You can see the difference in size. The smallest are most likely female, and the largest are most likely the alpha males - the ones most likely to get to the food first, and who will be more assertive as they age. When looking for a pair, it's often best to select one of the largest (most likely, a male) with one of the smaller ones (most likely, a female). Getting two of the largest often means you've selected two males, which can result in territorial issues.
