When New Pet Meets Old
When introducing a new pet to an already established resident animal, the less threatening the newcomer is, the better the chances he will be accepted. To animals, non-threatening means younger, smaller, and usually a different sex. For instance, a seven-year-old spayed female cat would probably feel intensely threatened by another adult female, even if she were also spayed. Chances that the two would ever get along are slim. On the other hand, she would probably accept a male kitten, who would pose no initial threat and would be young enough for the older cat to "train." Keep in mind, however, that a youngster can be very stressful on an elderly pet, since animals, as they age, become increasingly less able to cope with change. Animals also tend to get along better when they are all spayed and neutered.
Timing is also important. Since introducing a new animal is stressful to the residents, it makes sense not to do it at a time when they are already under stress-when they have just moved to a new house or apartment, for instance, or when one is recovering from an illness or injury.
Since "you never get a second chance to make a good first impression," how you stage the initial introduction is important. You can help dogs start off on the right paw together if you introduce them on neutral territory, perhaps in a park, instead of at home, so that your resident dog first regards the stranger as a possible friend rather than as a threat to his territory and dominance. Have someone else bring the new dog, so that your dog has no immediate reason to feel jealous, either. Let the dogs get to know each other, and play together, before you take them home. Once together, adjustment is mostly a matter of coping with feelings of jealousy and sibling rivalry. Lots of reassurance and attention are in order. Make a fuss over your resident dog and downplay the presence of the newcomer.
With cats, many of the same principles apply. Cats are very territorial and will resent the presence of an intruder, so the trick to cat introductions is to give them the opportunity to become familiar with each other and each other's scents without giving them a chance to slug it out. Dr. Randall Lockwood of the Humane Society of the United States suggests having a neutral third party bring the new cat into the house in a cage. Put the cage in a room where the resident cat can come up to it and smell the newcomer and leave the new cat in the cage for about an hour.
After all the cats have had a good sniff, let the newcomer out. Expect a certain amount of chasing, hissing, spitting, and growling-it's natural. But there's not much you can do to help the adjustment process, except referee and step in if it looks as though any cat could get hurt. One way to stop a fight is to throw a blanket on the combatants or spray them with water. Unless you are certain that the cats will not fight, put them in separate quarters when you are away from home.
It may take six to eight weeks or longer for cats to settle down. The same rules apply as for dogs: try not to give the resident cats cause to be jealous and don't force the animals to be together if they do not get along. Cats, like dogs, will rotate their schedules to accommodate the presence of another cat if they're not particularly fond of each other. Don't be disappointed if they never seem close. It may be necessary to provide separate litter boxes, separate food bowls, and separate toys and beds to maintain a harmonious household, and give the cats separate individual time with you, too.
Source: The Humane Society of the United States