Social Behaviour of Mountain Gorillas
The social organization and behavior of gorillas is dominated by results from research made by researchers on mountain gorillas. The minimum group size for all subspecies is usually a silverback and a female except for males ranging alone, while maximum group size varies slightly for each subspecies. The maximum group size for mountain gorillas can exceed 20 individuals, while eastern and western lowland gorillas generally are not found in groups larger than 17 to 20 individuals and western lowland gorilla groups are always observed in groups smaller than 20 individuals.
The Mountain Gorilla is diurnal, most active between 6:00 a.m. and 6:00 p.m. with a nap around a lunch time. Much of these hours are spent eating, as large quantities of food are needed to sustain its massive bulk. It forages in early morning, rests during the late morning and around midday, and in the afternoon it forages again before resting at night.
Each gorilla builds a nest from surrounding vegetation to sleep in, constructing a new one every evening. Only infants sleep in the same nest as their mothers. They leave their sleeping sites when the sun rises at around 6 am, except when it is cold when they stay longer in their nests. The males’ strength is 10 times stronger. The silver backs arms will stretch out seven feet. Mountain Gorillas have longer and darker hair than their lowland cousins since they live in colder climates at much higher altitudes.
In the Virunga Mountain Gorillas, the average length of tenure for a dominant silverback is 4.7 years. 61% of groups are composed of one adult male and a number of females and 36% contain more than one adult male. The remaining gorillas are either lone males or exclusively male groups, usually made up of one mature male and a few younger males. Group sizes vary from five to thirty, with an average of ten individuals. A typical group contains: one silverback, who is the group’s undisputed leader; one or two black backs, who act as sentries three to four sexually mature females, who are ordinarily bonded to the dominant silverback for life; and from three to six juveniles and infants. Most males and about 60% of females, leave their natal group. Males leave when they are about 11 years old, and often the separation process is slow: they spend more and more time on the edge of the group until they leave altogether.
The Silverback & Its Roles
The dominant silverback generally determines the movements of the group, leading it to appropriate feeding sites throughout the year. He also mediates conflicts within the group and protects it from external threats. When the group is attacked by humans, leopards, or other gorillas, the silverback will protect them even at the cost of his own life. He is the center of attention during rest sessions, and young animals frequently stay close to him and include him in their games. If a mother dies or leaves the group, the silverback is usually the one who looks after her abandoned offspring, even allowing them to sleep in his nest. Experienced silverbacks are capable of removing poachers’ snares from the hands or feet of their group members.
Mountain gorillas live in age graded groups of 9.2 individuals on average, with one adult male however much there could be more than one adult and multiple adult females, and their offspring. Natal dispersal is much more common for females than for males in mountain gorillas. Generally, females emigrate from their natal groups to avoid inbreeding. They do not always stay in their new groups throughout their lives and secondary transfer is common. Males either remain in their natal groups or disperse. If young males remain in their natal groups, they will be subordinates to the silverback, but may have the opportunity to mate with new females or become dominant if the silverback dies.
The other strategy for mountain gorilla males is to leave their natal groups and become solitary, attracting emigrating females and starting new social groups. In groups containing only one silverback male, females disperse and find new social groups upon the death of the silverback. This may be related to high rates of infanticide documented among mountain gorillas at Karisoke. Infants deprived of protection by an adult male are almost certain to be killed and as a tactic to protect against this, females join new groups in the absence of a silverback.
Finally, males can live in all-male groups, although this seems to be a strategy generally employed by young males when their group disintegrates before they reach adulthood.
It is important for female mountain gorillas to develop strong relationships with males because males offer such services as protection against predators, protection against infanticide by other males, and mating opportunities. It is common to see aggressive behavior between males and females, though rarely is it intense or does it lead to serious injury. Female-female relationships vary, but generally differentiate along lines of relatedness; maternal relatives associate closely and often affably interact.
Generally, though, female gorillas have limited friendly relationships and multiple aggressive encounters. These aggressive encounters often revolve around social access to males, and males intervene in contests between females. Relationships between male gorillas are generally weak, especially in heterosexual groups where the dominance hierarchy is quite apparent and there is strong competition for mates. Relationships between members of all-male groups, in those subspecies where they occur, are slightly more affiliative and they socialize through play, grooming, and close proximity.