aggressive behaviour

Question
Hello!, I have here in Madrid two turtles, she is 4.7 inches and he 5.2 inches, they look like red ear slider, but they do not have that red spot under the eyes. They are healthy, strong, beautiful and funny, have a lot of sun and eat well. The male has being doing this "nail dance" to his friend since a year now, no problem with that, they have always been together, but this summer he trys to mount her and bites her neck really strong, so I have to separate them. The second one is starting to defend by menacing with biting. Problem is I am not even sure if the friend is a female, for the front nails are also long... Is this normal, should I let them together?, Will I have to buy a second tank?, so sorry because the one they have now is new and so nice and big! Thanks for your help!

Answer
Hello Elsa, this behavior is mating behavior the male will do his "nail dance" to entice mating then he will mount her and bite her neck while they mate.

However if you assume that the smaller one is also a male (if the front claws are longer then the back ones seen in the link below) then it is a territorial dominant behavior and yes they need to be separated. You have the larger of the two trying to say "This is my territory, my home, there is only enough room for one of us, and you need to leave" He is trying to show that he is strong and does not want another male trying to steal his territory.

Link on telling male from female:
http://redearslider.com/determining_sex.html

Good luck
Yexalen