If you're wanting to do their natural habitats then zoology or wildlife biology would be good starts.
You'll need a PhD to really do anything in those fields.
Most likely a bachelor's degree in biology, zoology, or wildlife ecology, those are the most common to start in. There are tons of them that you can do though depending on your exact area of study. For example if you wanted to work with marine animals marine biology would be a good one, just depends what your preference is.