You could do all females but as far as I understand it some of the girls can call too... although not as loud. But maybe its only in response to the males. Hopefully someone who keeps them will clarify for us shortly. Cobalts can't be heard outside their tank. I have a pair of them on my desk at work. Although they are very territorial so you can only keep them in pairs. But they are very bold and quiet. Do not mix the tree frogs with the darts....they have different environmental requirements. And it will eventually mean doom for one group or both.