August 24, 2012
An Interesting Interaction: Giraneza and Bwenge's Group
On August 20th, Dian Fossey Gorilla Fund field staff reported that Bwenge’s group interacted with a lone silverback while they were ranging on a mountainous ridge between Karisimbi and Bisoke. Unseen by Bwenge, the silverback entered the group and approached Akaramata, a 4-year old juvenile female. He began making displays at her, causing her to scream, after which he hit her. Bwenge ran to her assistance and chased the lone silverback, but without making any physical contact with him. The rest of the group hurried after Bwenge. However, the interaction was not over yet. The lone silverback and Bwenge’s group stopped a couple of minutes later, and the two silverbacks began exchanging displays for about half an hour. No further physical contact was made, and eventually Bwenge led his group away from the other silverback in peace.
The trackers who observed the interaction were not able to identify the lone silverback at first. It was only later, by following his trail through the forest, that we were able to confirm that the lone silverback was in fact Giraneza. Giraneza's group’s trackers were having trouble locating him that morning, and when they did reach him in the afternoon, the interaction with Bwenge had already ended and he was back in the company of his females Taraja and Nyandwi. By using the trails to reconstruct the course of events, we believe that Giraneza must have left the two females behind before trackers found them, in order to interact with Bwenge.
Bwenge’s group was an interesting choice for Giraneza to target. Bwenge has only three adult females, all of whom are currently caring for unweaned infants. In general, females who have not yet had their first infant or whose offspring are weaned are more likely to transfer out of their groups than those who still have dependent offspring. To persuade one of these females to transfer would require a new silverback to kill her infant. Because of the potential for infanticide in Monday’s interaction, Fossey Fund field staff are relieved that no infants were injured, and that the interaction resolved itself without physical contact between the two silverbacks.
Although it seems odd in some ways that Giraneza would leave his females behind temporarily to interact with Bwenge’s group, given that none of Bwenge’s females are particularly likely to transfer, over the last month Giraneza and Bwenge’s groups have been linked in several occurrences. A couple of weeks ago, Giraneza’s two females, Taraja and Nyandwi, seemed to be following Bwenge’s trail, and we wondered if they might try to join Bwenge’s group. A couple of days later, Taraja’s infant Akarusho, who had been missing for some time, turned up in Bwenge’s group. It is difficult to know whether these encounters are just casual, as a result of ranging in the same area, or whether there is more to them that we aren’t able to piece together yet. In any case, the relationship between these two groups is definitely a fascinating one to follow.