C: Unexplained death

[Diana Arnett]

I had a pair of female gerbs, one one year old, the other one older (exact age unknown).  Both died late last night.  I found them curled up together in the tank when I came home from school.  The black was quite dead and the DEH was nearly dead.  She died very shortly after I found her.  The black may have been old enough to die of old age (she was a rescue), but the DEH just turned a year this month.  I haven't done anything new with them--same food, toys, bedding.  They were fine yesterday playing and running in the wheel.  I'd like to get another pair (the cats really miss watching them run) but I'm a bit concerned since I don't know why they died.  Any ideas?   Diana