Gerbil biting its own tail

Question
I have 2 gerbils and they have lived happily together since birth, now being about 7 or 8 months old.  One has just started biting the fur off his tail causing it to bleed?  Why has he started doing this and how can I make him stop?

Answer
he is becoming aggressive towards the other gerbil and the only way to stop it is to seperate them. He is fighting for dominance of the cage and will continue to pick on the other gerbil. you need to seperate them. that's the best and only way to do really help out both gerbils.