The female orangutan had been at the zoo since 2005.

The Denver Zoo announced the unexpected death of the female matriarch of the zoo’s Sumatran orangutan family, Nias. She had been at the Denver Zoo for 15 years and was well-loved among staff and visitors. Nias died on Thursday, December 17.

Currently, the cause of her death is unknown, as the zoo is awaiting the results of a necropsy from the Colorado State University College of Veterinary Medicine and Biomedical services.

Nias came to the zoo in 2005 at age 17 and gave birth to two daughters during her time in Denver. She was an important ambassador for her species and was close to many of the zoo staff. By all accounts, she was a delight to be around and never stopped surprising those who cared for her.

Nias is survived by her mate, Berani, her daughters, Hesty, 10, and Cerah, 2, as well as the team of care staff and volunteers that worked to make her life a happy one. She was known as the “Queen Bee” of the orangutan exhibit, according to the Denver Zoo.

Cindy Cossaboon, Nias's keeper from the very first day she arrived at the zoo, wrote a very touching tribute to her, referring to Nias as her “Heart Animal.” She describes her as an “amazing mother”, and fiercely protective. Read her full tribute here.

We are deeply saddened at the loss of this amazing animal that brought so much joy to many people. Our condolences go out to the Denver Zoo staff and to Nias's care team.

What is your favorite memory with Nias? Please share your experiences visiting her at the Denver Zoo in the comments.