Environmental enrichment is a vague concept referring to improvements
to captive animal environments. Some authors have applied the term to
an environmental treatment itself, without any concrete evidence that
the treatment represented an improvement for the animals. Others have
used the term when the main beneficiaries may have been people rather
than their captive animals. The criteria used to assess enrichment hav
e also varied according to animal use (e.g. laboratory, farm or zoo an
imals). In this paper, environmental enrichment is defined as an impro
vement in the biological functioning of captive animals resulting from
modifications to their environment. Evidence of improved biological f
unctioning could include increased lifetime reproductive success, incr
eased inclusive fitness or a correlate of these such as improved healt
h. However, specifying an appropriate endpoint is problematic, especia
lly for domestic animals. Potential methods of achieving enrichment th
at require further investigation include presenting food in ways that
stimulate foraging behaviour and dividing enclosures into different fu
nctional areas. The quality of the external environment within the ani
mals' sensory range also deserves greater attention. A common shortcom
ing of attempts at environmental enrichment is the provision of toys,
music or other stimuli having little functional relevance to the anima
ls. Failure to consider the effects of developmental factors and previ
ous experience can also produce poor results. Environmental enrichment
is constrained by financial costs and time demands on caretakers, and
providing live prey to enrich the environment of predators raises eth
ical concerns. Future research on environmental enrichment would benef
it from improved knowledge of the functions of behaviour performed in
captivity and more rigorous experimental design.