There is a wide array of aggressive behaviors that any dog has the potential to exhibit. Ethologists identify over a dozen different kinds of aggression, ranging from maternal aggression, the hormonally mediated defense of offspring, to food-related aggression, a subset of resource guarding. Defining the term itself is complex and controversial, and its definition is still hotly debated by ethologists.
Aggression in dogs can be described as the threat of harmful behavior directed toward another dog or human to resolve conflicts due to threat or challenge. Many aggressive threats are ritualized social behaviors used to communicate information in order to avoid combat, since actual fighting is not a good long-term strategy for survival of the species.
Although aggression is one of the most commonly diagnosed behavior problems in dogs, it can be difficult to effectively modify. Effective modification requires experience in identifying what kind of aggression the dog is displaying, determining how long the behavior has been occurring, setting up good management and designing modification plans to change the behavior.
The more common types of aggression that the average Chihuahua owner will be exposed to include fear or stranger aggression (when threatened by people or strange dogs), territorial aggression (when people enter an environment the dog considers defendable, including houses, cars, and yards), and resource guarding (when retaining resources including food, toys, his people, or places such as beds, couches, or chairs).
Chihuahuas have a reputation that precedes them when it comes to aggressive tendencies. But its fierceness does not mean that a Chihuahua is a bad or mean dog. Aggression is not a moral statement made by the dog; it is a behavioral response to a social situation. Your Chihuahua is for some reason threatened in that context and is trying to put a little distance between him and the perceived threat by showing those teeth, or perhaps he feels a resource is threatened and will fight to retain ownership of it.
So, what do you need to know? If your Chihuahua is threatening a person or another dog, take note of the situation. If it happens on a regular basis or is predictable, it’s time to get the help of a professional positive-reinforcement trainer or behavior consultant. Do not try to follow the advice of well-meaning, but often off base friends that are “sure” of how to fix things. It’s likely you will end up threatening your Chihuahua more and making it worse. Chihuahuas are tiny, but a bite still really hurts.