I agree with @joshgrossglaza this may in fact be a non breeding transfer - Binti Jua's line is quite well represented so they may be choosing to hold off breeding for a while. A silverback is still essential to a dynamic group though, so they were always going to swap in a silverback once Zachary was old enough for transfer.
According to St Louis Zoo, it is a breeding move:
"Jontu (seen in the first picture) is well known at the Zoo for his regal appearance and confidence as a leader in the bachelor group — with a playful and gentle side. His move from St. Louis to Chicago was based on a future breeding recommendation by the The Association of Zoos and Aquariums Species Survival Plan for western lowland gorillas, a conservation breeding program that manages a genetically healthy population of this species in North American zoos. This move was made with plans of growing and strengthening the western lowland gorilla population by providing Jontu the opportunity of creating a family. He will be joining a group with adult females in the hopes of future breeding to create a family group at the Brookfield Zoo."
Source: 登录 Facebook
Jontu is pretty valuable, Koola and Kamba are even more valuable. Considering just genetics, how close they are isn't a big issue, however, there are other males completely unrelated to them.