Blog 3: Mountain Gorilla Social System

A troop of Mountain gorillas resting during the day. Photograph by Joachim Huber.

Social Organization

        Mountain gorillas live a social lifestyle that includes multi-male and multi-female groupings. Within the Mountain gorilla “society,” these apes form a strong, tight social network group known as a troop. The average number of individuals creating a single troop can be around 20 or greater (Lang, 2005). Most gorilla troops are composed of family units or natal groups and are led by a single, dominant male silverback (Downey, 2019). Additionally, a troop comprises a majority of sexually mature females (roughly 3-4 individuals), a few subadult male blackbacks (roughly two individuals), and juveniles (a maximum of 6 or more offspring) (Downey, 2019). Interestingly, a 2015 scientific study revealed that nearly 40% of the Mountain gorilla troops inhabiting the Virunga Mountain range were multimale groups (Rosenbaum et al., 2015). Inside the troop, there is a clear understanding that kinships between biologically related gorillas are determined through the relatedness of their mother (maternal) rather than the father (paternal). For example, it has been observed that full siblings sharing the same mother will interact more frequently with each other than siblings sharing the same father (Grebe et al., 2022).

Internal composition of a Western lowland gorilla troop from Ngaga Forest in the Democratic Republic of the Congo. Although a different species, the Mountain gorilla has a similar social organization. Diagram illustrated by Forcina et al., 2019.

Social Structure

        A Mountain gorilla troop's social structure is complex yet similar to other multi-male/multi-female primate social groups. When examining the hierarchical patterns, each troop is controlled by a single leading male silverback. The dominant male’s role is to protect his position of power from rival blackback bachelor groups and troop members from mating with other females. The goal for the silverback is to mate as many females as possible to pass down his genes or increase fitness to the next generation. However, if the silverback dies, the subordinate males from his troop and bachelor groups have a greater chance of mating with the sexually mature females and compete to become the next leader of the troop (Lang, 2005).  When emigrating from a troop, sexually mature female gorillas frequently leave their natal group,  an innate prevention of inbreeding (Downey, 2019). Mothers also leave the troop when the father silverback dies, as well, to protect their offspring from rival males who may attempt to kill their infants (Downey, 2019). Subordinate, or lower-ranking, males can leave their silverback-led troop and form into bachelor or new multi-male/ multi-female troops. Females can be accepted into these newly formed troops and mate with the new dominant male to increase genetic diversity (Lang, 2005).

Mating System 

        Upon closer examination of the physical characteristics of Mountain gorillas, the primatologist sees that males and females possess extreme cases of sexual dimorphism or features that can distinguish the difference between the two sexes. For example, males have generally larger body sizes (height, mass, and robust, muscular morphology) and taller sagittal crests than females. Additionally, the males possess more profound or longer canines due to competition with rival males to gain access to mating opportunities with receptive females (Plavcan & Ruff, 2008).

        Compared to other mammalian species, such as elephant seals, California sea lions, mandrills, and elk, Mountain gorillas compose a mating system known as polygyny. Commonly known as a harem, this mating pattern occurs when a dominant male can copulate with multiple females. Due to the alpha male’s persistence in reproducing with as many females as possible, Mountain gorilla natal groups have a high reproductive skew (Grebe et al., 2022). However, the leading silverback is in direct competition and oversight with other bachelor blackback males within and outside his troop.

        Female Mountain gorillas have been recorded to have distinct reproductive strategies. Unlike other great apes such as chimpanzees and bonobos, female gorillas have minimal visual signals of ovulation, such as sexual swellings, and this receptive or estrus period lasts for around a month (Lang, 2005). Potential ideas for why gorillas have slight swelling include reducing the risk of parasitic entry, bacterial infection from cuts, or signaling reproductive status from blackbacks and silverbacks. Primatologists have previously explained that it is difficult to provide detailed observations when examining male reproductive strategies because blackback males can reproduce without displaying physical changes (Lang, 2005).

Care System 

        The offspring is raised through maternal care throughout the developing Mountain gorilla’s lifetime. The mother provides her young with lactation, protection, and shelter from birth to weaning (approximately when the juvenile is 3.5 years old) (Gorilla moms and dads: Being together, 2020). When the offspring grow up, their dependency on their mother decreases as they continue to explore and interact with other troop members and the environment. According to a 2018 research article, primatologists have recorded fathers demonstrating paternal care. Leading silverbacks have been known to protect infants, especially if the offspring are biologically related. This protection ensures the dominant male's genes will be passed on and prevents infanticide (Rosenbaum et al., 2018).

Mountain gorilla mother beside her infant offspring. Photograph by Charles J. Sharp.

References:

Downey, K. (2019). Mountain Gorilla, Gorilla beringei beringei. New England Primate Conservancy. https://neprimateconservancy.org/mountain-gorilla/#:~:text=As%20the%20second%2Dlargest%20primate,between%2040%20and%2050%20years.

Forcina, G., Vallet, D., Le Gouar, P. J., Bernardo-Madrid, R., Illera, G., Molina-Vacas, G., Dreano, S., Revilla, E., Domingo Rodriguez-Teijeiro, J., Menard, N., Bermejo, M., & Vila, C. (2019). From groups to communities in western lowland gorillas. [Diagram]. Proceedings of the Royal Society. B, Biological Sciences, 286(1896), 20182019–20182019. https://doi.org/10.1098/rspb.2018.2019

Gorilla moms and dads: Being together. Dian Fossey Gorilla Fund. (2020, May 7). https://gorillafund.org/uncategorized/gorilla-moms-and-dads-being-together/ 

Grebe, N. M., Hirwa, J. P., Stoinski, T. S., Vigilant, L., & Rosenbaum, S. (2022). Mountain gorillas maintain strong affiliative biases for maternal siblings despite high male reproductive skew and extensive exposure to paternal kin. eLife, 11. https://doi.org/10.7554/eLife.80820

Lang, K. (2005, October 4). Primate factsheets: Gorilla (Gorilla) taxonomy, morphology & ecology. Wisconsin National Primate Research Center. https://primate.wisc.edu/primate-info-net/pin-factsheets/pin-factsheet-gorilla/ 

Plavcan, J. M., & Ruff, C. B. (2008). Canine size, shape, and bending strength in primates and carnivores. American journal of physical anthropology, 136(1), 65–84. https://doi.org/10.1002/ajpa.20779

Rosenbaum, S., Hirwa, J. P., Silk, J. B., Vigilant, L., & Stoinski, T. S. (2015). Male rank, not paternity, predicts male–immature relationships in mountain gorillas, Gorilla beringei beringei. Animal Behaviour, 104, 13–24. https://doi.org/10.1016/j.anbehav.2015.02.025

Rosenbaum, S., Vigilant, L., Kuzawa, C. W., & Stoinski, T. S. (2018). Caring for infants is associated with increased reproductive success for male mountain gorillas. Scientific Reports, 8(1), 15223–15228. https://doi.org/10.1038/s41598-018-33380-4