Cruelty to performing and circus animals includes over-discipline involving excessive beatings and withholding of food, excessive hours of training, cramped quarters (often small cages on trailers), and extermination when biologically fit but ageing. Transportation between locations subjects the animals to extreme changes in temperature and disruption of natural cycles, causing considerable stress.
Learning of tricks goes against an animal's natural instincts, especially when taught under demeaning conditions. During any performance animals may be subject to ridicule and contempt in order to conform to the public's stereotype of them.
By experience and training circus animals become accustomed with many things other animals, inexperienced in circus, would find distressing.