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I have 2 Red Sliders both females one is much older approx. 24 years the other approx. 12 years of age they had been together in harmony for at least 5 years with no issues. Now the smaller younger female is being quite aggressive with the older turtle chasing and biting and I have had to separate them.. Does anyone have any idea why this may be happening.. both turtles shed eggs.
thx v

No idea why but it does happen. Almost always with males, but some females too. You will need to separate them before serious injury occurs.

Thanks Steve.. I have isolated them.. I am thinking it has something to do with territory and breeding frustrations from the younger female (maybe since there is no male) Their territory is quite large (200 gal water tank attached to a large 2 by 5 foot dry area. It seems to have started around the time when the young one started shedding eggs... I also noticed the smaller one trying to confine the older turtle to the dry area and all attacks were happening in the water.
The ironic thing is I got the second turtle to provide companionship, which did work for the longest time..
v

I'm not sure if the female aggression is due to mating, though there is a lot we don't know. RES do have strong and competitive personalities which I think can make them aggressive. I'm glad you're able to separate them. I have to keep my male and female RES in separate tanks, I've been wondering if I should move them to a single (divided) tank.