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 gorillas.
The largest group of mountain gorillas observed in the Virunga Massif contained 65 members, whereas the largest group seen in Bwindi had about 35 gorillas.
Groups consist of the dominant silverback (silverback gorilla trekking Uganda), several adult females, and immature offspring 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 (silverback gorilla trekking Uganda), but multi- male groups rarely contain more than two silverbacks (silverback gorilla trekking Uganda).
In multimale groups the silverbacks (silverback gorilla trekking Uganda) are usually related as brothers, half-brothers, or father and son. However, sometimes unrelated silverbacks (silverback gorilla trekking Uganda) 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 (silverback gorilla trekkingUganda) 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 (silverback gorilla trekkingUganda) 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 of 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 will split into two.
In Bwindi, the Habinyanja group fashioned in 2002, to create the Rushegura Group. The Shongi group also has fashioned in recent years.
Males follow one of two strategies to become the leader of a group: either remain in the group and attempt a takeover 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 compared to lowland gorillas is the occurrence of multimale groups. Lowland gorillas almost never form multimale groups, whereas about 40-50% of 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. 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 the 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.
For more information about social groups in gorillas, do not hesitate to ask us.