I would say it's a great symbiotic relationship, but it all comes down to plants. I suggest reading the book, and I'm sorry if I get the name wrong, as I haven't read it in a while, and don't own a copy, "The Bontany of desire" or something like that. But in this book. It talks about this topic, really. Plants were here before animals, and provided food for animals in the Precambrian and Paleozoic eras. Then in the Mesozoic, flowering plants evolved, which in turn gave rise to fruits, seeds, etc. This gave more food/energy to the natural world to help warm blooded animals. It all comes back to plants.