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Mountain Gorillas were first discovered in the year 1902 by a German explorer, Captain Robert von Beringei.

Mountain Gorillas; are only found in two small island habitats: the Virunga Massif (450sqkm), straddling the boundaries between Uganda, Rwanda & the Democratic republic of Congo and in Bwindi Impenetrable National Park, Uganda (330sqkm; with a small section of the forest extending into Sarambwe in Democratic Republic of Congo). These two locations are located only about 30km apart from each other at the nearest point. Both places are surrounded by some of the highest densities of populations living in a rural setting (as many as 700 people per sqkm). The people that live in the areas between the two populations are largely reliant on the land for subsistence agriculture. There is almost no forest cover between the two Gorilla habitats.

No body knows for sure when these two populations became isolated from each other, but genetic analysis suggests that the split began approximately 10,000 years ago. The split was due to climatic conditions as well as human influences. Some scientists suggest that the two populations should be considered separate subspecies, but it is generally agreed that they belong to only one subspecies, despite some differences in genetic makeup, physical appearance, and ecological conditions.

Mountain Gorillas live at some of the highest altitudes of any living primates in the world. The Virunga Massif spans an altitude range of approximately 2,200 m to 4,500 m. Bwindi is lower, with an altitude ranging from ,1,160 m - 2,500 m. Several things vary along with the altitude, namely; temperature and vegetation.


Unlike back in the 80s when the Mountain Gorillas were nearing extinction today, their population is back on the rise, thanks to Conservation Agencies such as The Dian Fossey Gorilla Conservation Initiative in Rwanda and Uganda Wildlife Authority in Uganda, Gorilla Doctors in both Uganda & Rwanda, International Gorilla Conservation Program (IGCP) and several other more. Their population as of date is estimated to be around 1,063 (source, Gorilla doctors). Thus alleviating their status from Critically Endangered to Endangered as per the International Union for Conservation of Nature (IUCN) conservation status.


Mountain Gorillas Live in stable, cohesive social units referred to as groups. The average size of Mountain Gorilla groups is 10 individuals, but there can be a lot of variation. Group size may range from 2 to 30 or more Mountain Gorillas. The largest group of Mountain Gorillas observed in the Virunga Massif contained 65 members, whereas the largest group of seen in Bwindi had about 35 gorillas. Groups consist of the dominant silverback, several adult females, and immature off-spring of various ages. Groups may be one-male, multimale, or all-male (non-reproductive, containing no adult females). Multimale groups contain more than one silverback, but multi-male groups rarely contain more than two silverbacks. In multimale groups the silverbacks are usually related as brothers, half-brothers, or father and son. However, sometimes unrelated silverbacks live in the same group.

The variability in the social system of mountain Gorillas is due to the following transitions. New social groups form when females transfer to solitary males. Such groups remain one-male until male offspring mature into silverbacks and the group is then multimale.

Multimale groups return to a one-male structure if all except one mature male emigrate, the original adult male dies, or the group fissions. When the silverback of a one male group dies, the group disintegrates, as the adult females and immature offspring will join a solitary male or another group. If a breeding group loses all its adult females, it becomes an all male-group. All-male groups may also form through a merger of immature males (evicted from a heterosexual group taken over by new silverback following the death of the previous silverback) and a solitary silverback. All male groups occur only rarely and they provide a better social setting for individuals than being solitary. All male groups can become heterosexual if females transfer into them. If a dominant male loses all of his group members, he becomes a solitary male. Fissioning is when a multimale group splits into two. In Bwindi, the Habinyanja Group fissioned in 2002, to create the Rushegura Group. The Shongi group also has fissioned in recent years. Gorillas are one of a few primate species in which both males and females may be philopatric (stay in the group where they were born) or disperse. Males that emigrate to become solitary males do so when they are blackbacks or young silverbacks, approximately 11-15years of age.

Males follow one of two strategies to become the leader of a group: either remain in the group and attempt a take-over or emigrate to become a solitary male and eventually form a new group.

