Cats do not need the company of other cats, and many only tolerate it. Littermates and kittens adopted together are the pairs most likely to get on.
A new cat is introduced gradually, by scent before sight, and each cat needs its own food, water, tray and resting places, with one extra of each.
Domestic cats descend from a solitary hunter, and unlike dogs they have no built-in need for company of their own kind. Some cats form close bonds with others; many merely tolerate them, and some are unhappy sharing a home at all. A second cat is not a reliable cure for loneliness, and the decision is best made with the resident cat's temperament in mind.
The pairs most likely to succeed are littermates or kittens adopted together, which grow up as one social group. Introducing a kitten to a young adult is usually easier than introducing two adults. An elderly or timid cat may be distressed by a boisterous newcomer, and a cat that has always lived alone or has fought with neighbouring cats is a poor candidate for sharing.
All cats in the household need to be neutered, and a newcomer is examined by a vet, vaccinated and treated for parasites before it meets the others.
Cats that belong to the same social group groom each other, sleep touching, rub against each other and greet with tails raised. Cats that simply share a house keep their distance, use different rooms and take turns at favourite spots.
Conflict between cats is often quiet. Signs include staring, one cat blocking doorways, stairs or the litter tray, a cat that spends its time on top of cupboards or under beds, and a cat that waits to eat until the other has left. Urine spraying, over-grooming, cystitis and soiling outside the tray are common results of this kind of stress.
Competition is reduced by providing each key resource in several separate places, so that no cat has to pass another to reach it. The usual guide is one for each cat plus one extra:
A cat flap that reads microchips keeps neighbouring cats out, which removes another source of tension.
Each stage lasts as long as the cats need. Some pairs settle in a week; others take months. Hissing, growling or a stiff, staring posture means the process has gone too fast and is taken back a step. Cats are never left to fight it out, because a bad first encounter can set the relationship for years.
Synthetic pheromone diffusers are sold to ease introductions and may help some households.
Some cats never accept each other. Dividing the house so that each has its own territory and resources can make life tolerable, and a vet can refer the owner to a qualified behaviourist. Where one cat remains fearful or the fighting continues, finding one of them a new home is sometimes the kindest outcome.
Each additional cat brings its own food, litter, veterinary fees and insurance, and its own share of time. Most landlords and some housing providers limit the number of pets, and cats in a crowded household are more prone to infectious disease and stress-related illness.