Our two chihuahuas Cocoa (male) and Niki (female) when disturbed taking a nap, or guarding their food, will show their teeth, and growl first and go "RAR-RAR"! at whichever dog is intervening-Does anyone else have a dog that does that?
As for food guarding. When you feed them, make sure that each dogs bowl is on the opposite sides of the room and make sure that each dogs rear end is facing each other.
We were extremely lucky with out first set of dogs. They both ate each meal slowly out of the same bowl together without a single quarrel for 13 years. The bowl was always full of food 24/7 and they only ate when needed. Both were both very slender for their size and breed.
With the modern day dogs, the Britney is a chunker and food must be regulated twice a day or she'll eat herself to the point of explosion. She also inhales each and every meal. The Springer will growl at the Britney during mealtime even though the Britney is 20 yards away at the other end of the house, not paying attention to the Springer or her food.
They say the first set of dogs will be the best animals you'll ever own. I believe this to be true!