Cover photo: Barbary macaque (Macaca sylvanus) photo by Mary Lassanyi.

Kreger, Michael D. "Environmental Enrichment for Nonhuman Primates Resource Guide January 1992 - February 1999." AWIC Resources Series No. 5. Beltsville, MD: U.S. Department of Agriculture, Agricultural Research Service, National Agricultural Library, Animal Welfare Information Center. March 1999. 114 pp. http://www.nal.usda.gov/awic/pubs/primates/primate.htm