Mountain Gorilla (Gorilla beringei beringei)

Harald Parzer

Dominant Silverback with wound due to a fight with a lone male. Photo credit to Harald Parzer.

Dominant Silverback with wound due to a fight with a lone male. Photo credit to Harald Parzer.

Mountain gorillas (Gorilla beringei beringei) are a subspecies of the Eastern Lowland Gorilla (Gorilla beringei) and are endemic to (or only found in) the mountainous region of the Albertine Rift in East Africa at an altitude from around 2200m to 4000m. They are found in two disjunct areas: the Virunga mountains, Rwanda and in Bwindi Impenetrable Forest, Uganda. Interestingly, these two populations behave quite differently. Bwindi Mountain Gorillas tend to climb much more and eat plenty of fruits, while the Virunga gorillas mostly stay on the ground and replace fruits with herbaceous stems. In fact, because of this, and morphological and genetic differences, some scientists argue that these populations represent two species, or at least two subspecies. Unfortunately, due to poaching and habitat destruction, only 880 individuals of this critically endangered species are left on this planet.

Mountain gorillas are incredibly powerful specimens to behold. They can grow to be as tall as an average man, with the same muscle and fat distribution, and weigh up to 430 lbs. Bodybuilders could only dream of such numbers. And yet, the mountain gorilla diet consist solely of plant matter! To maintain such weight, male mountain gorillas eat up to 34 kg of plants. Although they do not consume animal protein, about 18% of their overall food intake is protein from plant matter they select. Thus, an adult mountain gorilla can eat of up to 612 grams of protein every day far surpassing that of a bodybuilder's diet. Higher ranking females tend to have a higher caloric intake, not because they get nutrient richer plants, but because they tend to eat faster and are less active than their male counterparts.

Female mountain gorilla with her offspring. Photo credit to Harald Parzer.

Female mountain gorilla with her offspring. Photo credit to Harald Parzer.

Research shows that the home range of mountain gorillas changes with the seasons. In the dry season, mountain gorillas may increase their home ranges to 18 square miles in order to find their favorite plants (in Bwindi they have been observed to eat 107 species of plants). During the rainy season, the home ranges of mountain gorillas shrink dramatically as plants are more abundant and foraging doesn't require them to travel as far.

Mountain gorillas, which are either left or right handed in about the same proportions, live in small groups of around 10 individuals. The groups are composed of one to a few adult males, as well as females and juveniles of either sex. The dominant male, or silverback (named after his grey short hair on his back, which develops as a teenager), is almost twice as heavy as an adult female, and sires about 85% of the offspring if a second ranking silverback is in the group (who sires the remaining 15%). Thus, a silverback benefits from a large harem, as this means a lot of offspring. At times, like the group visited by the author in Bwindi Impenetrable Forest, older silverbacks are still hanging onto their group (and regularly get lost due to their slower pace...) and can be observed at the edge of it, old men observing the youth.

Infant mountain gorilla (about half a year old). Photo credit to Harald Parzer.

Infant mountain gorilla (about half a year old). Photo credit to Harald Parzer.

On average, a female produces 2.1–3.6 surviving offspring in her lifetime. Dominant females, which have higher lifespans, are producing more than lower ranking females. Infant mortality is high (21% of all infants won't make it to adulthood), mostly because of infanticide - yes, the ugly side of the mountain gorilla. When a new male takes over the group, after a vicious fights (see picture above) or natural death of the dominant silverback, the newcomer tends to kill all offspring to make sure that his harem is receptive for his own offspring. Thus, females prefer strong dominant males, which can protect their group as long as possible. Young males leave their groups when they are about 11 years old, and wander through the forest, mostly as lone males, to fight for a new group for themselves and that can take time.

Mountain gorillas, like all other gorillas, have not been observed to use tools in the wild, and have so far not been kept in zoos. Thus, if you want to meet this gentle giants, you will have to travel to Uganda or Rwanda, and join a gorilla trekking tour. The prices for such tours are steep ($600 for Bwindi National Park and $1500 for the Virunga Mountains), but they allow you to stay very close to one of the habituated groups for one hour. And you may even be hugged by a juvenile, as it happened to the trekking group of the author! These tours are well worth it and at least some of the money goes into protecting the habitat of the gorillas and to protecting other national parks, which are less frequently visited. So start saving money (cancel your TV subscription and write an essay for our upcoming competition to gain a few extra bucks), and get ready for a fantastic adventure! 

References

Bradley, B. J., M. M. Robbins, E. A. Williamson, H. D. Steklis, N. G. Steklis, N. Eckhardt, C. Boesch & L. Vigilant. 2005. Mountain gorilla tug-of-war: silverbacks have limited control over reproduction in multimale groups. Proceedings of the National Academy of Sciences of the United States of America 102: 9418–9423.

Robbins, M. M., A. M. Robbins, N. Gerald-Steklis & H. D. Steklis. 2007. Socioecological influences on the reproductive success of female mountain gorillas (Gorilla beringei beringei). Behavioral Ecology and Sociobiology 61: 919–931.

Rothman, J. M., A. J. Plumptre, E. S. Dierenfeld, & A. N. Pell. 2007. Nutritional composition of the diet of the gorilla (Gorilla beringei): a comparison between two montane habitats. Journal of Tropical Ecology 23: 673–682.

Zihlman, A. L., & R. K. McFarland. 2000. Body mass in lowland gorillas: a quantitative analysis. American Journal of Physical Anthropology 113: 61–78.