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Forming a Bachelor Group at the Louisville Zoo's Gorilla Forest

Compiled by Roby Elsner

Tremendous advancements in the care and management of captive western lowland gorillas (Gorilla gorilla gorilla) have been made over the past twenty-five years. Many of today’s zoos strive to house their gorillas in complex, spacious, and natural exhibits, as well as provide them with adequate mental stimulation, nutrition, and veterinary care. Another important aspect of modern gorilla programs is the provision of appropriate social stimulation. A better understanding of the social structure of wild gorillas has encouraged zoos to maintain gorillas in species typical groups.

As a polygynous species, wild gorillas most typically live in long-term, established social units comprised of a leading silverback, several adult females, and their immature offspring. Groups vary in size from two to twenty individuals. Before breeding, females transfer to other social groups or reside with solitary silverbacks. Maturing male gorillas, or blackbacks (yet to develop the characteristic silverback graying), leave their natal groups as well, living solitarily or occasionally in all-male groups – known as bachelor groups -- until being able to create cohesive family units of their own (Parnell 2002).

Maintaining gorillas in social groups similar to that of their wild counterparts and employing other methods to improve the captive environment have led to improvements in reproductive success. In the 1970’s, captive gorilla births were uncommon and a majority of the infants born were pulled for hand-rearing. Not only has breeding success increased since then, but more females now demonstrate maternal competence. Many of them were raised in or introduced to social groups themselves, and it is hoped that they will raise their offspring successfully within their own groups. For females that need some assistance raising their offspring, zoos have developed training programs to foster maternal care. Sometimes because of health or social constraints, hand-raising is unavoidable. In these instances, some zoos have implemented surrogacy programs, where adoptive gorillas provide care-giving behaviors to the infants and help them integrate successfully into social groups at a young age. Hand-raised females integrated into social groups at a young age have a likely better chance of demonstrating species-typical breeding behavior and exhibiting adequate maternal skills toward any potential offspring (Beck and Power 1988; Ryan et al. 2002).

Captive gorilla population management has become increasingly important with the growing number of births, especially considering that the birth ratio for this polygynous species is non-sex biased. Approximately 50 percent of gorillas born in captivity are females and 50 percent males. With the same number of male and females born in captivity, maintaining gorillas in species-typical groups means that some males will inevitably be without female social partners.

As young male gorillas of the wild mature within their natal group, they represent potential competition for the leading silverbacks, who become less tolerant of the blackbacks. At this point the young males would likely leave their natal groups to live a portion of their lives outside mixed-sex social groups. Captive blackbacks within family units often become less tolerated by the silverbacks as well, resulting in possible aggression. Borrowing the example from the wild several years ago, the American Association of Zoos & Aquariums’ Gorilla Species Survival Plan Management Group suggested the formation of all-male groups, or bachelor groups, within zoos to address the needs of the captive gorilla population (Hutchins et al. 2001) Through various methods with males of varying ages and numbers, several zoos have successfully formed bachelor groups of males and continue to maintain them as such. In forming these groups, zoos tried combining males of adult ages, one or more males of an adult age with one or more juvenile males, or all juvenile males. While the former strategy failed, the latter two have worked. St Louis Zoo formed a bachelor troop in 1990, Cleveland created one in 1994, and Zoo Atlanta followed in 1996. Other zoos with bachelor groups include Disney’s Animal Kingdom, Santa Barbara Zoo, Knoxville Zoo, Kansas City Zoo, St Paul’s Como Zoo, and Birmingham Zoo. Several zoos with gorilla exhibits in the construction phase, such as Sedgwick County Zoo and Henry Doorly Zoo, plan to house bachelor groups.

There is still much to be learned on the requirements necessary to successfully form and maintain potentially long-term, stable groups of all-male gorillas. Just as much was learned from field and zoo research and applied toward improving the environment of captive gorillas, much ongoing research has focused on all-male groups and findings are anticipated to apply toward the successful management of bachelors. The research examines all-male gorilla groups from a variety of perspectives, including behavior, physiology, personality, and exhibit design. Responsible for a multi-year inter-institutional study on captive male gorillas has been Zoo Atlanta’s Tara Stoinski, Cleveland Metroparks Zoo’s Kristen Lukas, and Disney’s Animal Kingdom’s Chris Kuhar. Currently, 14 zoos participate in the study by collecting behavioral datat on their gorillas. Data are collected from males of all-male groups as well as those in mixed-sex groupings. These institutions plus a number of additional zoos assist in the collection of physiological data for the study as well by collecting urine samples from males. Louisville Zoo is currently participating with both aspects of this research.

With the growing number of captive gorilla births, it is likely that zoos will need to create and maintain an additional ten to twenty (or even more) bachelor groups in the next few years. The Gorilla SSP strongly encourages zoos to house bachelor groups when possible.

In playing its part to help the captive gorilla population, Louisville Zoo accepted the responsibility of trying to successfully form and maintain a bachelor group of gorillas. In deciding how to form this bachelor group, recent findings from research were investigated and revealed that some of the most successful bachelor groups thus far have been those created with young males during their formative years. However, to more closely approach species-typical norms, males deemed for allocation into long-term bachelor groups, such as those that are genetically well represented, should still be able in their early years to reap the benefits of social or family groups before forming all-male groups of their own at approximately 8-10 years of age (Stoinski et al., in press).

