Leo was brought to the Seattle zoo in 2008 and became the adoptive father of 2-year-old Yola. He leaves her and female gorillas Nadiri and Akenji behind.

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Leo, a silverback gorilla and the leader of his family group at Woodland Park Zoo in Seattle, died in his sleeping den at age 40 after a brief illness.

The gorilla had been feeling unwell since Monday, when he began staying in his den and shunning food and drink, according to the zoo. He was placed on a 24-hour treatment plan that included medications, hydration, hand-feeding and observation — and had seemingly rallied on Thursday when he ate and drank a lot, according to Nancy Hawkes, the zoo’s director of animal care.

“Unfortunately, he died suddenly in the presence of one of his gorilla keepers and in close proximity to his family without any warning,” she said in a statement on Friday.

Leo’s family group included 22-year-old female gorilla Nadiri, her 2-year-old daughter, Yola, and 16-year-old female Akenji. His body was kept overnight at the gorilla building so that the animals and people who knew him could say goodbye, the zoo said.

Gorillas are social animals, explained Hawkes. “This is a devastating loss for Leo’s family, our gorilla keepers and our zoo family. We’re shocked by his sudden death and will provide extra support and TLC for his family group. This is a very difficult time for our staff and volunteers.”

For the immediate future, Nadiri, Yola and Akenji will continue to be in the outdoor exhibit during zoo hours.

Leo, whose full name was Leonel, was brought to the Seattle zoo in 2008 and had become the adoptive father of Yola, the zoo said. He had exceeded the median life expectancy of 32 for male western lowland gorillas.