Leah and Andrew described how their two cats, 5-year-old littermates Ashe and Skye, had always been wonderful playmates, sisters and friends. Since coming into the home together as kittens, they had snuggled for naps, shared toys, food and owner affection easily, and even seemed distressed if they were separated from each other for any reason over all these years.

Now, the only way the cats could be in the home together was with constant separation — Leah and Andrew had installed a screen door in their hallway to keep the cats apart for the last month or two. This was necessary because the cats had suddenly started fighting, and now they bristled, hissed and soon fought whenever they were in each other’s company. Why had these fast friends turned on each other? And what could we do to help them?

It all began for Ashe and Skye when a roaming outdoor cat started coming around. One day, Ashe and Skye were watching the stray cat outside their window when the stray hissed at them, startling Ashe who accidentally jumped back, landing on Skye, who responded to Ashe with teeth and claws. A loud but brief fight ensued, until Ashe had run under the bed and Leah had carried Skye to another room. Now, the cats seemed tense and ready to fight every time they were in the same room, and fur had flown on the few occasions the cats had accidentally gotten together since that day. Leah and Andrew could not believe that one startling incident like that could cause such ongoing animosity between their cats. I explained that these circumstances arise again and again in my work as an animal behaviorist.

Many cats are sensitive to “one-trial learning,” especially when it comes to fearful or startling stimuli. This means that when a cat experiences a frightening or aversive event, he or she may continue to show ongoing defensive aggression or fear around stimuli that were associated with or are similar to that event. When the frightening event happens while two otherwise friendly cats are in close proximity to each other, one or both cats can associate the other cat with that scary thing, resulting in ongoing defensive or aggressive behavior between them.

In my practice, I have had many cases where the scary event was precisely that which startled Ashe and Skye — another family’s cat or stray cat on the outside of a window who startles indoor resident cats who then turn on each other with fear or aggression. I have also had cases where the frightening trigger event was a bee sting, a loud noise outside, a sprinkler turning on, or an owner yelling at one or both cats.

In all these cases, the cats started to fight in the moment, almost reflexively, but then continued to fight when they encountered each other later, well after the original frightening stimulus was gone. Even when things have returned to an otherwise calm and happy state, the cats persist in their distrust or defensive behavior toward each other, and the resulting aggression between them can be quite serious.

Leah and Andrew were already two steps ahead of the game in setting up a screen door as a barrier between the cats. For Skye and Ashe, the screen was enough to keep them from exhibiting aggression toward each other. (For other cats, the screen door can be too open, and we must begin with a solid door between them, for example, with just a crack underneath for them to hear and smell each other.)

When we have arranged the appropriate barriers and schedule for bringing the cats together under controlled conditions, we begin a systematic desensitization program for reintroducing them. To do this, we accumulate a stockpile of each cat’s favorite things (e.g., toys, treats, tuna, brushes, owners’ laps, heated bed) and arrange for each cat to access these things on his or her respective side of the barrier, as we watch for signs of increased relaxation and calm replacing signs of tension, fear or other precursors to aggression.

When we have stable, relaxed behavior from each cat at this stage, we reduce the barrier by, for example, moving to a baby gate with wider slots for some physical contact between them, then some open access while an owner sits between them or uses a small object as a partial barrier.

This process involves repeated sessions each day when possible, to allow the cats to spend increasingly more time in each other’s presence, but arranged so that at each stage we ensure aggression is not likely to occur and both cats are exhibiting all the signs of relaxed and content behavior. When conducted carefully in these stages, we may see resolution in as early as one to two weeks, where other cases can take several months. In a few of the most difficult cases, cats have remained unreliable around each other and owners have chosen either to rehome one of the cats or to maintain separation in the home as needed to ensure the safety and happiness of both cats.

What is especially hard for owners in these situations is that the cats were reliably such good friends for so long. Yet we know from the animal behavior and learning literature that cats and other animals can indeed show these large and sudden changes in behavior after a particularly frightening event. Even those stimuli that seem relatively benign to us, of course, may be interpreted quite differently by our pets. While some cats can rebound and resolve the situation on their own, many others require behavioral intervention under the guidance of a board-certified animal behaviorist and with the help of devoted and patient owners who are able to help the cats bond again and bring back the trust between them.

Megan Maxwell, Ph.D., is a Certified Applied Animal Behaviorist whose column appears on the first Tuesday of every month in Extra. Volume may prohibit individual replies to emails. The information presented here may not be applicable for every pet and is not intended to serve in place of an individualized behavior or training plan.

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