These snakes represented here were emaciated and spines showing, it probably didn't matter as to what they were eating, as long as it was alive and it was meat.
As for them in the wild, natural selection tells them what they need to eat, as the ones that survive will probably eat from the same source, and the ones that don't die, creating an ecosystem most likely that doesn't need thought or calibration on the prey size they want to eat.
More knowledgeable people please correct me if I'm wrong.