Predator/Prey simulation

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I want to create two simulated robots which will engage in predator/prey behavior. I don't actually care what kind of robots they are, the simpler the better. But I do need a way to detect when the predator collides with the prey in order to remove the prey from the simulation (due to being eaten).

I've managed to get a few turtles running in the turtlesim, but how would I go about doing collision detection? Would I have to roll my own node or is there something existing I can use?

If I switched from turtlesim to either stage or gazebo, would this be easier to leverage pre-built tools do the collision detection?