Nance, Susan: Entertaining elephants: animal agency and the business of the American circus. Baltimore 2013. Internet: https://books.google.de/books?id=k2DvAkBB7l4C&printsec=frontcover&dq=Entertaining+Elephants.+Animal+Agency+and+the+Business+of+the+American+Circus.&hl=en&sa=X&redir_esc=y#v=onepage&q=Entertaining%20Elephants.%20Animal%20Agency%20and%20the%20Business%20of%20the%20American%20Circus.&f=false.