The Cincinnati Zoo & Botanical Garden and hippo fans across the country are mourning the loss of Henry, one of the zoo’s beloved hippopotamuses.

The zoo had to euthanize Henry, who was the father of the zoo’s popular baby hippo, Fiona, on Tuesday, after he spent months suffering from an unknown illness.

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The 36-year-old Nile hippo had been battling chronic health issues that included a loss of appetite. He had lost hundreds of pounds over the past months, according to a zoo press release.

“The blood work from Henry’s last exam gave us some hope that he was on the mend, but his appetite never returned and his condition declined rapidly,” Christina Gorsuch, the zoo’s mammal curator, said in a statement.

“Vets and his care team worked tirelessly to keep him comfortable and help him fight this illness. Nothing – antibiotics, favorite foods, extra TLC – seemed to turn his condition around,” Gorsuch said.

“We are sad to lose him. Everyone loved him. He was a sweet, gentle giant with a big personality,” she said.

Cincinnati Zoo vet Dr. Jenny Nollman said Henry’s decline was irreversible.

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“Regardless of the cause, Henry had lost too much weight to bounce back. He was weak and starting to become disoriented,” Nollman said. “There was nothing else we could do for him.”

The zoo is planning a necropsy to see if they can determine the cause of Henry’s illness.

The average lifespan for a Nile hippo is 35 years. Henry was 36.

The zoo still has two Nile hippos at its Hippo Cove exhibit — Henry’s mate, Bibi, and their baby, Fiona.

Say your goodbyes: zookeepers laid to rest a beloved hippo father Twitter/CincinnatiZoo