ORIGINAL PAPER Socioecological influences on the reproductive success of female mountain gorillas (Gorilla beringei beringei) Martha M. Robbins & Andrew M. Robbins & Netzin Gerald-Steklis & H. Dieter Steklis Received: 18 September 2006 / Revised: 19 November 2006 / Accepted: 26 November 2006 / Published online: 24 January 2007 # Springer-Verlag 2007 Abstract Over the past few decades, socioecological models have been developed to explain the relationships between the ecological conditions, social systems, and reproductive success of primates. Feeding competition, predation pressures, and risk of infanticide are predicted to influence how female reproductive success (FRS) depends upon their dominance rank, group size, and mate choices. This paper examines how those factors affected the reproductive success of female mountain gorillas (Gorilla beringei beringei) of the Virunga Volcanoes, Rwanda from 1967–2004. Reproductive success was measured through analyses of interbirth intervals, infant survival, and surviv- ing infant birth rates using data from 214 infants born to 67 females. Mountain gorillas were predicted to have “within- group scramble” feeding competition, but we found no evidence of lower FRS in larger groups, even as those groups became two to five times larger than the population average. The gorillas are considered to have negligible “within-group contest” competition, yet higher ranked mothers had shorter interbirth intervals. Infant survival was higher in multimale groups, which was expected because infanticide occurs when the male dies in a one- male group. The combination of those results led to higher surviving birth rates for higher ranking females in multi- male groups. Overall, however, the socioecological factors accounted for a relatively small portion of the variance in FRS, as expected for a species that feeds on abundant, evenly distributed foliage. Keywords Mountain gorilla . Female reproductive success . Dominance rank . Group size . Feeding competition . Socioecological model Introduction Over the past few decades, conceptual models have been developed to explain the links between ecological con- ditions, the social systems of primates, and reproductive success of their individuals (Wrangham 1980; Isbell 1991; Sterck et al. 1997; Isbell and Young 2002; Koenig 2002). This socioecological model indicates that social groups are shaped primarily in response to the abundance and distribution of food (Wrangham 1980; Isbell 1991), predation pressures (van Schaik 1989), and infanticide risks (Sterck et al. 1997). The abundance and distribution of food can influence female reproductive success (FRS) through contest or scramble feeding competition (Table 1), which can occur within or between groups (Janson and van Schaik 1988; van Schaik 1989; Sterck et al. 1997; Koenig 2002). Scramble competition occurs when consumption by any one individual reduces net energy intake of all others. Within-group scramble competition (WGS) is expected when limiting food resources occur in large patches, or when such food is highly dispersed and/or quickly depleted (e.g., Thomas langurs, Steenbeek and van Schaik 2001). Larger groups may need to travel farther for food (Janson and Goldsmith 1995), increasing the energetic costs and reducing reproductive success for all females (Srivastava Behav Ecol Sociobiol (2007) 61:919–931 DOI 10.1007/s00265-006-0321-y Communicated by D. Watts M. M. Robbins (*) : A. M. Robbins Max Planck Institute for Evolutionary Anthropology, Deutscher Platz 6, 04103 Leipzig, Germany e-mail: robbins@eva.mpg.de N. Gerald-Steklis : H. D. Steklis Dian Fossey Gorilla Fund International, 800 Cherokee Avenue SE, Atlanta, GA 30315-1440, USA