For much of its history, veterinary science operated within a primarily biomedical paradigm. Disease was a lesion, a pathogen, or a biochemical imbalance. The animal was a patient to be physically examined, vaccinated, or surgically corrected. However, the last three decades have witnessed a paradigm shift, driven by ethology (the scientific study of animal behavior) and welfare science. This review argues that —as essential as pharmacology or pathology.

: Recent research highlights how metabolic processes and gut health directly influence an animal's mood and reactions.

| Presenting Behavior | Possible Medical Cause | Behavioral (Functional) Cause | |---------------------|------------------------|-------------------------------| | House-soiling in cat | Feline lower urinary tract disease, CKD, diabetes | Litter box aversion, territorial marking | | Aggression in dog | Hypothyroidism, brain tumor, pain (e.g., dental) | Fear, resource guarding, redirected aggression | | Compulsive tail chasing in dog | Epilepsy (focal seizures), neuropathic pain | Stereotypic coping with chronic stress | | Feather plucking in parrot | Lead poisoning, skin infection, Psittacine beak and feather disease | Boredom, social isolation, learned habit |