Cats are territory-sensitive, and a newcomer's scent and resource competition trigger chronic stress in the resident even without open fights.
Stress signals include out-of-box elimination, overgrooming and hair loss, reduced appetite, more hiding, and a shrinking range, often misread as revenge.
Introductions should be gradual: isolate and swap scents, feed across a door, then brief supervised contact, a process of weeks rather than days.
A resident showing symptoms needs a vet check for conditions like stress-linked cystitis, plus more vertical space and resource stations, with a behaviorist if needed.
