Named simply for its original size of 13 individuals when it was discovered, it has since maintained the name, with 21 individuals: 1 Silverback; 10 Adult females; 1 Sub adult female; 1 Juvenile and 8 Babies.
Formerly this group was commanded by Nyakarima who was challenged by Agashya meaning "the news". Agashya indeed made news by first watching and estimating Nyakarima's strengths and eventually challenging him to a fierce fight by taking off with his whole group. This was a shock to Nyakarima and an unprecedented event in observed gorilla history. Agashya then moved up the volcano to secure his group and make sure Nyakarima did not track them. Agashya has since increased his group's numbers by snatching from other groups and assimilating other lone gorillas, rapidly increasing the group from 12 to 21 individuals.
Agashya is known, at the first sign of trouble, to take his whole group up to the top of the volcano. Once, while we were taking a group of tourists to see Group 13 this exact scenario unfolded. Agashya felt that there was another silverback who was about to challenge him. In response, he took the group up the volcano. Every time we thought we were close to see them, Agashya would move the group further uphill. We did not reach them until the very top, where we finally got a chance to see them. By the time we got back to the car we have walked for 12 hours. While it was an exhausting day, it was magical to see gorillas at the top the world.