Why do bison body-slam trees?

Did you know that American bison help prevent grasslands from turning into forests by body-slamming young trees? And that they once lived in herds so enormous they stretched all the way to the horizon? This week, we travel to the grasslands of Montana with Dr. Andy Boyce, a research ecologist at the Smithsonian's National Zoo and Conservation Biology Institute. Discover why bison live in herds, how they defend themselves from predators, and why scientists call them a keystone species. Along t...