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Atlanta, GA, United States

Rosenbaum S.,University of California at Los Angeles | Rosenbaum S.,University of Chicago | Maldonado-Chaparro A.A.,University of California at Los Angeles | Stoinski T.S.,The Dian Fossey Gorilla Fund International

Relationships between conspecifics are influenced by both ecological factors and the social organization they live in. Systematic variation of both—consistent with predictions derived from socioecology models—is well documented, but there is considerable variation within species and populations that is poorly understood. The mountain gorilla (Gorilla beringei) is unusual because, despite possessing morphology associated with male contest competition (e.g., extreme sexual dimorphism), they are regularly observed in both single-male and multimale groups. Both male–female and male–infant bonds are strong because males provide protection against infanticide and/or predation. Risk of these threats varies with social structure, which may influence the strength of social relationships among group members (including females and offspring, if females with lower infant mortality risk are less protective of infants). Here, we investigate the relationship between group structure and the strength of proximity relationships between males and females, males and infants, and females and offspring. Data come from 10 social groups containing 1–7 adult males, monitored by the Dian Fossey Gorilla Fund’s Karisoke Research Center in Volcanoes National Park, Rwanda. After controlling for group size and infant age, association strength was similar for male–female pairs across group types with both dominant and nondominant males, but male–infant relationships were strongest in single-male groups where paternity certainty was high and animals had fewer social partners to choose from. The male:female and male:infant ratios better predicted both male–female and male–infant associations than the absolute number of males, females, or infants did. The fewer the number of males per female or infant, the more both pair types associated. Dominant males in groups containing fewer males had higher eigenvector centrality (a measure of importance in a social network) than dominant males in groups with more males. Results indicate that nondominant males are an important influence on relationships between dominant males and females/infants despite their peripheral social positions, and that relationships between males and infants must be considered an important foundation of gorilla social structure. © 2015 Japan Monkey Centre and Springer Japan Source

Eckardt W.,University of Chester | Fawcett K.,The Dian Fossey Gorilla Fund International | Fletcher A.W.,University of Chester
Behavioral Ecology and Sociobiology

Weaning marks an important milestone during life history in mammals indicating nutritional independence from the mother. Age at weaning is a key measure of maternal investment and care, affecting female reproductive rates, offspring survival, and, ultimately, the viability of a population. Factors explaining weaned age variation in the endangered mountain gorilla are not yet well understood. This study investigated the impact of group size; group type (one-male versus multimale); offspring sex; and maternal age, rank, and parity on weaned age variation in the Virunga mountain gorilla population. The status of nutritional independence was established in 69 offspring using long-term suckling observations. A Cox-regression with mixed effects was applied to model weaned age and its relationship with covariates. Findings indicate that offspring in one-male groups are more likely to be weaned earlier than offspring in multimale groups, which may reflect a female reproductive strategy to reduce higher risk of infanticide in one-male groups. Inferior milk production capacity and conflicting resource allocation between their own and offspring growth may explain later weaning in primiparous mothers compared to multiparous mothers. Sex-biased weaned age related to maternal condition defined by parity, rank, and maternal age will be discussed in the light of the Trivers-Willard hypothesis. Long-term demographic records revealed no disadvantage of early weaning for mother or offspring. Population growth and two peaks in weaned age within the Virunga population encourage future studies on the potential impact of bamboo shoots as a weaning food and other environmental factors on weaning. © 2016, Springer-Verlag Berlin Heidelberg (outside the USA). Source

Vigilant L.,Max Planck Institute for Anthropology | Roy J.,Max Planck Institute for Anthropology | Bradley B.J.,Max Planck Institute for Anthropology | Bradley B.J.,George Washington University | And 3 more authors.
Behavioral Ecology and Sociobiology

As in other highly sexually dimorphic, group-living animals, reproduction in gorillas has been largely viewed as the outcome of competition among males. However, females may exert choice via dispersal decisions or choice of partner in multimale groups, and males may also mate selectively. Here, we examine the paternity of 79 wild mountain gorilla offspring born into four groups characterized by stable dominance hierarchies and the presence of mature offspring of the dominant male. We found that on average the dominant male sires the majority (72 %) of the offspring in stable multimale groups and subordinate males also produce offspring, particularly when dominant males become older or the number of competing males increases. Although expected to disperse to avoid inbreeding, only half of the maturing daughters of dominant males left the group in which their father maintained dominance. However, in all five cases of reproduction by a resident daughter of a dominant male, a subordinate male was the sire of the offspring. As females commonly initiate and end copulations, and dominant males may prefer mating with fully mature females, both male and female mate preferences in addition to male competition apparently play a role in reproductive patterns in multimale groups, emphasizing the complexity of social dynamics in one of our closest living relatives. © 2015, Springer-Verlag Berlin Heidelberg. Source

Rosenbaum S.,University of California at Los Angeles | Silk J.B.,University of California at Los Angeles | Stoinski T.S.,The Dian Fossey Gorilla Fund International
American Journal of Primatology

We examined the pattern and possible functions of social interactions between adult males and immatures in three free-ranging, multi-male groups of mountain gorillas (Gorilla beringei beringei). Previous studies conducted during the 1970s when groups contained one to three adult males concluded that male-immature relationships were likely to be a form of low-cost paternal investment [Stewart, Mountain gorillas: three decades of research at Karisoke. Cambridge, UK: Cambridge University Press, 2001]. We evaluated whether this hypothesis still held in groups containing six to nine adult males, or if male-immature relationships might serve other functions (e.g. mating effort, kin selection, or alliance building). Overall, we found that immatures spent the most time near, and interacted most with, the alpha silverback. These behaviors peaked during the period when infants were still quite vulnerable but increasing their independence from their mothers. Such findings suggest that parenting effort remains the primary function of male-immature relationships; however, there is some evidence for the mating effort hypothesis as well. © 2010 Wiley-Liss, Inc. Source

Caillaud D.,The Dian Fossey Gorilla Fund International | Caillaud D.,Georgia Institute of Technology | Ndagijimana F.,The Dian Fossey Gorilla Fund International | Giarrusso A.J.,Georgia Institute of Technology | And 2 more authors.
American Journal of Primatology

Since the 1980s, the Virunga mountain gorilla population has almost doubled, now reaching 480 individuals living in a 430-km2 protected area. Analysis of the gorillas' ranging patterns can provide critical information on the extent and possible effects of competition for food and space. We analyzed 12 years of daily ranging data and inter-group encounter data collected on 11 gorilla groups monitored by the Karisoke Research Center in Rwanda. During that period, the study population increased in size by almost 50% and the number of groups tripled. Groups had small yearly home ranges compared to other known gorilla populations, with an average 90% kernel density estimate of 8.07km2 and large between-group variations (3.17-23.59km2). Most groups had consistent home range location over the course of the study but for some, we observed gradual range shifts of up to 4km. Neighboring groups displayed high home range overlap, which increased dramatically after the formation of new groups. On average, each group used only 28.6% of its 90% kernel home range exclusively, and in some areas up to six different groups had overlapping home ranges with little or no exclusive areas. We found a significant intra-group positive relationship between the number of weaned individuals in a group and the home range size, but the fitted models only explained 17.5% and 13.7% of the variance in 50% and 90% kernel home range size estimates, respectively. This suggests that despite the increase in size, the study population is not yet experiencing marked effects of feeding competition. However, the increase in home range overlap resulting from the formation of new groups led to a sixfold increase in the frequency of inter-group encounters, which exposes the population to elevated risks of fight-related injuries and infanticide. Am. J. Primatol. 76:730-746, 2014. © 2014 Wiley Periodicals, Inc. Source

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