I was not aware of Basel. Could you please elaborate?
Father/son combinations: I thought zoos wait as long as possible before they move a son to another zoo. (For instance, a keeper from Munich told me they won't move Sadiki until the conflicts with his father Roututu increase intolerably. Besides, they want to wait as long as possible so they can move Sadiki together with his younger brother Kajolu, in order to keep the two brothers together - probably when moving them to Schmiding Zoo.) Do you want to say the EEP recommends transfers at a certain age, regardless of the respective father/son relationship?
When Basel opened their current house- circa 1967/8 all the Gorillas were put together in one group for the first time. The young male 'Pepe' (originally the mate for 'Goma') was about 12 years old and silverbacked by then. However when I saw them like that the older silverback 'Steffi' was obviously stressed by this arrangement and kept 'Pepe' confined to the furthest enclosure on his own, while preventing the others joining him there. This arrangement was later terminated by giving Pepe just Goma as a companion (Pepe later became group leader on Steffi's death). But being his sons, the young males 'Jambo' and 'Migger' both lived with their father quite successfully until they were nearly adult and sent elsewhere.
Howletts occassionally had two silverbacks together in their earlier days too, Djoum and Mumbah were one example but it wasn't very longterm.
Age at which males are removed. Some zoos do leave a son in the natal group until he's nearly mature, others don't. I don't know of any EEP recommendation about this. It seems to depend on individual circumstances, as in your Munich example. Similarly, 'Kesho' was still with his father at Dublin at age 11 and silverbacked, perhaps because there was no place for him until ZSL took him, whereas his two younger brothers were removed at much younger ages(particularly the youngest) because they could go to Longleat. 'Matadi' left Twycross aged 8 to join Paignton's male group because the new silverback 'Oumbie' rejected him(EEP had said they should get along okay but they didn't). On the other hand Bristol have said they want to keep their young male 'Komale' in the group as long as possible- a good move IMO. I think it very much depends on individual circumstances every time but efforts should be made to keep them as long as is feasible.
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