(CNN) The last of the "Gorillas in the Mist" -- made famous by renowned American primatologist Dian Fossey -- is believed to have died.

A gorilla known as Poppy, who would have turned 43 on April 1, has not been seen by trackers since August last year , according to Fossey's namesake nonprofit organization.

Poppy was born nine years after Fossey established a camp within Rwanda's Volcanoes National Park in 1967 as part of an effort to study the area's vanishing mountain gorilla population.

Fossey was killed in Rwanda over 30 years ago but her nonprofit said in a statement about Poppy's death that Fossey often wrote of the young gorilla in her journals. She described Poppy as a "little darling ... winsome and appealing. She could do no wrong."

Poppy pictured here in August 2015 was the last living mountain gorilla studied by famed zoologist Dian Fossey.

The Fossey Fund said Poppy hailed from one of the area's "royal families." Her mother, Effie, was the powerful matriarch whose members are now spread across many gorilla groups in the national park. Poppy's other well-known relatives include two silverbacks, Cantsbee and Isabukuru, as well as her sister, Maggie, who was a favorite of Sigourney Weaver while filming the Academy Award-winning film "Gorillas in the Mist," which was adapted from a book by Fossey.

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