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Thu, September 2, 2010

Silverbacks Share Leadership

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Two Silverbacks Collaborate in Leading Largest Mountain Gorilla Group

After more than 15 years of undisputed leadership by silverback Cantsbee, a younger silverback is trying his chances at leading Pablo’s group, the largest group of mountain gorillas ever recorded. And it seems to be working — the dominant silverback, Cantsbee, seems to be accepting the behavior of the younger male.

CantsbeeThis large group is named after the late silverback Pablo, who died in 2008 but had already ceded leadership to Cantsbee by that time. Currently the group includes 44 members, with Cantsbee the oldest male, supported by an especially large number of younger males. Five of those males have already matured enough to be called “silverbacks,” as a saddle of hair across their back has turned silver-grey, while another eight are still in the “blackback” category, as younger adult males.

Cantsbee is one of the last gorillas of the Dian Fossey generation and the last one among the three legendary silverbacks she observed, including Titus and Shinda, leaders respectively of Beetsme’s group and Shinda’s group. Cantsbee is also the subject of a BBC Wildlife documentary filmed in Rwanda last year and airing this year on BBC Two to great acclaim.

GicurasiDespite his age, Cantsbee’s massive strength still discourages other silverbacks from challenging him for the top position. Many males have left the group and become lone silverbacks instead, hoping for a better chance to attract females and form a group of their own. On the other hand, time is leaving its mark on the mighty silverback. He depends on the help of younger males to ensure group cohesiveness and protection, which means he must also allow them increased opportunities for sexual approaches to females.

As a result, the second-ranking silverback, Gicurasi, is taking his chances, accounting for the majority of copulations recorded this year, while Cantsbee apparently took no such opportunities. Researcher Dr. Cyril Grueter (post-doctoral student at the Karisoke™ Research Center) observed that “on one occasion when Gicurasi was mating with female Nyabitondore, Cantsbee disapproved and charged the young silverback but he soon stepped back, allowing him to continue…”

Cantsbee with his groupThere are two possible explanations why Cantsbee did not react to Gicurasi’s ambitious behavior. One is that he is avoiding physical confrontations, due to his age. The other is that he finds it convenient to allow sexual opportunities for the young male in order to ensure efficient collaboration in protecting the group.

Cantsbee is 31 years old, and Gicurasi is only 15. Gicurasi has gradually obtained the privilege of being Cantsbee’s right arm and, unlike other males, has always been tolerated by the dominant silverback in the center of the group, surrounded by females and immature gorillas. Sexually active females are attracted to the young silverback and some of them followed him during four occasions since the beginning of the year when the group split temporarily. The subgroup led by Gicurasi always rejoined the main group after one day of independent travel, but it still represents the very first split from the group since the death of silverback Pablo.

Sub-grouping and eventually group splits have been observed in the other two historical groups monitored by the Karisoke Research Center, Beetsme’s and Shinda’s. The events were related to a change in dominance between silverbacks caused by the death of the previous dominant silverback, as in the case of Shinda’s group, or by the challenge of a younger male, as in the case of Beetsme’s group.

Pablo's group on the moveOnly a few behaviors can be used as indicators of dominance in mountain gorillas. These are rare events which, if observed, can mean that a gorilla’s rank in the group is starting to rise. One of these behaviors is the “passing” of gorillas in front of others during group movement. The group movements are always initiated by the dominant silverback. All gorillas in the group follow him in a line and a few fights may occur to compete for the position closest to the head. Young Gicurasi tried a few times to overtake Cantsbee, and he did succeed once. “This was the first time that I observed a silverback passing in front of Cantsbee,” says Francois Xavier Ndungutse, data technician in Pablo’s group since October 2001.

It is still early to announce any change in Pablo’s group, as Cantsbee remains the undisputed leader. Nevertheless, the recently observed dynamic among the males may suggest that the exclusive leadership of such a large group may have become much too hard a task for a silverback of Cantsbee’s age. For now, though, the coexistence of more than one male is an advantage for all group members, as they can make the most of the experience of the old leader and of the strength of younger males.

Submitted by Veronica Vecellio, Gorilla Program Coordinator, Karisoke™ Research Center

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