The world’s oldest known gorilla died in her sleep overnight, the Columbus Zoo announced Tuesday.

With a heavy heart we report that #ColoGorilla passed away in her sleep. She recently celebrated her 60th birthday. https://t.co/bkHnyLRZv2 pic.twitter.com/DA3ehFdAlV — Columbus Zoo (@ColumbusZoo) January 17, 2017

Colo, a 60-year-old Western lowland gorilla, was the first gorilla ever born in a zoo, according to the Columbus Zoo. A necropsy is planned to determine Colo’s cause of death, but the zoo notes that she had a malignant tumor removed under her arm on Dec. 3.

Francis Miller via Getty Images Colo as an infant, being held by the Columbus Zoo director's wife.

Colo, whose name was short for “Columbus, Ohio,” had just celebrated her 60th birthday on Dec. 22, which zookeepers rang in with a gifts and a cake made out of apples and tomatoes.

“She was the coolest animal I’ve ever worked with and caring for her was the highlight of my career,” assistant curator Audra Meinelt said in the zoo’s statement. “It was not just about what she meant for the gorilla community but for whom she was as a a gorilla. I’m heartbroken but also grateful for the 19 years I had with Colo.”

Columbus Zoo Colo in an undated photo from the Columbus Zoo.

The average lifespan of a Western lowland gorilla in the wild is 35 years, according to National Geographic. And although gorillas in captivity may have longer lifespans, some primatologists and animal advocates question whether the large, intelligent creatures ethically can be kept in zoos at all.

Primatologist Sarah Bluffer Hrdy told The New York Times in June that she has to “turn off” her feelings when she goes to a zoo.

“Otherwise, I can’t really justify keeping great apes in cages,” she said.

Colo is the second Ohio zoo gorilla to die this month. Bebac, a 32-year-old western lowland gorilla at the Cleveland Zoo, died of heart disease on Jan. 6.