To accommodate the construction of its new African ape facility, Lincoln Park Zoo in 2002 loaned to Louisville Zoo twelve gorillas (four males and eight females) for the latter’s newly constructed Gorilla Forest. Until Debbie’s death August 2003, the social composition of the gorillas consisted of a social unit of five (JoJo and his four adult females, now with infant Azizi), a social unit of six (Frank, Debbie, and the four juveniles in the group), and a solitary-but-content Helen. All of JoJo’s group and Rollie and Mumbali are scheduled to return to Lincoln Park Zoo in May 2004, but remaining behind will be Frank, Jelani, Bengati, and Helen.

Genetically well-represented males, Jelani and Bengati are good candidates to go into a bachelor group. Frank is very paternal toward these two males, and he has historically surrogated other youngsters in his group as well. Because of the young ages and social histories of mother-raised Jelani (born January 1997) and Bengati (born June 1998) and the paternalism of Frank, the addition of two young males to the group was deemed optimistically successful. Previously, Debbie with juveniles Rollie and Mumbali occupied this group as well. Males integrated into this group would have been exposed to the “family” elements that the females provided. Unfortunately, Debbie’s death was untimely, and she was unable to loan her excellent surrogacy skills to likely help integrate the chosen males into the group. However, many attributes of this group still constitute a family group, albeit not a typical one. Rollie and Mumbali showed no signs of separation anxiety from Debbie, and they continue to demonstrate many prosocial behaviors in Frank’s current troop. The friendly behavior is often reciprocated from the males. Additionally, Frank is paternal toward and very tolerant of the females.  Until their return to Chicago, Louisville Zoo gorilla and management staff felt that Rollie and Mumbali should remain in their current group to provide what social contributions they can in that brief time. They, too, will benefit from the males’ presence and even learn skills valuable to gorilla introductions. Once they return to their new home, Rollie and Mumbali are scheduled for integration into JoJo’s group.

Louisville Zoo collaborated with the Gorilla SSP and Lincoln Park Zoo to research the availability of two mother-raised gorillas of similar age to Jelani and Bengati that resided in a North American Zoo. Ideally, these gorillas would be genetically well represented as well. It took little time to find the perfect candidates, Cincinnati Zoo’s Kicho (born March 1997) and Cecil (born November 1998). Both mother raised and well socialized in the same troop, they are almost the same ages as Jelani and Bengati, and are genetically well represented as well. Making their potential transfer to Louisville even more attractive was the fact that they had been exhibiting reproductive behaviors toward members of their family group, and Cincinnati hoped for their relocation to another zoo. Rapid and cooperative communication between Cincinnati Zoo and Louisville Zoo (and owning institution Philadelphia Zoo), as well as timely authorization from the Gorilla SSP, resulted in the transfer of Cincy’s males only a couple of months after the candidates were identified.

Adhering to zoo policy, Kicho and Cecil spent approximately one month in quarantine. In working with them during that time, their personalities hinted to staff that Louisville Zoo indeed received the correct pair of males to introduce to Frank, Jelani, and Bengati. In addition to five males expected to get along, the design of the gorilla facility provides elements essential to forming a bachelor group. High flexibility and spaciousness, as well as high degree of accessibility of staff to gorillas, are definitely advantageous to any gorilla introductory process. Gaining skills as gorilla keepers for the past two years, Gorilla Forest animal staff has on a daily basis demonstrated proficiency in rotating the gorillas throughout their spaces, training them for flexibility (in shifting and separating, something very advantageous during a gorilla introduction) and prosocial behaviors, and granting them control and the ability to make choices as much as possible. Other aspects of the area’s comprehensive enrichment program that are employed each day, and that scheduled for implementation, will likely help the process of forming a bachelor group of gorillas well.

In playing its part to assist the captive gorilla population, Louisville Zoo Gorilla Forest staff is confident and determined to ensure success in conducting this introduction to form and maintain a successful bachelor group of gorillas.


Beck, B., and Power, M. 1988. Correlates of sexual and maternal competence in captive gorillas. Zoo Biology 7: 339-350.

Hutchins, M., Smith, B., Fulk, R., Perkins, L., Reinhartz, G., and Wharton, D. 2001. Rights or welfare: A response to the Great Ape Project. In Great Apes & Humans: The Ethics of Coexistence, eds. B. Beck, T. Stoinski, M. Hutchins, T. Maple, B. Norton, A. Rowan, E. Stevens, and A. Arluke, 329-366.Washington,D.C.: Smithsonian Institution Press. 

Parnell, R. 2002. Group size and structure in western lowland gorillas (Gorilla g. gorilla) at Mbeli Bai, Republic of Congo. Am. J. Primatol. 56: 193-206.

Ryan, S., Thompson, S., Roth, A., and Gold, K. 2002. Effects of hand-rearing on the reproductive success of western lowland gorillas in North America. Zoo Biology 21(4): 389-401.

Stoinski, T., Kuhar, C., Lukas, K., and Maple, T. In press. Social dynamics of male western lowland gorillas living in bachelor groups.  Behaviour.