I can't believe you nod to cows, they have like 3 or 4 stomachs.
As far as I'm concerned, anything with more than one stomach is just posing.
Cats, apes and pigeons get a wave. Bats and lemurs get a nod, one stomach so I guess they are ok.
But think of the 20oz NY Strip! I refuse to wave to pigeons based solely on the fact that they've **** on my bike.