Predictors of Matrilineal Overthrows in Large Captive Breeding Groups of Rhesus Macaques (Macaca mulatta)

The purpose of this retrospective case-control study was to identify and assess biologically plausible variables that may predispose a captive rhesus macaque breeding colony to a matrilineal overthrow. Matrilineal overthrows are the result of members of multiple matrilines jointly attacking the highest-ranking matriline. Matrilineal overthrows in captive rhesus macaque colonies result in significant morbidity, mortality, and loss of genetic diversity. The following variables were investigated as potential determinants of overthrows: season, cage density, demographics, sex ratio, age of the alpha and beta animals, absence of the alpha and beta animals, pregnancy status of the alpha and beta females, number of adult females in the alpha matriline, recent changes in the male hierarchy, time since group formation, and number of adolescent males in the alpha matriline. Data were collected from January 1996 through January 2007. Univariate analysis indicated that absence of the alpha female from the group was associated with matrilineal overthrows, but multivariate analysis was not totally supportive. Conditional logistic regression identified number of juvenile males and number of adolescent males as associated with an overthrow; exact logistic regression was supportive. Principal component analysis followed by multivariate logistic regression identified 2 marginally nonsignificant predictors (the density and alpha factors). Our results suggest a possible association between the occurrence of a matrilineal overthrow and the following factors: absence of the alpha female, decreased housing density, number of juvenile males, and number of adolescent males.