The artificial line between "medical" problems and "behavioral" problems is dissolving. In modern , behavior is a vital sign. It is as important as temperature, pulse, and respiration. An animal's withdrawal, aggression, or repetitive pacing is a clinical symptom, just as a fever or a lump is.
If you're looking for a story that bridges the gap between how an animal acts and what is happening inside its body, the case of The Pacing Polar Bear zooskool animal sex better