Non-recognition aggression in cats
If you’ve ever lived with more than one cat at a time, you may have experienced non-recognition aggression, without knowing the behavior has a name.
It’s one of those things you can’t believe is happening until you see it with your own eyes. Your cats, who may have been best friends until now, are suddenly fighting with each other. And it only happens after one cat has returned home after leaving for a period of time.
It’s a really hard behavior for anybody to understand. In fact, cats are the only animals who seem to experience non-recognition aggression. Dogs don’t. Birds don’t. Horses don’t. And weirdly, cats don’t become aggressive to people who’ve left and come back, only other cats.
It’s not a behavior that scientists really understand. Although there are some credible theories, we don’t know exactly why cats become suddenly aggressive to other cats they know well and usually like.
We also don’t know exactly how to prevent non-recognition aggression from happening, or what to do when it does.
But there are some good ideas out there that are worth trying. These things work for some cats, some of the time.
The purpose of this post is to tell you what we know about feline non-recognition aggression, and to present the best tactics we currently have for dealing with it.
What is feline non-recognition aggression?
Feline non-recognition aggression happens between cats who know each other and get along, until one cat leaves the house.
Perhaps the leaving cat has an appointment at the vet. Maybe he has to see the groomer. Or maybe an indoor cat just slipped through the front door.
When the two kitty roommates are finally reunited, it’s like they never knew each other. The stuck-at-home cat reacts violently to his former friend: hissing, growling, and swiping. The returning cat responds defensively, increasing the tension.
An all-out battle can ensue.
The worst part of non-recognition aggression is that it can lead to an ongoing feud between the two cats who once enjoyed each other’s company. Fearfulness and territoriality intensifies, perpetuating a conflict between the two cats that can be ongoing.
It’s sad when two cats who once enjoyed each other become mortal enemies. It’s also extremely difficult to live with, and it can be dangerous. Cats can really hurt each other, and they can hurt you, too, if you get in the middle of it.
(If you get scratched or bitten by a cat, read this post, and head straight for your doctor’s office.)
Cats and aggression – why it’s important to do something about it
I wanted to say a quick word about cats and aggression here, before we move on.
Aggression is an unfortunately common behavior problem in cats. Statistics show that 27% of cats placed in shelters for behavioral reasons were given up because they were aggressive.
It’s important to understand why cats become aggressive, and to do everything possible to prevent or manage aggression, because aggression is literally a matter of life or death for cats. 71% of all cats in shelters are euthanized. Cats labeled aggressive in a shelter probably don’t stand a chance.
There are at least nine different types of cat aggression
Aggression is a common response to a lot of uncomfortable situations for cats, unfortunately. Cornell College of Veterinary Medicine lists at least nine different varieties of cat aggression, from play aggression, to petting aggression, redirected aggression, status-induced aggression, pain-induced aggression, and more.
Why is aggression such a “go-to” response for cats?
While we like to think of our cats as fierce predators, they are also prey. Your cat is small enough to become dinner to a coyote, raccoon, fox, alligator, or even a dog. Aggression is a natural, self-protective response that may be life-saving in the wild.
Unfortunately, aggression that is self-protective in the wild, is problematic for cats who live indoors with us.
Why don’t cats who know each other, recognize each other?
The kind of aggression we’re talking about today is called “non-recognition aggression” because that’s what it looks like to us. It looks like cats acting out because they don’t know each other.
The left-at-home cat, instead of welcoming her old friend with open paws from a trip to the vet, treats her former pal like stranger. She seems to be saying, “Who the heck are you, trespassing in my house?”
Maybe the returning cat doesn’t smell like herself
Maybe the left-at-home cat fails to recognize her old friend’s scent.
Cats view the world through their noses, more than their eyes. Many unusual cat behaviors that we cat guardians observe are all about scent:
- depositing scent, such as scratching furniture
- sharing scent, such as sticking their butts in our faces
- communicating with scent, such as urine spraying, and
- creating group scent, such as head bunting.
A cat returning from the vet most certainly does not smell like himself. He may smell like the alcohol, anesthetic, or disinfectants that are ubiquitous in a medical office.
A cat returning from the groomer, on the other hand, may smell like shampoo or cologne spray.
