Complex Social Lives of Mountain Gorillas (Including Sex)

Who Can Be a Silverback?

A silverback is an adult male gorilla, typically over 12 years old and named for the distinctive patch of silver hair on his back. They have large canine teeth that come with maturity. Silverbacks are the strong, dominant troop leaders but not all have this position; some may have never led a troop while others have either retired or been deposed. Groups of male silverbacks have been observed though the dynamics can suddenly change when the leader dies or is overthrown.

Each typically leads a troop (group size ranges from 5 to 50) and is in the centre of the troop’s attention, making all the decisions, mediating conflicts, determining the movements of the group, leading the others to feeding sites and taking responsibility for the defence, safety and well-being of the troop.

What Is the Role of Blackbacks?

Blackbacks are sexually mature males up to 11 years old; they usually serve as backup protection but often maintain distance from the leader. They often strike out independently and can form transitory groups of their own. If alone, they sometimes join other troops for short periods. It is only when they become silverbacks that they will attempt to form their own group or take over an established one.

Multi-male groups appear to be more sexually successful than single males, which results in the formation of blackback troops with individuals of different origins. However, it is also thought the reason that males leave these groups is the innate need to reproduce. This is somewhat contradictory, and may be due to how well they bond as individuals within the group; those who don’t bond end up leaving.

How Loyal are Gorilla Females?

There is no such thing as life-long troop loyalty among females either: they usually migrate away from their natal troop. The avoidance of in-breeding and the search for a suitable mate must be innate but other factors can be a trigger, i.e. a lack of males or too much female competition.

Social relations between females are strong and status is defined by seniority. Friendly behaviour between females is the most common though aggression can occur over competition for food or males. They can form coalitions to protect themselves from aggression from other females or males. These tend to be among relatives as the natal bonds remain into adulthood.

How Do Gorilla Groups Interact?

Research in the 1920s indicated an average group size of 13 while Batwa estimates were 15–20. Nowadays groups of 30 with five silverbacks have been recorded, as have groups that are male only; 40% in the Virungas are multi-male. The latter have weak relationships and may join another group if there is an opportunity to mate with females; though this can be difficult as the lead silverback has this prerogative and is likely to be the father of most of the group’s infants.

Conflict between gorilla troops is rare, unlike chimpanzees and other monkeys, because they are not territorial. If two groups happen to meet there is a lot of chest-thumping, charging and other dramatic displays of dominance but rarely any physical contact. Avoidance is the main strategy.

During such a meeting females may try to defect to a new group, which may be prevented by the silverback leader. Defectors can be harassed by related senior females from the group they joined. The silverback leader may intervene by calming the situation down; he usually does not take sides. His interest is to limit the damage as disruption of social harmony may cause females to migrate.

Who Gets to Make Babies?

Dominance and sex relations are only loosely linked, though competition among silverbacks can centre around females, initiated by the male who doesn’t have any. Attitudes of established leaders towards their mates vary from protective dominance when younger and complete tolerance when older.

Genetic studies have shown that the leader does not sire all the troop’s children (85%); the second-in-command, if there is one, is responsible for the balance. However once they have established a position they brook no opposition. If challenged by a younger or even by an outsider male, a silverback will scream, beat his chest, break branches, bare his teeth, and then charge forward.

If the leader is killed by disease, accident, fighting or poachers and there is no other silverback, the group will split up, as they disperse to look for a new protective male. There is a strong risk that the new male leader, if it is a new silverback, will kill the infants of the deceased. Uncertainty is created when a female has mated with more than one male, which preserves her children.

How Does Gorilla Sex Look Like?

Female gorillas reach sexual maturity at 8 years. Before they become sexually active, they will normally leave their original troop and thus avoid inbreeding; inbred offspring can have crossed eyes and webbing between the fingers. They are fertile for up to three days a month and often initiate contact; they can choose among all adult gentlemen in the group.

Gorilla sex is rare and short, sometimes less than a minute. George Schaller described it like this:

The female rises and stands by the rump of the male. He glances up and they stare at one another. The process of pulling her into a sitting position is repeated. At about seventy-five thrusts he begins his copulatory sound. His eyes are closed, and the thrusts rock her back and forth, a motion added by his hands on her hips and the swaying of her body. His lips are pursed and hers are slightly parted. At about one hundred and twenty thrusts the male suddenly opens his mouth with a loud sighing ‘ahh’, the female opens her mouth at the same time. He relaxes, she rises and leaves.

photo by Marcus Westberg

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