Now that you have your breeding pair, all you have to do is introduce them.
Gerbils are very territorial creatures and will not tolerate being put together without any sort of introduction. Improper introduction will usually result in fighting and your gerbils can get severely hurt.
In the wild, one gerbil would chase the intruder away, but in a tank there is nowhere to go and so they will fight to the death.
There are some instances, however, where an introduction period may not be necessary. If both gerbils are under the age of 8 weeks, or if one is a lone adult male gerbil and the other is a female gerbil under the age of 8 weeks, a split cage may not be needed. In both these cases, the gerbils may take to each other immediately, but if you see any hostility, you will need to use one of the two methods below.
Main methods of introducing gerbils
Split cage method
For this method you will need a tank and a piece of wire mesh (hardware cloth) that is cut to fit diagonally across the tank, from top to bottom.
You want to use wire mesh with small holes, no larger than 1/2 inch, otherwise the gerbils can harm each other through the divider.
I personally use 1/4 inch wire mesh and have had no problems with biting injuries.
You also want to make sure that there is no space that a gerbil can squeeze through to get at the other gerbil.
I place a heavy object, such as a dictionary, on top of the tank to weigh down the lid. This prevents the gerbils from sneaking over top of the divider. Make sure the tank has been thoroughly cleaned with regular soap, rinsed with water and dried completely. Then you should add fresh bedding. I like to keep the tank empty except for shavings, a toilet paper roll and some tissue for bedding on each side of the split. I find the introduction process takes longer if there is additional toys and housing in the tank that could distract the gerbils from the introduction.
Now that your tank is ready, place one gerbil on each side of the split cage. The gerbils may exhibit aggressive behaviour such as chewing at the divider or they may just calmly sniff each other through the holes. Don't be worried that they are acting aggressively, it just means they will likely need to be split for a longer period of time. You want to switch the gerbils to each other's side of the tank about 4 times a day. This allows them to learn each other's scent and eventually no longer recognize the other gerbil as a threat. The split cage can take anywhere between 2 days and 2 weeks. Even if it takes longer than that, it is okay, just have patience. I have never had a split cage fail yet!
This method is very similar to the split-cage method, except you use a small wire cage to split the gerbils rather than a divider.
Just like the split-cage method, you want to clean the tank and the cage very well with soap and water and dry them completely.
Then you can add shavings, some cardboard and some facial tissue in both the tank and cage.
Place one gerbil in the tank and the other in the cage. Switch them about 4 times a day until they are ready to introduce.
I don't like the cage-within-cage method as much because the cage bars tend to be spaced more far apart than the holes in the wire mesh. A very determined gerbil could manage to bite the other gerbil through the cage bars. Some people have had great luck with the cage-within-cage method, but I've always used the split-cage method instead.
Putting your gerbils together
The best indicator that your gerbils are ready to have the split removed, is if you seem them sleeping next to each other through the divider.
Another great sign is if you observe them grooming each other.
Some gerbils never exhibit this sort of behaviour, however. In that case, you may just need to introduce them when they stop marking everything frantically when you switch their sides.
If they seem to be nonchalant about the existance of the gerbil on the other side of the split, then it may be time to try removing the barrier.
When you do remove the split, make sure to wear thick leather gloves and have an extra container to put the other gerbil in if they fight. If the gerbils do decide to fight, you don't want to stick your bare hand in there, because they will bite you. I can tell you from experience that it really hurts!
It is perfectly normal for gerbils that have been introduced to sniff each other's rear ends a lot. There may be some thumping and often the gerbils will try to mark each other with their scent gland (looks kind of like mating, but isn't). Boxing behaviour is pretty normal too, where they stand on their hind legs and swat each other with their paws. These behaviours are quite harmless unless they get out of hand. You need to watch the gerbils for at least 2 hours to make sure that they are going to get along. If they start grooming each other or you see them sleeping in a nest together, then they probably will be fine.
If you see any overly aggressive behaviour, you will need to resplit your gerbils and try again. Please don't lose hope, these things happen. With patience, you should see your gerbils together eventually. I have never had a split-cage fail before. The general rule is to try split-caging your gerbils for 2 full weeks. If they fight, then split cage them again. If the third time fails, then they will probably never live together. I cannot stress enough that you need lots of patience.
Aggressive behaviour includes actual fighting, where the two gerbils roll into a ball and are biting each other's throats. If they roll into a ball, you must seperate them immediately! This is a fight to the death! Another warning sign is if you see one gerbil chasing the other gerbil crazily, and the gerbil being chased is squeaking wildly. These gerbils would need to be resplit. Other things to watch for are aggressive marking where one gerbil won't leave the other alone and overgrooming where the gerbil being groomed is squeaking a lot.