Tue, February 12, 2019

Episode Two: Male hierarchy and a gorilla group’s success

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This is the second in a series of articles chronicling the lives of the gorillas in a legendary mountain gorilla group named after former silverback Pablo, in honor of the group’s 25th anniversary. Pablo formed the group in 1993, but was its leader for only one year, before yielding dominance to silverback Cantsbee, who would reign for the next 20+ years.

 Mountain gorillas live in groups that are led by a dominant silverback (adult male), who must lead the group’s daily travels and activities, keep the group cohesive, and defend the group from outside intruders. He usually also has the best access to the females for mating. Cantsbee was so successful at this that he oversaw the group’s growth to a record 65 members, sired more offspring than any other male we’ve documented, and held the longest reign of power of any other silverback.

Cantsbee was a very impressive and successful leader and was able to maintain dominance and respect throughout most of his adult life. Our observers described him as strong but gentle, charismatic and authoritative. His clear leadership meant he was strongly respected by all members of the group, providing a clear sense of hierarchy. And respecting the hierarchy is the first and most important rule in keeping a gorilla group cohesive. So in this episode, we are focusing on male hierarchy, since it is the most critical factor in group life. But hierarchy exists in both males and females, becoming relevant when gorillas reach sexual maturity, around age 8.

“Hierarchy among the silverbacks is so important that when there is more than one silverback there is literally a number one, number two, number three and so forth,” says Veronica Vecellio, the Fossey Fund’s gorilla program senior advisor. And Pablo’s group is a prime example of this, with its four silverbacks and three slightly younger blackbacks all involved.

Hierarchy dictates a variety of behavioral rules within the group, such as: deciding where the group will go; access to feeding spots; who can walk in front of whom; and male access to females for mating and for proximity during rest periods. Hierarchy also becomes evident during interactions with other groups, where males have to show their alliances with each other while facing an opponent.

Gicurasi is current number one

Cantsbee left his mark on the group in so many ways, not least of which was helping to raise son Gicurasi, who now leads the group. Gicurasi’s mother left the group when he was still quite young, and Cantsbee helped watch over his young son. But even before Cantsbee’s death in 2017, Gicurasi began testing out the possibilities of leadership, challenging his elderly father more and more often, eventually taking over.

Gicurasi, 23 years old, is in his prime and is the dominant silverback in the group.

One time, while the group was resting in 2015, Gicurasi approached Cantsbee and another silverback who was nearby. These two loudly grunted at Gicurasi but he did not stop approaching, and Cantsbee then stepped back to make room for Gicurasi to pass through. This kind of “crossing in front” of a dominant silverback (officially called a “displacement”) may seem like a small act, but our expert gorilla trackers and researchers know that it is an important sign that leadership is being challenged. Another sign of Gicurasi’s growing dominance desires was his ability to gain sexual attention from several females in the group around this same time.

By 2016, Gicurasi was already leading much of the group’s protection and mating activities, though he did this without “official” recognition from all the group members. And the number of displacements by Gicurasi increased, even though Cantsbee was sometimes still able to show strong opposition when they ended up fighting physically. Now, as the number-one silverback, Gicurasi is a strong and undisputed leader, in his prime at age 23.

Numbers two, three and four

The current number two silverback in the group is 15-year-old Dushishoze, also a son of Cantsbee. He has the trust of Gicurasi and plays an important role as a sentry for detecting outside dangers, and being second in charge when facing an intrusion. Yet, he spends time on the periphery of the group and is always last when the group is moving, which is not unusual for the number-two silverback. He also doesn’t get much access to the females.

Dushishoze, 15 years old, is the current number two in the group.

Number three silverback is 13-year-old Ubwuzu, a grandson of Cantsbee, who is less shy with the females and a very active subordinate. His good relationship with Gicurasi allows him to show off to the females, and his favorites so far are young Shishikara and Umwari. They seem to like him too!

Ubwuzu, 13 years old

Number four silverback is another son of Cantsbee, 12-year-old Agahebuzo who is just maturing from his adolescent stage. He seems well aware that there are three silverbacks above him and that he needs to respect them. A few months ago he made some challenges to Ubwuzu but lost, mainly because Ubwuzu has a better relationship with number one, Gicurasi. Now Agahebuzo is calmer and knows to accept his lower rank.

Agahebuzo, 12-years-old

We are also watching the maturation of three younger males, blackbacks ranging in age from 8 to 12 years old. They are too young to be in the running for a position in the leadership hierarchy so far, but they are becoming sexually mature. One of these — Imfura — has grown a lot recently and some of the females are showing interest in him. But this may be temporary so we are watching closely to see what these three young males do, as they shift from playing sessions to flirting to displays of power!

Imfura, 10 years old

Learning all the time

Even though we have been observing mountain gorilla groups for more than 50 years, we have only witnessed a few examples of dominance shifts between silverbacks, and most of those occurred due to a unique trigger (such as death of a silverback), rather than due to old age. So the Cantsbee hierarchy change was special to observe.

And now, with all the males in Pablo group, we have an incredible opportunity to study all kinds of additional details about male hierarchy, including individual variability, resilience in the face of change, and various strategies for success.

“Being able to observe the gorillas for many hours every day gives us the opportunity to accurately document dominance hierarchy, because it allows us to detect rare behaviors and reactions that last just a few seconds,” says Vecellio. “Most of the time, the males actually keep a peaceful tolerance and a certain distance between themselves, so it would be hard to see that something is going on if you just observe them for short time!”

Of course, male gorillas are not the only ones who establish hierarchies within a group. Adult females do this too, among themselves within a group. On very rare occasions, we have also seen an adult female take over leadership of a group, but only for short periods of time, when the dominant silverback had died and there were no other adult males to take over.

 

 

 

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