I recently saw a video that brought up an interesting point.
Pneumatic bones, or bones that are hollow, are light but not very strong. On the flip side, solid bones are stronger, but much heavier. All birds have hollow bones, which only makes sense because they need a light frame to enable them to fly. However, their bodies are fragile and can't take much punishment. It's a logical trade-off.
Mammals, on the other hand, need stronger bones to hold up their weight against the surface, to run from or after other animals. Again, it only makes sense that mammals that don't need to fly should have heavy, dense bones.
However, there are a handful of exceptions in the animal kingdom. Ostriches, for example, never spend a minute of their lives in flight, and yet they have hollow bones, and in fact suffer from a higher incidence of bone fractures than other land animals of the same size and weight. Likewise, bats spend an inordinate amount of time in the air, and yet they have the added weight of solid bones like their land-based mammalian cousins.
So it seems that someone decided that if you are a bird, then you get hollow bones, whether you need them or not, and if you are a mammal, then you get solid bones, whether they help you or not. Why would that be the case?
The theory of evolution has an answer, and it's remarkably simple. Ostriches descended from birds of flight and thus inherited their hollow bones, to their disadvantage. Also, bats descended from mammals, and thus received solid bones and their extra weight.
Now it seems to me that if I were in charge of designing the animal kingdom, I wouldn't parcel out bone structure based on whether you were a bird or a mammal. I would divvy up hollow bones to flying creatures and solid bones to land-based creatures, regardless of their class.
Essentially, this is William Paley's watch argument in reverse: If you are tromping through a dense woods, and you come upon an ostrich corpse lying on the ground next to a bat corpse, what can you infer? Looking at the feathers, wings, and hollow bones of the ostrich, you might conclude that it is a flying bird. And looking at the hair, teats, and solid bones of the bat, you might conclude that it is a land-based mammal. And you would be dead wrong on both counts.
Yes, we could argue that our mythical Intelligent Designer must have had a good reason to create these two creatures with these handicaps--it's not a suboptimal design if you designed them that way on purpose. But it seems very strange to me that the Intelligent Designer would intentionally design something to appear as if it wasn't intelligently designed.