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DENVER (KDVR) — Nias, the matriarch of a Sumatran orangutan family at the Denver Zoo, died unexpectedly last Thursday, the zoo confirmed Monday.

Nias was delighting guests at the zoo for the past 15 years, according to the zoo. She came to the Denver Zoo in 2005 when she was 17 years old.

“We do not know the cause of Nias’ death at this time, and are awaiting the results of a necropsy from our partners at Colorado State University’s College of Veterinary Medicine & Biomedical Services,” the Denver Zoo tweeted.

Nias, according to the Denver Zoo facebook page, “was often seen caring for and playing with her two daughters, Hesty, 10, and Cerah, 2. Though Cerah is young, she is expected to continue to thrive in the company of Hesty, her father, Berani, and her care team.”

Great Apes Keeper Cindy Cossaboon cared for Nias since she arrived at the zoo. She wrote a lengthy tribute, saying, in part, “Nias really made you work hard to earn her friendship. As the years went by, I was always surprised how deep and strong my friendship was with her. In 2010, the very first time she was pregnant and had gone into labor, she built her nest in the far corner of her den. It was a busy day at work, but once it quieted down and it was just us left, she moved her nest to where I was sitting and gave birth in front of another keeper and me. It was truly one of the most humbling experiences of my life.

“When Nias accepted you into her circle, she was truly a silly, goofy girl. She had the cutest giggle and she knew I loved it. She would somersault and do headstands all while giggling away. Sometimes we would be sitting in our office and hear giggling and playing sounds. We would sneak back and see the entire family playing and wrestling together.”

Nias went by a lot of nicknames, according to Cossaboon, including Mrs. Neeners, Ladybug, Queen Bee and Sweetpea.