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so i today i picked up my turtle stella and my other turtle cosmo jumped off to see what was happening and i held stella up to cosmos tank and cosmo started fluttering!

so does this mean cosmo is a male? that stella is a female? although i know they can flutter out of agression so stella could still be a male i guess?

I'm afraid it's not that simple! Both males and females will flutter without regard to the genders. And they won't just flutter at other turtles - sometimes they flutter at rocks, decorations, or their keepers.

Your little guys are way to young to tell whether they are males or females. They need to be at least 4" long before you can tell. The most dramatic thing that will happen is that the males will grow enormously long front toenails.

Fluttering is frequently a sign of aggression and also is part of the courtship ritual. I gather you've got your turtles in separate tanks, which is a good thing.