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Section C
Environmental Enrichment
Environmental Enrichment

Providing novel or complex stimuli to encourage species-specific
behaviors in a laboratory setting, and/or to avoid distress and
stereotypical behaviors resulting from boredom or fear
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Enrichment—Group Housing Considerations

Is the species social or solitary in the wild?

Normal complex socialization in the wild, e.g., rhesus macaques—
different for males and females
Photo by Kai Yan, Joseph Wong. Creative Commons BY-NC-SA.
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Enrichment—Group Housing Benefits and Costs

Benefits include support of species-specific behavior, social
buffering, increased resistance to disease, increased immune
response

Costs include increased aggression and wounding, food competition,
infant mortality, increased variability (dominance rank in NHP
groups), separation effects, e.g., depression
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Enrichment—Immunological Impact of Housing in NHP

Single housed (SH) gorillas have elevated cortisol

SH rhesus and African green monkeys develop long-term
immunosuppression

Pair housing marmosets reduces cortisol response to novelty

Social separation of cynomolgus macaques exacerbates
atherosclerosis
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Enrichment—Immunological Impact of Housing in Mice

SH induces immunosuppression

Minimal stress with four per cage compared with two or eight
per cage

SH behave differently in behavioral tests

Group housing influences expression of heat shock proteins,
chemotherapeutic efficacy, tumor growth, hematopoiesis
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Environmental Enrichment in Rodents

Brain structure and function are affected by environment [enriched
cages (EC) vs. impoverished cages (rats kept in barren, individual
cages) (IC)]
-  The number of synapses per neuron is 20–25 percent higher in
EC rats
-  There is an equally substantial increase in the sizes of dendritic
fields of neurons
Source: Greenough, W. University of Illinois.
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Environmental Enrichment in Rodents
Source: Greenough, W. University of Illinois.
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Environmental Enrichment in Rodents

There are differences in synapse morphology and architecture
between EC and IC rats

Volume of capillary per neuron is increased in EC rats

Astrocytes in EC rats are increased in size and number

EC rats have more myelinated axons in the corpus callosum than IC
rats
Source: Greenough, W. University of Illinois.
10
Environmental Enrichment in Rodents

Other effects include …
-  Greater body weight in IC than in EC rats
-  Greater food consumption by IC rats
-  More rapid maturation of the long bones in IC rats
-  Higher kidney/body weight ratio in EC rats
-  Lower thymus/body weight ratio in EC rats
Source: Greenough, W. University of Illinois.
11
Environmental Enrichment in Rodents

His conclusion
-  Environmental enrichment can affect research outcomes—it may
not be valid to compare results from animals kept under
different environmental conditions
Source: Greenough, W. University of Illinois.
12
www.ilarjournal.com
13
http://dels.nas.edu/ilar
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