I have two female guinea pigs, a 6 year old (R) and a 4 year old (P). As R has gotten older, since she turned 5 or so, she has seemed to have lost weight. The last time I weighed her, I think she was around 800g, which isn't extremely low compared to before, but she definitely looks and feels to be on the skinnier side. P eats the same amount as R, if not less (R is the dominant one), but P is definitely on the fatter side (over 900g last time I weighed her). Should I supplement R's diet with formula to give her extra calories, or is weight loss/redistribution common in older guinea pigs?