January 9, 2012
A Most Persistent Solitary Silverback
The Fossey Fund’s Karisoke™ Research Center routinely monitors nine groups of mountain gorillas. Ranging in the same area are six well-known solitary silverbacks that make appearances from time to time when they are searching out social groups in hopes of attracting females. By the end of 2011, Karisoke researchers had recorded 82 encounters with a lone silverback, 54 of which were active interactions with a social group. These interactions happened under a variety of different scenarios and lasted anywhere from mere seconds to an entire day, to weeks on end. While these silverbacks are frequently alone, they are almost always on the trail of another group, biding their time to make a move.
The behavior of the lone silverback
Solitary silverbacks spend several years – and sometimes their entire adult life – traveling alone. The young males are born and raised in the safety of a gorilla group, but as they grow into adults and the characteristic silver hair begins to show on their back, the desire to become dominant can cause them to strike out on their own. The silverback will then begin a lonely quest that may last for years, peppered with dramatic interactionss. Throughout that time, the lone silverback will periodically pursue a social group, usually with his eye on a specific female he would like to acquire. Displaying and vocalizing dramatically, he can push the group’s dominant silverback to react aggressively, sometimes resulting in violence or injury. More frequently however, the field staff observes "auditory interactions" between social groups and the lone silverbacks. This occurs when the lone silverback announces his presence with intimidating chest beats and hooting vocalizations, to which the silverbacks within the group will respond accordingly. Sometimes the interaction will end there, if the group is successful in discouraging the outsider.
Will persistence pay off?
Without a doubt, the most tenacious of these six lone silverbacks has been Gwiza, whom Karisoke trackers encountered 31 times in 2011. Gwiza left Shinda’s group in April 2004 when he was 16 years old. During the past eight years he has been observed traveling alone. Interestingly enough, since the death of dominant silverback Shinda and the subsequent group split, Gwiza’s interaction frequency has increased dramatically. The lone silverback routinely targets Ugenda’s and Ntambara’s groups (the two groups that resulted from thebreakup of Shinda's group). It seems that, despite his decision to live and travel alone, silverback Gwiza still does not want to stray too far from his origins.
In October of 2011, the young female Ubufatanye transferred from Ugenda’s to Titus’s group, spurring an impressive competition between Gwiza and two other lone silverbacks. Several weeks later, in mid November, Titus’s group endured a two-day interaction with lone silverback Tuyizere, followed by a one-day interaction with lone silverback Turatsinze and finally, a strenuous interaction with lone silverback Gwiza which lasted an exhausting 18 days.
During these encounters, Ubufatanye (better known as “Fat” by the Karisoke field staff) showed interest in all of the males involved. She was documented as part of an isolated pair with each lone silverback at different times. On one such occasion, she spent an entire three days separated from her group with lone silverback Gwiza before rejoining Titus’ group leader Rano and the other group members. The three days that they spent together seemed to give Gwiza hope that his solitary life had finally come to a close.
When Fat rejoined Titus’ group, Gwiza was not ready to give her up and he persistently followed the group both day and night for nearly three weeks. With numerous displays, chest beats and vocalizations, Gwiza tried hard to persuade Fat to follow him. There were also many dramatic displays by the two silverbacks and two blackbacks of Titus’ group towards Gwiza. During this time, Fat still seemed to be interested in Gwiza, as the two gorillas often shared night nests and were observed in physical contact. The Karisoke staff was very hopeful that Gwiza, the silverback who had been solitary for far too long, had finally found a mate.
However, to the field staff’s great surprise (and a bit of disappointment), Gwiza was not with the group when the trackers reached them on the morning of December 1, but Fat was still among the group members. It seemed that Gwiza and Fat’s tumultuous affair had come to an end.
The interactions in 2011, coupled with the 44 years of mountain gorilla observation by the Fossey Fund’s Karisoke Research Center, gives a good idea of how difficult it is for a lone silverback to form a new group. Little is known about the life of the solitary males because they are not monitored daily as the social groups are. The encounters recorded by Karisoke give an essential, if partial, understanding of their ranging pattern and frequencies of interaction with social groups. More observations are needed to better include the solitary males as an important element of the reproductive strategies of the species.
Submitted by Veronica Vecellio, Gorilla Program Manager and Jessica Burbridge, Field Communications Officer, Karisioke™ Research Center