but let’s face it, the smiles are because they are perspiring, and they are perspiring because they are
joyfully playing. When their mouths are closed, owners equally project upon them “expressions” of thought, of melancholy, of gloom, of selfpity.