In June 1989, Loon began rapidly losing weight, and in July of that year he was diagnosed with diabetes. Treatment began immediately, with multiple daily blood draws and insulin injections; however, this led Loon to become extremely aggressive, and the zoo was forced to restrain him in a small cage, and to sedate him before procedures. As a result, he developed neurotic and stereotyped behavior, which decreased his quality of life so much that euthanasia was considered.
With the help of a former SeaWorld animal trainer, zoo personnel used operant conditioning techniques so that Loon would associate venipuncture and other medical procedures with rewards such as getting fed or groomed. He subsequently learned to participate in getting himself weighed, and to provide daily urine samples by "go[ing] potty" on command.
Loon eventually became a role model for human children with diabetes who were likewise reluctant to receive multiple injections, and the techniques which had been used to train him were adapted by other institutions to train their animals to accede to medical procedures.
Samples of Loon's tissues have been cryopreserved in a frozen zoo. Due to physical consequences of his diabetes, Loon was unable to breed naturally; however, his semen was assigned to the Columbus Zoo, where female drills are available for artificial insemination. As well, he was one of the first two animals of an endangered species to have his tissue samples used to make induced pluripotent stem cells as an attempt at conservation.
- Ben-Nun IF, Montague SC, Houck ML, Tran HT, Garitaonandia I, Leonardo TR, Wang YC, Charter SJ, Laurent LC, Ryder OA, Loring JF (2011). "Induced pluripotent stem cells from highly endangered species" (PDF). Nature Methods. 8 (10): 829–31. doi:10.1038/nmeth.1706. PMID 21892153.
- Zoo farewells pioneering monkey, at the Australian Broadcasting Corporation; published June 16, 2003; retrieved June 14, 2016
- Training a Diabetic Drill (Mandrillus leucophaeus) to Accept Insulin Injections and Venipuncture, by Gary Priest, in Laboratory Primate Newsletter; volume 30, number 1 January 1991; archived at Brown University; retrieved June 14, 2016
- End of life near for pioneering monkey at zoo, by James Steinberg, in the San Diego Union-Tribune; published June 16, 2003; retrieved June 14, 2016
- Kicked, Bitten, and Scratched: Life and Lessons at the World's Premier School for Exotic Animal Trainers, by Amy Sutherland; published 2006, by Penguin Books
- NEW TECHNOLOGIES HELP RARE ANIMALS REPRODUCE, by Michele Ostrove, in Deseret News; published March 18, 1990; retrieved June 14, 2016
- Stem cell research could save endangered rhinos and monkeys, by Janet Fang; at ZDNet; published September 7, 2011; retrieved June 14, 2016
- Ben-Nun IF, Montague SC, Houck ML, Ryder O, Loring JF (2015). "Generation of Induced Pluripotent Stem Cells from Mammalian Endangered Species". Methods in Molecular Biology (Clifton, N.J.). 1330: 101–9. doi:10.1007/978-1-4939-2848-4_10. PMID 26621593. Retrieved 2016-06-20.