We had a guinea pig that was approximately one year old. We recently added another (both are female), but we didn't have any problems with them getting along. Everything seemed fine and then this morning the year-old guinea pig was dead. There were no bite marks or any sign of problems. What could have caused this? I can't give much more detail because we haven't been watching her as close as usually over the past few weeks because of a death in our family. Can you give me some ideas so this doesn't happen to the other one. Plus, we are buying another to replace the one that died and really don't want to go through this again. Thank you!      David Droll