Population and group structure of western lowland gorillas (Gorilla gorilla gorilla) at Lokoué, Republic of Congo

Abstract

During a 17-month study at the Lokoué clearing in Odzala National Park, Republic of Congo, we identified 377 western lowland gorillas. This population included 31 solitary males, 37 breeding groups, and eight nonbreeding groups. Its age- and sex-class structure was similar to those observed at two other clearings in the same forest block. However, the size of breeding groups varied with site (either clearing or forest sites). At Lokoué, breeding groups (mean size: 8.2 gorillas; range: 3-15) included a single silverback male and, on average, 3.2 adult females. Nonbreeding groups (mean size: 5.5; range: 2-15) were devoid of adult females. Five of the nonbreeding groups were composed predominantly of blackbacks, subadult males, and juveniles, and thus fit the definition of all-male groups previously observed in mountain gorillas. Our study confirms that 1) one-male breeding groups are the norm in western gorillas, and 2) all-male groups occur in this species. Despite frequent changes in members due to migrations of the males, the persistence of these all-male groups indicates that they may play an important role in the life of migrating males. Variations in population structure, and group composition and type among gorilla populations are discussed. However, a further understanding of the evolution of group-living in gorillas requires detailed ecological studies conducted in parallel with studies of the population structure and dynamics of these groups.

Origin and Biology of Simian Immunodeficiency Virus in Wild-Living Western Gorillas

“…Whether this occurred primarily by sexual or other (e.g., exposure to infectious saliva) routes is not known; however, it is likely that gorilla behavior and social structure facilitated virus dispersal. Western lowland gorillas live in family units of 2 to 30 individuals which typically comprise one dominant male (silverback), three or more sexually active adult females, their offspring, and possibly a few nondominant males (27,42,62,67). The silverback mates with all adult females in the group (27,48).…”

“…Western lowland gorillas live in family units of 2 to 30 individuals which typically comprise one dominant male (silverback), three or more sexually active adult females, their offspring, and possibly a few nondominant males (27,42,62,67). The silverback mates with all adult females in the group (27,48). Adolescent males typically leave their natal groups and become solitary (and highly mobile) until they form their own harems (23), while females also transfer between groups (63).…”

Molecular Epidemiology of Simian Immunodeficiency Virus Infection in Wild-Living Gorillas

“…Interestingly, we observed a predominance of SIVgor infection in females, and whether this is related to a sample bias and/or the social organization of gorilla groups needs to be studied further. Western lowland gorillas live in social breeding groups usually formed by one silverback male, three adult females (on average), and their offspring (7,9,20,24). Before reaching sexual maturity, male gorillas leave their natal group and go through a bachelor stage that can last several years (9,19).…”

The Effect of Nonindependent Mate Pairing on the Effective Population Size

“…For example, in humans, paternity is correlated with paternal confidence in paternity (Anderson 2006), and married individuals tend to repeatedly produce offspring with each other more frequently than expected by chance. Nonindependent mate pairing among breeding seasons occurs in many other species as well-for example, longterm pair bonding in prairie voles (DeVries et al 1995), harems in gorillas (Gatti et al 2004), and sperm storage in fruit flies (Neubaum and Wolfner 1999).Current theoretical models that allow calculation of the effective population size with overlapping generations and age structure make several simplifying assumptions, notably constant sizes of each age class, sufficiently large numbers of individuals in each age class that second-order terms in their reciprocals can be neglected, and independent sampling of offspring genotypes from the same individual reproducing at different times (Hill 1972;Nunney 1991Nunney , 1993Caballero 1994;Charlesworth 1994Charlesworth , 2001). The latter assumption in particular makes it difficult to provide accurate expressions for species such as humans and Drosophila, which reproduce nonindependently because of long-term pair bonds and sperm storage, respectively (Charlesworth 2001).…”

“…For example, in humans, paternity is correlated with paternal confidence in paternity (Anderson 2006), and married individuals tend to repeatedly produce offspring with each other more frequently than expected by chance. Nonindependent mate pairing among breeding seasons occurs in many other species as well-for example, longterm pair bonding in prairie voles (DeVries et al 1995), harems in gorillas (Gatti et al 2004), and sperm storage in fruit flies (Neubaum and Wolfner 1999).…”