The following poem says it all. This is the main reason for me why I dislike zoo’s and keeping animals in cages. But behind the obvious is for me the human being in his/her own cage. 
The Panther
in the Jardin des Plantes, Paris
His vision, from the constantly passing bars,
has grown so weary that it cannot hold
anything else. It seems to him there are
a thousand bars; and behind the bars, no world.
As he paces in cramped circles, over and over,
the movement of his powerful soft strides
is like a ritual dance around a center
in which a mighty will stands paralyzed.
Only at times, the curtain of the pupils
lifts, quietly-. An image enters in,
rushes down through the tensed, arrested muscles,
plunges into the heart and is gone.
The selected Poetry and Prose of R.M. Rilke by Stephen Mitchell