Female Red Eared Sliders are Biting Eachother

Question
I've had two red eared sliders for about 3 years. They are both females as far as I can tell, Harriet and Timothy (I named Timothy before I realized he was a girl#. Harriet is slightly older than Timothy #just by a few months) but they've always gotten along. Earlier this year, Harriet got this wound on the back of her back leg and the place between her tail and her foot. I wasn't sure where it came from so I just made sure her tank was clean and she plenty of basking time and it cleared up. Now It's back and worse than ever and I'm trying to do the same thing but the other day I came in and saw Timothy come up behind Harriet and bite the healing wound. Harriet got all freaked out and I separated them as quickly as I could, and they've been in separate containers since then but it's really a temporary solution. I don't know if that is how the wound got there or if Timothy is trying to help somehow, or if they have suddenly developed a hate for one another but I'm kind of afraid. Do you know why this is happening or what I can do to stop it?

Thanks, Emma

Answer
Hi Emma,

If you got your turtles as hatchlings and they're only three years old, it's possible one is a male, since young RES all look female.  Timothy may in fact be a maturing male, so watch for signs (most obvious would be lengthening claws).  However, females can also develop aggression, most typically in situations of overcrowding.  You didn't give any details on your setup, but RES require a great deal of space.  If your turtles are about 4" now, they need at least an 80 gallon tank, and as adults if both are female you really need about a 200 gallon tank, with corresponding filter, large basking area, etc.  If conditions are too crowded, it's much more likely that a more dominant turtle will bully a tankmate and hog the best basking areas, food, etc., along with biting and chasing.  

If you aren't able to provide one large tank, you should probably separate them so that Harriet doesn't become too stressed (and you should certainly separate them while the injury heals).  In any case, if Timothy does turn out to be male, separation is still a good idea because a male is very likely to constantly harass a single female even in a very large tank.

For now, the wound can be cleaned with diluted betadine and an antibiotic ointment applied; keep her dry for a couple of hours afterward.