An escaped cat may smell like the whole wide world.
Maybe the returning cat doesn’t smell like the group
The other scent a cat may lose when he leaves the house is “group scent.”
An “affiliated” or bonded group of cats will behave in ways that cats who are strangers will not do. They will groom each other. They will entwine their tails. But the most important thing about a group of cats is that they will smell like each other.
Cats can help achieve group scent by allorubbing, in which one cat rubs the side of her body against another cat. I’ve written a whole blog post about allorubbing, which you can read here.
Cats who have been out of the home environment for some time, may lose that group scent, especially if other strong scents have overlaid the group scent.
Maybe the returning cat does not look like himself
A cat who returns from the vet might not look like himself, either. He may be wearing a bandage, or a cone.
A cat who returns from the groomer could be sporting a bandana, or have clipped fur.
So, even though cats rely on scent to know one another, appearances probably count, as well.
Maybe the returning cat does not act like herself
A cat returning from the vet may not act like herself. Pain and illness can certainly change a cat’s behavior, as can the stress of a veterinary visit.
Anesthesia, which might not have worn off completely by the time a cat is brought home, can also alter behavior.
It’s possible, from the left-at-home cat’s point of view, that the cat waltzing through the front door, looks, acts, and smells like a complete stranger.
Maybe the returning cat smells of danger, or bad memories
It’s equally possible that the scents the returning cat brings in with him signal something negative to the at-home cat.
Perhaps the returning cat smells of stress or fear, which is likely if the cat just returned from the vet or groomer. Some cats will discharge their anal glands in times of stress or fear, and it’s possible that the pheromones these glands contain signal danger to the at-home cat.
It’s equally possible that the smell of the returning cat just reminds the at-home cat of unpleasant experiences, like her own trip to the vet. We just don’t know.
Cats are enigmas. But I didn’t have to tell you that.
How to prevent non-recognition aggression
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One of the problems with non-recognition aggression is that it doesn’t always immediately go away on its own after the “away cat” starts smelling and looking like herself again.
In some kitty families, this non-recognition event seems to trigger a new, unhappy phase in their relationship. The fighting seems to become ingrained, causing a chronic rivalry that can become impossible to live with for human and cat alike.
Prevention, if possible, is key.
Since we don’t really know what causes non-recognition aggression, there are no scientifically proven ways to prevent it. However, there are some tactics that work well for some cats some of the time.
They’re worth a try.
Take all the cats. If possible, bring both (all?) of your cats together when you go out. This is not always practical. But it will most definitely prevent the separation and reuniting that seems to be a hallmark of this kind of cat aggression.
Apply the family scent. Bring something along with you that carries the cat-family scent on it, like a blanket that has been used as a cat bed. Rub it all over the returning cat before he enters the house.
Find a vet that does house calls. One way to reduce the number of times a cat has to leave and return is to seek out a vet who comes to you.
How to manage reintroductions after one cat has been away
Even if your cats have always gotten along after one has been to the vet or groomer, don’t assume it’s going to go well every time. It’s worth monitoring reintroductions, and taking precautions:
Quarantine a sick or groggy cat. If one cat returns from the vet feeling unwell, or is still woozy from anesthesia, put her in another room, by herself, with her own litter box, and food and water dishes.
No cat should have to deal with potential aggression when they’re not feeling well.
In addition, if illness or behavioral changes are a trigger for non-recognition aggression, this may not be the best the time to reintroduce cats.
Monitor reintroductions. Don’t just plop the returning cat out of the carrier and walk off. Stick around and see how things go.
First, leave the away cat in the carrier and let the at-home cat sniff around. Evaluate that reaction before deciding whether to let the away cat out of the carrier.
If the cats show any sign of wariness or aggression, such as hissing or growling, quarantine the returning cat for 12-24 hours, to let him reacquire his normal scent.
If, however, cats appear friendly or politely curious with each other, you can let the returning cat out of the carrier. But WATCH.
Bathe both cats. If your cats have a history of non-recognition aggression, and you’re concerned about it happening again, you can try one suggestion offered by Dr. Nicholas Dodman, one of the world's most famous veterinary behaviorists: bathe both cats.
