One of the Taiping Four gorillas flown from South Africa to Cameroon last year has died, the International Fund for Animal Welfare (Ifaw) announced on Tuesday.
The female gorilla, known as Oyin, died of "unknown causes" in Cameroon on Monday afternoon, said Ifaw spokesperson Christina Pretorius.
"She had apparently been sickly for about 10 days. At this stage it is unclear what caused her illness and, following an autopsy, blood and tissue samples have been sent to appropriate laboratories to ascertain the cause of death," she said.
Ifaw would announce the cause of death as soon as it was known.
The four western lowland gorillas endured an 18-hour flight from South Africa to the Limbe Wildlife Centre in Cameroon, their country of origin, in December after a five-year political tussle.
They had lived in specially constructed habitats at the Pretoria Zoo since 2004 after their confiscation in 2002 by Malaysian authorities who found them being illegally imported to its Taiping Zoo from Nigeria.
The surviving gorillas are known as Tinu, Izan and Abbey.
At the time they were relocated, Pretorius said the gorillas were all tested and declared free of disease.
However, they were going to be kept in quarantine for several months while they acclimatised.
"They are sociable animals and you can't just release four adolescent animals into a family group. They have to adapt," she said.
Opponents of the move argued that the apes would be safer in South Africa, where they would be protected from poachers who might hunt them for meat.