Gerbils are social animals that live in small family groups in the wild. They will cuddle, groom, chase, and wrestle with each other. As with other social animals, such as wolves, the family has a hierarchy - one dominant pair mates, and the older offspring help care for the young. As the offspring reach adolescence and adulthood, they leave their social group to form another.
Gerbils in captivity also prefer to live in social groups. However, it is up to the you to keep the group at an optimal size. The American Gerbil Society has made the following recommendations:
Gerbils are very territorial, and will be aggressive towards other unfamiliar gerbils. Ideally, you should pick out two gerbils that are already living together, or that are between six and eight weeks old. This will result in a much smoother transition for them to live together in the new cage you will provide. However, if one or both gerbils are over eight weeks old a Split Cage Introduction is recommended. Split cage introductions work only for two lone gerbils. Gerbils that have been separated for more than a day need to be gradually reintroduced using a split cage.
Scent is important to gerbils, and the Split Cage Introduction Method is based upon this. The split cage allows each gerbil to become accustomed to the scent of the other without allowing them to become aggressive towards each other. Ideally, the gerbils will recognize and accept each other's scent and live together peacefully once the divider is gone.
You will need a cage or tank (15-20 gallon aquarium) that can be divided into two sections. To split the cage, simply divide the cage or tank diagonally using stiff, heavy hardware cloth with small holes. Place the divider from corner to corner, so the cage is divided into two equal triangles (this makes it more secure). Alternatively, your hardware store may be able to make a plexiglass® divider. By drilling holes in the plexiglass® you will allow each gerbil's unique scent to travel through to the other. In either case, be sure the divider fits tightly and securely, since each gerbil will try to crawl under, over, and around the divider to get to the gerbil on the other side. Also, be sure the cover of the cage fits snugly. Monitor the gerbils carefully after placing them in the divided cage until you are convinced that the cage divider and cover are indeed secure.
Once your gerbils are secure in their respective cage halves, you must transfer scents between gerbils. To do this, carefully switch the gerbils to opposite sides of the cage four or five times a day. After a week of these daily transfers, remove the divider. Keep heavy gloves handy and be prepared to separate the gerbils if a fight should occur. You will need to watch them carefully for at least five to six hours, perhaps longer. (Remove the divider on a day when you will be available to monitor them closely.) Do not leave the gerbils alone together if you cannot monitor them. If you must leave prior to being sure the gerbils are compatible, replace the divider. You will know your gerbils are coexisting peacefully when they are grooming each other and sleeping together in the same nest.