Aggression between cats can be resolved in most cases. Here are some tips.
*This presentation was written by Kelley Bollen of Animal Alliances on behalf of AlleyCats.org.
Ongoing aggression can cause litter box issues or rehoming;
Cats prefer the company of their relatives but don’t always end up that way.
Distrust forms when one kitty is agitated (such as when s/he sees another cat outside) and other cat jumps up or is in the area.
Slow intros include scent exchange; Pet cat with sock or towel and pet the other cat with it. Do not rush introductions. They can take a few days or a few weeks. A rushed introduction can damage the relationship permanently.
Try tuna fish on either side of a baby gate. Treats for both cats when in the presence of each other will help.
Make sure there are plenty of resources for the cats spread around the home – several hiding places, escape routes, water sources, litter boxes, enrichment, etc.
If you punish the aggressor, the victim is punished because it is a negative association.