One of our gerbils ( about 10 weeks old ) has suddenly become quite aggressive. They had both stopped nipping, and were letting us stroke them quite happily and pick them up, though they don't stay in our hands very long. The one who has become aggressive was even crawling up my daughters arm and up onto her head! This week though she has started biting or trying to bite whenever someone puts their hand in the cage or tries to handle her. As soon as we go over to talk to her she leaps up on to the bars, and will try to bite if we put our fingers anywhere near the bars. If we put our hands in she runs straight over to 'attack'. Does any one know why this could be happening? She is the bigger of the two and seems to be more dominant and more active than the other one. They have chewing toys, tunnels and a wheel (which they don't run on!). Any advice would be very welcome.