Males may be solitary for several years and some silverbacks never obtain groups or become the dominant male of a group. One of the most noticeable differences in the social structure of mountain gorillas and lowland gorillas is the occurrence of multimale groups. Lowland gorillas almost never form multimale groups, whereas about 40-50% of Mountain Gorilla groups in both Bwindi and the Virunga Massif are multimale. The reason for this difference is not fully known, but it is believed to be linked to differences in ecological conditions. If females do not remain in the group where they were born, they will transfer directly to another group when they are 6-10 years old. This initial dispersal from their natural group may be to avoid inbreeding, although genetic studies have shown that even when a daughter does not disperse, she will not breed with her father. Females will not transfer to another group if they have an unweaned offspring. Females may transfer several times in their lives. As a result, females are typically in social groups containing unrelated individuals, although mother-daughter and sister pairs are not uncommon because females do not always transfer or related females may transfer into same group. Intergroup encounters (when two groups meet) are the only time when females transfer, so they are an important time for female choice (females choosing between silverbacks) and male-male competition (males retaining or losing females). However, female choice may be limited in cases where males herd females away from a neighboring group to prevent transfers.


Demography and life history involves features of the life cycle for an animal pertaining to development & reproduction including factors such as birth rates, mortality rates, gestation length, the number and sex of offspring, the interbirth interval (time between births), age at first breeding, and life span (longevity). Furthermore, knowing these factors enable us to study the changes in the size and age composition of populations, and the biological and environmental processes influencing those changes.

Gorillas, like all other apes, are known for having what's called a 'slow life history'. This means that it takes a long time for them to reach maturity, they live for a long time, and they have very low rates of reproduction. As a result of having a slow life history, the rate of growth for a gorilla population (how much a population can increase in size over a given period of time) cannot be very high. Furthermore, if the rate of mortality is too high (too many deaths over a short period of time), the overall population size will decline because it takes such a long time for the production of new individuals.

in recent decades, the mountain gorilla populations of the Virungas and Bwindi have either stayed stable in size or increased.

They have had a population growth rate of 1-2% per year, which is partially due to the extreme conservation efforts in these locations.

It is important that we continually collect detailed data on the patterns of births and deaths as well as dispersals in and out of the gorilla groups. This information is one way that we can monitor whether the population is increasing, decreasing or stable over time. Routine censuses of the entire populations also tell us how many gorillas are found in each protected area. Understanding the changes in gorilla populations helps us to understand what park management strategies are effective and helps planning for future conservation activities.

The age/sex classifications typically used for mountain gorillas are below.

Infant (0-3.5 years):

Infants are dependent on the mother for survival. Infants are not yet weaned, although the frequency of nursing declines as infants get older. Infants are in constant physical contact with their mothers for approximately the first six months of their lives. Infants begin to eat vegetation when they are about 9 months old and incorporate more plants into their diet as they age. Infants are carried by their mothers regularly until they are about 2 years old. Initially they are carried on their mother's chest and belly (ventrally) and then they shift to being carried on the mother's back when they are about 9-12 months old.

Infants share their mother's night nests.

Juvenile (3.5-6 years):

Juveniles are immatures that are still heavily reliant on their mother, but can survive if the mother dies or disperses to another group. This is because they have stopped nursing and they are capable of foraging independently.

They typically also construct their own night nest every night, but may sometimes still share their mother's nest.

Subadult (6-8 years):

Immature gorillas are fully independent from their mothers, but they are not yet fully grown adults. Subadults play much less than juveniles and infants.

Adult female (> 8 years):

Females that are reproductively mature, even if they have not yet had an off-spring. They weigh approximately 100 kg.

Blackback (8-12 years):

Males that are starting the process of becoming mature adults. Blackback males are initially the size of adult females, but continue to grow larger.

They tend to have more square shoulders than adult females. As blackbacks mature, the hair on their backs starts to turn silver and their heads develops into the large sagittal crest.

Silverback (> 12 years):

Adult males. Mature silverbacks have the characteristic silver back, large crest on their heads and are nearly double the size of adult females. All male gorillas that live into their teens become silverbacks. The process of becoming a silverback takes a few years and males do not obtain their full adult size until they are 15 years old or later. They weigh approximately 150 kgs.


The likelihood of mortality (death) varies depending on the age of gorillas, which is a typical pattern in animals. Infants suffer high mortality, with one in three infants born dying before they become 3 years of age. Infants may die from a variety of causes including the mother having inadequate milk or bad maternal skills, infanticide, accidents, disease, or congenital (genetic) defects. Once a gorilla reaches age 4, the likelihood of dying greatly decreases. Most gorillas that attain 4 years of age will survive until adulthood. Female gorillas may live into their late 30's or early 40's. Adult males appear to have slightly shorter lifespans than females, typically living into their late 30's, but sometimes longer.


Females become sexually active at approximately age 6 (as subadults) and then go through what is called 'adolescent infertility' for usually at least 2 years, during which time they will mate but will not become pregnant. The average age of first birth is approximately age 10 years (range 8-13 years).

