Whilst I'm unsure on the success with older castrated males (as I don't think any of the trial individuals are old enough yet), I know it is a successful method for adolescents/blackbacks. I believe that the only castrated male that would have been old enough to be considered an adult has unfortunately passed away from non-castration related issues.
Update; The oldest castrated male in European Zoos is now 18 years old- Loango at Gaia Park, the next oldest is at Chessington (Mbula, 13) and both places also have a second castrated male(12 & 10 respectively). Vallee de Singes in France also have a 12 year old in their group. The 18 year old is certainly fully adult and the others getting close. Currently (afaik) all are still living in their natal groups, but another young castrated male in Apenheul (Jabari, also 10) is currently split from his, as he challenged/hassled a newer old and quiet silverback.
The situation in Gaia Park is also interesting as a new silverback moved in recently after the castrated males' own father died. But they seem to have been accepted in this new group too.
The original fully adult male which died of non-related issues was Kukuma, who died aged 23 in Belfast. He was the original single 'trial' male, born at Apenheul he then spent most of his life in the Belfast group without incident.
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