Blog 4: Mountain Gorilla Communication, Social Behavior, and Cognition

A male Mountain gorilla displaying his teeth while yawning. Photograph by Dave Proffer.


Communication

     

            Mountain gorillas can produce vocal calls to communicate with other troop members to indicate a particular activity, emotion, and behavior. This behavior is similar to other mammalian species, such as elephants, orcas, beluga whales, and walruses. Due to their large sizes and dense forested environments, Mountain gorillas produce low-frequency sounds to communicate at short distances (Watson et al., 2020). These vocals are generally categorized as whines, whimpers, grunts, barks, belches, and hoots (Lang, 2005). In recent studies, researchers have discovered that these gorillas “sing” to appreciate their botanical meals (Nature on PBS, 2020). According to previous studies, primatologists have identified up to 25 unique acoustic signals communicated by Mountain gorillas (Downey, 2019). For example, the apes’ most auditory signal is a belch. The leading silverback utters a low, rumbled “clearing the throat” sound, which is a sign of comfort. The call is reciprocated by his troop’s blackbacks and female members (Gorilla vocalizations, 2019). Other calls that can be heard within the troop include copulatory grunts, cough grunts (signaled during intimidation displays), and dog grunts (signaled to warn a neighboring gorilla to stay away from one’s food source). If a silverback perceives a sign of outsiders, such as a rival bachelor group, the alpha male will vocalize higher frequency sounds such as hoots and barks (Lang, 2005). The Mountain gorillas produce roars and screams if tensions escalate into aggressive behaviors between the silverback’s troop, outsiders, and predators (Downey, 2019).

            Besides vocal communication, documentation shows that Mountain gorillas use semi- and non-auditory signals within their troop. The silverback's chest-beating behavior is the most recognized visual signal, accompanied by a small series of vocalizations. This signal occurs when the leading male conveys his body size and dominance to rivaling males and courting females (Wright et al., 2021). Similar to other ape species, gorillas are known to make facial expressions. For example, infant Mountain gorillas create play signals such as an open mouth with no bared teeth (Gorilla communication, 2020).

            In contrast, blackbacks can show teeth when yawning at the silverback. This expression depicts the younger male’s subordination to the leading male of his troop. It can be interpreted as a sign of distress (Gorilla communication, 2020). Tactile communication signals within Mountain gorilla troops include copulating, playing (between an infant and an adult, and two infants), and grooming. A viable method of communication amongst Mountain gorillas is the release of bodily biochemicals. For example, female gorillas in estrus periods produce pheromones to signal the silverback they are ready to mate (Downey, 2019)


A short segment of the PBS nature documentary Spy in the Wild depicting footage of "singing" Mountain gorillas.


Social Behaviors


            When analyzing the grooming behaviors of Mountain gorillas, most of the interaction is observed between mothers and infants (Reynolds, 1965). This behavior allows the mother and her offspring to form and strengthen a strong bond. Researchers have also documented this grooming behavior between the silverback and sexually mature females. This grooming behavior is initiated by the alpha male towards the reproductive female Mountain gorillas and indicates his fitness (Habumuremyi et al., 2018). Immature males can also demonstrate this behavior when selecting mating partners (Rosenbaum et al., 2010)

            If the silverback’s subordinate males do not want to leave the troop to form a new or bachelor group, the blackbacks ally with the dominant male (Rosenbaum et al., 2010). This alliance allows the immature males to learn social strategies led by the silverback and allows the other males to form “friendships” with one another. Moreover, these tight networks allow the males, from both ranks, to defend the harem from predators and bachelors (Rosenbaum et al., 2010). 

            Aggressive behaviors of alpha male silverbacks have been documented within the troop and outside by rival groups. Most of the troop interactions are non-life-threatening compared to encounters with rival outside groups. During mating, aggressive behaviors have been observed between males and females (Lang, 2005). For example, “courtship aggression” occurs when a male gorilla becomes aggressive toward estrus females, a sign of a readiness to copulate (Rosenbaum et al., 2010). 


Cognitive Abilities


            Similar to studies documenting chimpanzees and orangutans, Mountain gorillas have been observed using tools within their environment to support themselves and others. For example, two photographs from a 2010 study reveal a mother aiding her infant by using a bamboo pole to clear brush away (Grueter et al., 2013). Mountain gorillas live in a habitat with abundant food and are strong enough to pull up bamboo shoots with sheer strength (Grueter et al., 2013). For these reasons, Mountain gorillas do not heavily rely on tool usage like chimpanzees. Regarding ecological intelligence, Mountain gorillas have learned to examine a food item and remove thorns, spines, and bark before consumption. (Neufuss et al., 2019). It is important to note that other cognitive ability studies in Mountain gorillas have yet to be fully observed, researched, or published.


Figure diagram of a mother gorilla supporting her infant through a bamboo shoot. Captured by Grueter et al., 2013.


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.

Gorilla communication. Dian Fossey Gorilla Fund. (2020, May 22). https://gorillafund.org/uncategorized/gorilla-communication/ 

Gorilla vocalizations. Dian Fossey Gorilla Fund. (2019, December 13). https://gorillafund.org/uncategorized/gorilla-vocalizations/#:~:text=Belches%20are%20described%20by%20Fossey,the%20rest%20of%20the%20group. 

Grueter, C. C., Robbins, M. M., Ndagijimana, F., & Stoinski, T. S. (2013). Possible tool use in a mountain gorilla. Behavioural Processes, 100, 160–162. https://doi.org/10.1016/j.beproc.2013.09.006

Habumuremyi, S., Deschner, T., Fawcett, K. A., & Robbins, M. M. (2018). Male–female interactions in multimale groups of mountain gorillas. American Journal of Primatology, 80(11), e22910-n/a. https://doi.org/10.1002/ajp.22910

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/ 

Nature on PBS. (2020, April 13). Did you know gorillas can sing?. YouTube. https://www.youtube.com/watch?v=6yB51vdkZAE 

Neufuss, J., Robbins, M. M., Baeumer, J., Humle, T., & Kivell, T. L. (2019). Manual skills for food processing by mountain gorillas (Gorilla beringei beringei) in Bwindi Impenetrable National Park, Uganda. Biological Journal of the Linnean Society, 127(3), 543–562. https://doi.org/10.1093/biolinnean/bly071

Reynolds, V. (1965). Some Behavioral Comparisons between the Chimpanzee and the Mountain Gorilla in the Wild. American Anthropologist, 67(3), 691–706. https://doi.org/10.1525/aa.1965.67.3.02a00050

Rosenbaum, S., Silk, J. B., & Stoinski, T. S. (2010). Male-immature relationships in multi-male groups of mountain gorillas (Gorilla beringei beringei). American Journal of Primatology, 73(4), 356–365. https://doi.org/10.1002/ajp.20905

Watson, S. K., Heesen, R., Hedwig, D., Robbins, M. M., & Townsend, S. W. (2020). An exploration of Menzerath’s law in wild mountain gorilla vocal sequences. Biology Letters (2005), 16(10), 20200380–20200380. https://doi.org/10.1098/rsbl.2020.0380

Wright, E., Grawunder, S., Ndayishimiye, E., Galbany, J., McFarlin, S. C., Stoinski, T. S., & Robbins, M. M. (2021). Chest beats as an honest signal of body size in male mountain gorillas (Gorilla beringei beringei). Scientific Reports, 11(1), 6879–6879. https://doi.org/10.1038/s41598-021-86261-8


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