Webster, J. 1994. Animal Welfare - A Cool Eye Towards Eden. Blackwell Science, Oxford, UK.

Pragmatic, very useful ideas on environmental enrichment. It is reassuringly easy to state the obvious, i.e. sentient animals should not be kept in barren cages. ...Most 'toys' offered to animals in barren environments are ignored after a brief period of investigation because the animals perceive them to be useless.