This is because females are much more competitive and territorial than males are. Introducing adult gerbils often requires that special steps are taken. This means that the gerbils have to live in the same tank or cage with a barrier between them. Gerbils should never be introduced to a group of adult Gerbils. Even a simple pair will often fall out if another Gerbil is introduced as it disrupts the dominance hierarchy that the Gerbils have already established.
Gerbils normally live longer than mice, rats, hamsters and other small pets.
Typically they die shortly before they reach the age of three. However, some gerbils will live significantly longer and may get as old as five.