The best way I have found with out a split cage is to have a second cage and let the new male have time in it for 2 weeks so you know he doesn't have any problems like disease that he can pass to your other gerbils and this way he can make a scent in this cage, what you do after the 2 weeks is to put him in their cage and then put them in his cage and do this for a few days and then introduce them, if they fight then separate them and do this again. I only change the gerbils once a day so they get use to the smell in the cage. I have had lots of success with this. And no one has fault as of yet.