Why does my European pond turtle rest on the back of my red-eared slider?

Question

I have a European pond turtle and a red-eared slider as pets. They had been living together in harmony in an aquarium for about 8 years, but when I moved them both to a larger aquarium a year ago, I started noticing something weird.

Every now and then, the pond turtle (which is about half the size of the red-eared) would climb on the back of the red-eared slider and stay there. The red-eared doesn't seem to like it and tries to remove the pond turtle from its back, but the smaller pond turtle manages to stay there.

Is this normal behavior, or could it be harmful for the red-eared slider? Should they co-exist in the same aquarium?

Answer 1

The thing is, they lived together for 8 years in a smaller tank, so that does mean that they can live together. Ideally, you would mix turtles of similar size, as that reduces some risk, but from your description I don't think there's much in the way of aggression going on here.

What I do think is happening, and the larger tank is the key to having me think this, is that the smaller turtle is stacking on the larger one to get closer to the basking light(s), which is actually fairly normal. However, this irritates the larger turtle because your RES also wants (and needs) to bask. If it's happening all the time, this could be an issue for the RES since sufficient UV radiation is necessary for shell and bone health. If it really is infrequent, as you kind of indicate, then I don't think it's a risk.

However, you might want to help out the situation, perhaps ensuring a larger basking area and bringing the lamps closer in, if possible. If the tank is large enough, you might also want to consider a couple of basking areas.