Raising Gerbils?

I had 2 female gerbils of about 6 months old. One of them died suddenly today, with no obvious signs of illness, in fact, she was fine just yesterday evening. I had to move them to a friend's for a couple of days as my heating broke down and I didn't want them to freeze. So I was wondering if the trauma of moving could have been a reason?

The remaining one seems fine. She is the bigger and more dominant of the pair. Will she be OK on her own, or should I try to introduce a companion for her? I have read that females can be very territorial, but if she needs a "friend" I am aware of the split cage method.
Would be grateful of any advice.

By Mitch from UK