red eared slider and possibly red eared cooter breeding

Question
i have a 2 red eared sliders!they are the same age but on is very much smaller than the other and i think i have a river cooter!i dont know wat it is because my brother was under nourishing them and they had a bad kept home so i took them home with me!i woke up one day after leaving the window open and the"cooter" was giving the mating dance t my large slider"lily!"this confused me cuz i dont think they can breed but then agian i dnt know wat the one i think the cooter is?any help?

Answer
If it has red ears, it is pretty much a Red-ear Slider. Female Sliders are usually bigger than the males.

If they are not of mating age (4" or bigger), then this can be a sign of overcrowding and aggression.

Try http://www.redearslider.com for more info.