Hancocks, David: A different nature: the paradoxical world of zoos and their uncertain future. Berkeley 2001. Internet: https://books.google.de/books?id=Oaal_LfJTNAC&printsec=frontcover&dq=Hancocks,+David:+A+Different+Nature.+The+Paradoxical+World+of+Zoos+and+their+Uncertain+Future.+Berkeley/Los+Angeles/London+2001.&hl=en&sa=X&ved=0ahUKEwjEg4KtoLbVAhXM7RQKHdK8CIAQ6AEIPjAF#v=onepage&q&f=false.