As Violet Beauregarde noted, spitting is a dirty habitat (so is nose picking but that’s a fun fact for another day). Deer don’t have access to Willy Wonka rainbow drops so they can’t spit in 7 different colors.
What they can do is produce copious amounts of the bodily fluid. How much you ask? 2 gallons a day! Compare that to our meager 1 to 2 quarts (1/4 to ½ gallon) a day.
Saliva helps with chewing and swallowing and contains enzymes for breakdown of fat and starch but it’s most important function is to buffer the pH level in the reticulum and rumen. The right pH keeps the microbes happy. Happy microbes produce enzymes to digest cellulose and hemicellulose – the stuff that plants are made out of. Unhappy microbes revolt. They may be little but they be fierce. Revolting microbes swiftly put an end to the deer carrying them.
Lots of spit = Working microbes = Delighted deer
Fun Fact Extra: Cows produce over 12 gallons of saliva a day. That’s a lot of spit!