It seems like animals can climb on rocks, but their droppings can't. I have had multiple occasions when the dirty habitats notification keep popping up, tried very hard to find out where the poops are, and it turned out they're just stock-piling under those blasted rocks where the keepers cant reach. What are the odds of the animals repeatedly climb on top of a rock and take a dump? Highly likely it seems. It would be better if animals droppings can interact with the rocks, either stick on them or roll all the way down for your convenience.