By Lindsay Pulman, LVT, LATg

Adult rats of either gender may kill a baby, from either defending nest territory, or inadvertently with a bite that would not kill an older youngster. I counsel people to not get babies for adult rats unless they are prepared to house them separately until they are big enough to withstand a severe bite. This means the new youngster must to be 10 to 12 weeks old or at least 200 grams before attempting introductions to adult rats. Introducing youngsters to young rats around the same age, who are not more than 3 or 4 months old, is much less risky.

The following is somewhat graphic: The importance of delaying until a certain age or weight cannot be underestimated. The restriction of 10 to 12 weeks of age, or at least 200 grams, insures the youngsters have sufficiently tough skin and muscle development to protect their jugular vein and spinal cord. I personally know of many cases where a baby was put with an adult rat, and the adult then killed it with one bite. In one case, the baby male and adult male went through all the recommended introductions in neutral territory, for over a week, with no problems, but on the last day of introductions (in the bathtub), the male walked over to the baby and killed him with one bite, without any warning or body language indicators.

Proper Introduction Techniques for Either Youngster or Adult Rats:

Supervised play time, 15 minutes at a time, in completely neutral territory like the tub, the bathroom floor with the door closed, or some other area where there is no established rat scent. The area must be safe enough so the rats will not escape.

Watch body language. If you see fluffed fur ("poofiness" or piloerection, photograph here), or aggression, fights, or bites (many examples here, and video example here), this match-up may not work. Immediately separate the rats if fights occur. [Editor: To insure you can "immediately separate the rats," explore Techniques to Introduce Rats on JoinRats, in advance of beginning an introduction process.]

Try again the next day, and if they do not begin to relax with each other after two or three tries, you may want to consider keeping separate homes and getting a young companion for your new little one.

ON the other hand, if they do start to relax with each other, including beginning to explore the playground, sniffing, grooming, and doing regular rat stuff - or, even better, licking each other, or if one puts her head under the chin of the other and wants to be groomed, and they seem friendly - then you have a good chance of successfully introducing them.

Keep up the supervised playtime, if it's working as described above, for at least one week. You can extend the playtime gradually to a half an hour each session.

After a week to 10 days, if they are getting along well and give signals that they look forward to being together, then:

• Scrub the cage you want them to live in together, from top to bottom, and remove ALL scent.
• Resist the urge to keep even one item either is familiar with and has scented. That is a mistake. They must both feel that they are moving into a brand new apartment.
• Clean and disinfect the cage top, sides, and pan.
• Clean and disinfect all the permanent furniture (plastic, metal), all the hammocks, other cloth, etc.
• Dispose of all item that you can't wash, such as cardboard boxes and tubes, wooden play toys, wooden anything, or organic matter such as hay.
• Clean the water bottle. It's wise to use 2 food bowls and 2 water bottles, on separate floors, for awhile. Once they are settled in, you can remove the extra ones.

When everything is clean and the cage is completely rearranged, put them in together at the same time, and do not leave them alone. Give yourself time to watch them (a weekend is good if you work). Watch how they interact.

Make sure they are comfortable together, and have enough private areas that they can be alone for a nap if they desire.

Make sure the new cage is big enough: There is nothing that instigates fighting more than crowding into a too-small cage. Pittsburgh Rat Lovers Club recommends a minimum size of 17 inches x 30 inches x 24 inches tall for two rats.