As Cats Protection, a feline charity in the U.K. that has been saving the lives of cats for almost a century, says, cats are solitary hunters who can become territorial or aggressive if they live in close proximity to cats from different social groups. You’ll know cats are from the same social group if they share scents. (Watch for mutual grooming or sleeping next to each other.) Cats from different social groups can get cliquey and lash out at each other.
Depending on a cat’s personality, she may not take kindly to new additions to the family. Even cats from the same litter have been known to distinguish themselves as being from different social groups. Introducing a new kitty to an existing group can—and should!—be a longer process to make sure everyone fits well together. Forcing two (or more) cats who don’t get along to live together can cause emotional distress (one cat may not allow another to use the litter box), malnourishment (one may prevent another from eating) and potentially costly vet visits (one may start unfair fights with another).
It’s also important to note that many cats hide their unhappiness. Hissing and clawing may be absent, but distrust or fear may permeate your cats’ relationships with each other. Again, with a lot of cats in one household, it’s really hard to keep track of these behaviors—both subtle and not-so-subtle. The fewer the cats, the easier it is to monitor actions and make sure everything is copacetic.
Basically, five cats who get along well and love each other are healthier than two who don’t. And if you’re not sure how to predict your single cat’s social behavior, follow this advice from Rayer: “If you’ve had a cat living alone for years and they are now 14 with health problems, it might not be a good idea to put that additional stress on them.
On the other hand, if you have a dominant six-year-old and want to open your home, go for it. Just be sure that if you’re getting an adult cat, you don’t bring in one that is also dominant, unless your current cat is very chill. If you already have a dominant cat, it might be better to start with a kitten or consider switching up the new cat’s gender.” Whatever you decide, keep in mind that fostering is the best way to test the waters and see how a new situation will play out.