First Advisor

Richard Forbes

Date of Publication


Document Type


Degree Name

Doctor of Philosophy (Ph.D.) in Environmental Sciences and Resources: Biology


Environmental Science and Management




Baboons, Zoo animals -- Behavior



Physical Description

1 online resource (x, 181 pages)


Drills (Mandrillus leucophaeus) are an endangered species of African monkey (Cercopithecidae), and their sole congener the mandrill (Mandrillus sphinx) is vulnerable to extinction. Both species are threatened in the wild by deforestation and hunting.

Drills have a poor record of captive reproduction. Many individuals appear to have behavioral deficiencies which interfere with reproduction. Thus, the zoo population of drills does not serve as a “hedge” against the species’ total extinction: drills are endangered in captivity as well as in the wild. Mandrills, by contrast, reproduce well in captivity. Information on the behavior of mandrills in captivity may help zoo managers improve husbandry for both species.

The intent of this research was to study the relationship between aspects of the captive environment and behaviors which lead to reproduction. A review of the literature on both drills and mandrills, in the wild and captivity, was used to suggest “essential characteristics” of the captive environment that may encourage animals of both species to engage in natural, active behaviors, to form cohesive dyads with opposite-sex adults, to develop affiliative bonds, and to engage in sexual behavior. Sixty-two drills and mandrills in 14 groups in the U.S. and Germany were studied with behavior sampling methods, using the Drill Species Survival Plan ethogram.

Data were analyzed by multiple regression using transformed variables. No over-all species differences in behavior were found. Results suggested that two factors promoted natural activity: 1) environmental enrichment which provides positive reinforcement for active behaviors, and 2) an affiliative husbandry style by the animal’s keepers. Active animals were more likely to engage in social behaviors leading to copulation.

Many of the non-reproducing drills appeared to be “passive and withdrawn,” and shared a constellation of signs that appeared to be analogous to human clinical depression. An etiological model for “passive and withdrawn” Mandrillus, based on biobehavioral theories of human depressive disorders, was developed to link early rearing conditions and environmental enrichment. This model was then used to develop a historic-demographic hypothesis for why mandrills have historically had greater reproductive success than drills in zoos.


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