It's a jungle out there. And in here. We all know that animals play a role in medicine. But you may not have known what a wide range of animals are finding their way inside the human body—sometimes literally, in the form of transplanted tissue, but more often figuratively, in the way they help researchers develop treatments for humans. Our furry (and scaly, and feathered) friends are saving more lives, with each passing year. From apes to zebrafish, here's a slideshow of the new medical menagerie.