Nonja was brought to Miami from a zoo in Holland in 1983

Nonja, who had been resident at the Miami Metro Zoo since 1983, was found by her keepers on Saturday morning.

Experts said a brain tumour or aneurysm was the likely cause of death.

"She was a grande dame and I think she knew it," said zoo official Ron Magill, adding that most orang-utans died before reaching their mid-40s.

Nonja, whose name means "girl", was brought to Miami from a zoo in Holland in 1983, according to Mr Magill.

He said Nonja had mothered five offspring.

Sumatran orang-utans are listed as critically endangered by The World Conservation Union, with their population down to 7,300 in 2003, according to the Sumatran Orangutan Society.