Mine do this same thing (and I only have 2). In fact, I just fed them and then noticed they were making quite a rukus. I went back to check it out and Lucy (the smaller one who we swear is a female but ACTS like a male) was on top of Nini holding onto him in his huge bear hug in the water. I've also seen Nini have Lucy's entire leg in his mouth. When this happened we separated them because it was always happening during feeding times. Lucy wasn't getting her fair share of the eats and was losing weight.