In veterinary science, behavior is often the first clinical sign of a medical issue. Animals cannot verbalize pain or discomfort; instead, they communicate through behavioral shifts. A cat that stops using its litter box may not be "acting out," but rather suffering from feline idiopathic cystitis or arthritis that makes entering the box painful. A dog showing sudden aggression may be dealing with a neurological disorder or chronic dental pain.

If your pet suddenly develops a new behavior (aggression, hiding, house-soiling), assume pain first. Do not hire a trainer until your veterinarian has ruled out arthritis, dental disease, or internal pain.

When you visit the vet, ask for a behavioral screening. Report any changes in sleep, appetite, play, or social interaction. These are vital signs as important as temperature and pulse.