Females show no obvious signs of being pregnant and they usually mate while pregnant. Gestation (pregnancy) length is 8.5 months. Typically only one infant is born at a time, although twins have been born rarely. Females go through what is referred to as "lactational anestrous" for approximately 3 years, which means that they do not ovulate and cannot get pregnant when they are nursing an infant. As an infant gets older, the frequency of nursing will decline and eventually the mother will resume ovulating and can conceive the next offspring. In Bwindi, the typical interbirth interval (time between births of offspring that survive) is 5 years.

Interestingly, this is one year longer than the interbirth interval of mountain gorillas in the Virunga Volcanos, which is only 4 years.

If an infant dies before weaning, the female typically produces another offspring one year after the infant death.

Females will continue having babies throughout their adult life, although their fertility declines when they are in their mid to late 30's.

Despite this lower fertility, there is no evidence that female gorillas go through the equivalency of • human menopause (extended period late in life when ovulation stops). Because females have babies only about every

4-5 years and many infants die (about one in three), each female may have only 4 offspring that survive to adult-hood.

The estrous cycles of female gorillas are similar to those of humans, approximately 29 days long. Females are receptive for mating only for 1-2 days around the time of ovulation. Matings typically last less than two minutes in duration.

Females in multimale groups will commonly mate with more than one male.

Dominant males are more likely than subordinate males to mate with adult females, whereas subordinate males are more likely to mate with subadult females. Genetic studies have shown that dominant silverbacks sire (father) about 85% of all offspring. The fact that subordinate males sire some offspring (approximately 15%) may be one benefit of these males staying in a group.

Silverbacks may be dominant for 10 or more years and sire five, ten, or more offspring, depending on the number of females in the group and the length of the dominance tenure.

Diet and Feeding Ecology

Gorillas are vegetarians. Most people are surprised to learn that gorillas don't eat any meat, with the exception of occasionally consuming ants or termites. The gorillas select foods that are high in proteins and carbohydrates, but relatively low in fibre. They primarily eat leaves, stems, and pith of herbaceous vegetation. Gorillas spend about six hours a day eating so they consume a huge quantity of food. Silverbacks eat about 25 kg of vegetation each day and adult females consume about 15 kg.

Keep in mind that about 80% of that food is water, which explains why we only rarely observe the gorillas drinking water. Their stomachs are so big not because they are fat, but because they are digesting all that vegetation.

The diet of mountain gorillas varies quite a lot, depending on where they live and what is available (Table 1). As a general rule, the number of different species of plants found in an area decreases as altitude increases. For example, in the Rwandan section of the Virunga Massif, more than 80% of the gorillas' diet consists of only four species of plants, the common names being wild celery, thistles, nettles, and bedstraw. On the DRC side of the Virungas, the gorillas also eat these species, but because it is lower in altitude, they eat some other plants as well.

All of the foods consumed in the Virungas are available all year round, with the exception of bamboo shoots, which are one of the most preferred foods of the gorillas.

In Bwindi, which is lower in altitude, some plants that are quite common in the low altitude area of Buhoma are rare or absent from the higher altitude areas of the park, such as Ruhija. The foods that are most commonly consumed in the Virungas are almost entirely absent from Bwindi. Bwindi gorillas enjoy fruit and they will not hesitate to climb 20 meters or more to eat it, but it is available only at certain times of the year. There are about 30 species of tree that produce fruit consumed by Bwindi gorillas. Also, as altitude increases, fruit availability declines, so the gorillas in Buhoma have more fruit than those in Ruhija and the mountain gorillas in the Virunga Volcanoes consume almost no fruit at all. Most fruit is available only seasonally, for 1-3 months per year, but the patterns of seasonality vary depending on the species.

Gorillas are not territorial, meaning they do not defend a set area of habitat. Instead, gorilla groups have overlapping home ranges. The typical size of home ranges in Bwindi is 20 - 25 km?. A group will typically travel about 1 km per day, but they can travel as much as 2-4 km per day. In general, gorillas move more per day when they are seeking out trees with fruit.




Vrunga Massif (Rwanda Portion)

Basella alba

Basella alba

Basella alba

Galium sp.

Chrysophyllum sp.

Brillantasia sp.

Brillantasia sp.

Carduus nyassanus

Ipomoea sp

Cyathea manniana

Ficus sp.

Yushania alpine

Mimulopsis arborescens

Ficus sp.

