Rat terriers have a short average lifespan because of their propensity for jumping on anything be it Chihuahua or a Pit Bull.
My wife has a Jackrat, half rat and half Jack Russell, jumped in on a brawl my Labs were having with a local Boxer/mastiff mutt. If I hadn't been there he probably would have bought the farm. A few hours later he was still bloody and in the yard posturing and kicking up dirt, challenging the other dog who had went back to his own yard.
