I got 2 little girls about 4 months ago. They are my first guinea pigs. They were kept together with 2 other piggies before I got them. One, Yennifer, was missing half of 1 ear before I got them. I watched her being chased and bitten by 1 of the piggies I did not get. After I got them home, Yang chased Yennifer around mounting her for several weeks but nothing too terrible. No biting. Mostly just "head swaying". After a couple of weeks they settled in together. It took at least a month and a half for them to let me pet them and now they're both fairly comfortable with me also. But, now a new problem has arisen. When I take Yang out by herself she's great with me. It's when I put her back with her sister that she becomes aggressive. Their cage is very large from what I've read. But, Yang makes it a point to find her sister and jump all over her making poor little Yennifer scream. I admit Yennifer is quite a bit more emotional than Yang. She cries when she's left alone in the cage and squeels with delight when I bring in the veggies; whereas Yang is much more reserved and rarely makes any sounds aside from some"purring". And Yang is definitely the confident one. But, I don't understand why it isn't the other way around, as in they get excited and happy to be reunited. They are separated for no more than 20 minutes at a time and it's usually more like 10 minutes for just some 1 on 1 quality time with "mommy". Yennifer does not do this when she's put back in. Nor does she ever try to defend herself. She just squeels until I come rescue her. Could Yang be getting upset because she doesn't want put back in the cage and taking it out on her sister?