Gouania longispicata

Droguetia iners

Momordica foetida

Ipomoea sp.

Laportea aestuans

Peucedanum linderi

Myrianthus hostil

Maesa lanceolata

Maesa lanceolata

Rubus runssorensis

Rubus Sp.

Momordica foetida

Myrianthus hostil

Laporta alatipes

Triumfetta sp.

Myrianthus hostil

Palisota mannii

Urera hypselodendron

Triumfetta sp.

Triumfetta sp.

Xymalos monospora

Urera hypselodendron

Urera hypselodendron

Table1. Scientific names of most commonly eaten plants in the three areas of Bwindi (Ruhija, Rushaga, and Buhoma) and the Rwanda portion of the Virunga Massif.

Social Behavior

Gorillas are very social, with all group members cohesively coordinating thei daily activities with each other. Gorillas wake up as it becomes light, roughly about 6:30 - 7:00 am. They may not get out of their nests immediately, particu.

Tarly if it is cold or rainy. The first thing they do when they get out of their nests is to start feeding. They will feed for 1-3 hours, then rest for a few hours, then feed for a few hours. A typical day consists of alternating between several hours feeding and moving through the forest and then resting for a few hours.

Finally, as dusk falls around 6:00 pm the gorillas will settle down for the night by making their nests.

Interactions among individuals are usually peaceful, but occasionally conflicts may arise, with individuals giving aggressive "cough grunt" vocalizations, screaming, or even fighting. In social living animals, individuals typically compete for access to limited resources such as food and mating opportunities. Instead of fighting each time there is a conflict, they establish dominance relationships . Dominance can be defined as a pattern of repeated, agonistic (aggressive) interactions between two individuals, characterized by a consistent outcome in favor of the same individual, with the loser yielding to the winner rather than an escalation of a conflict or fighting. These repetitive interactions lead to the creation of dominance hierarchy among individuals.

Social relationships among males and females are the foundation of gorilla groups. Silverbacks are dominant over all other group members. Given males are clearly larger in size and dominant over females, why do they need to behave aggressively towards females?

Some of this aggression is due to feeding competition, but most of it likely serves as a means of reasserting their protective abilities as well as courtship aggression (showing their strength as a way to attract mates). During rest sessions the dominant silverback is typically surrounded by adult females and immature individuals. Grooming may occur between the silverback and adult females, but not among all of them.

Some silverbacks groom frequently and others almost never groom anyone else. The same is true for adult females (but they always groom their infants).

Males may compete intensively for access to females both within multi-male groups as well as between groups.

Males typically use displays, such as chest-beating, as a way to show their strength, but in some cases they will use physical aggression such as hitting and biting.

In multimale groups, silverbacks typically do not interact with each other often. Fights are more likely to occur when females are sexually receptive.

The dominant silverback is the one that is strongest and in his prime. Cases of younger males 'usurping' or taking over from an aging alpha male have been observed. Deposed older males may either stay in the group or become solitary.,

Adult females residing together in a group are typically unrelated, although mother-daughter and sister pairs sometimes live together because females do not always transfer or related females may transfer into the same group. Levels of aggressive and affiliative (friendly) interactions among female-female dyads are highly variable and may change over time. Dominance hierarchies exist among adult females, but they are not always easy to determine. Some females will spend a lot of time together and groom one another, whereas other females will not be particularly friendly with one another.

Despite food being very abundant, conflicts may occur over feeding spots. This is why the gorillas spread apart from one another more when they are feeding than when they are resting. Conflicts are more likely to occur when the gorillas are feeding on fruit in trees than when eating herbaceous vegetation.

Intergroup encounters occur about once a month, depending on how many groups use a particular area; this is the only opportunity for females to transfer between social units and hence the time for males to outcompete their opponents and appear the most impressive to attract females.

While the primary contribution of silver. backs to the care of immature gorillas is to provide protection, there are other positive benefits to silverback-immature associations. As the time spent near the mother decreases in late infancy, the time spent in close proximity to the silverback increases. Immature gorillas are attracted to silverbacks as the focal point of the group and spend considerable time in close proximity to him even in the absence of their mothers during both feeding and resting periods. Play likely serves as one of the most important behaviors for the development of social skills in gorillas. Play begins typically when infants are about nine months of age, peaks when gorillas are juveniles, and then decreases when gorillas are subadults. Sex differences in play exist, with males playing more than females, especially during adolescence. Occasionally adults will play.

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