Bathing physically removes outside scents from the returning cat, and makes both cats smell the same. It also has the advantage of being highly distracting. One bath and the at-home cat may forget all about the “stranger” who entered her home.
Why do some cats fight after one cat goes to the vet, but not others?
Fortunately, not all cat pairings result in non-recognition aggression. For some cats, it doesn’t matter how many times one cat goes in and out the door: the other cat is just never going to start a fight.
Dr. Dodman, once shared what he thought about this question with Veterinary Practice News.
“Some cats,” he said, “…are extremely stable and highly unlikely to fly off the handle. Others…are mercurial and may become enraged almost literally at the drop of a hat.”
Cats are just individuals, like all of us, with their own moods, personalities, and proclivities.
What to do if the cats are fighting
If a fight breaks out between your cats, in spite of all your attentions and best efforts, don’t get in between them. Injuries from cat bites and scratches are very serious.
Throw a towel or a blanket over the cats, to distract them momentarily, and so you can safely reach in.
Or, clap your hands loudly to create a momentary diversion.
Alternatively, grab a broom and use it as a tool to try to separate the cats.
What are the signs of an aggressive cat?
Cats who are feeling like fighting, wear their hearts on their sleeves. Look for:
- Constricted pupils, meaning that the black dot in the center of the eye gets very small
- Ears pressed back
- Hair standing on end on a cat’s back or tail
- Hissing, growling
- A cat facing the other head on, and making direct eye contact.
What are the signs of a frightened cat?
- Dilated pupils, meaning that the black dot in the center of the eye gets very large
- Ears pressed to the side
- An arched back
- Avoiding eye contact, or glancing at the other cat sideways
- Rolling on his back
- Tucking his tail under his body
- Hissing, growling,
How to deal with ongoing non-recognition aggression
If, before reading this post, or in spite of reading this post, you’ve experienced non-recognition aggression between your cats that does not seem to be resolving, there are still things you can do to improve the situation.
First, know that the problem your cats are experiencing right now can resolve, in time, by itself. Keep that hopeful thought in the back of your mind.
But, to help things along, treat the cats as the strangers they seem to think they are. Pretend you’ve just got a new cat, separate them, and re-introduce them very slowly, over a period days or weeks – whatever it takes.
Follow the instructions in this post about introducing a new cat to your existing cat, and remember not to advance too quickly, no matter how impatient you are to return things to normal:
When you finally bring the two cats together, do keep the aggressor on a harness for extra control, just in case.
In time, your home will be a peaceable kingdom once again.
If you enjoyed this post, you might also enjoy:Should I use a spray bottle to train my cat?
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Dawn LaFontaine is a lifelong animal lover who always seems to have a little pet hair in her keyboard. Her blog, Kitty Contemplations, helps cat guardians better understand and care for the special beings they share their lives and homes with. Her cat-products business, Cat in the Box, sells beautiful, well-made, and award-winning products that she designed to meet the biological needs of cats.
 Dodman, Nicholas. “Feline Non-Recognition Aggression.” Veterinary Practice News, 30 Nov. 2011, www.veterinarypracticenews.com/feline-non-recognition-aggression/.
 “Animal Shelter Euthanasia.” American Humane, 17 Oct. 2016, www.americanhumane.org/fact-sheet/animal-shelter-euthanasia/.
 “Feline Behavior Problems: Aggression.” Cornell University College of Veterinary Medicine, www.vet.cornell.edu/departments-centers-and-institutes/cornell-feline-health-center/health-information/feline-health-topics/feline-behavior-problems-aggression. Accessed 17 July 2023.
 Wilson, Julia.
 Hunter-Frederick, A. (2023) Non-recognition aggression in cats. The IAABC Foundation Journal 26, doi: 10.55736/iaabcfj26.9. https://iaabcjournal.org/non-recognition-aggression-in-cats/
 Schaible, Laci. “Feline Non-Recognition Aggression.” Embrace Pet Insurance, 22 June 2017, www.embracepetinsurance.com/waterbowl/article/feline-non-recognition-aggression.
 Dodman, Nicholas.
 Wilson, Julia.
 “Nonrecognition Aggression in Cats.” Vetstreet, 8 Sept. 2022, www.vetstreet.com/care/nonrecognition-aggression-in-cats.
 Wilson, Julia.