One of Honolulu Zoo's oldest animals has died.

Cleopatra, known as Cleo, was the second oldest hippopotamus in captivity in the U.S.

A zoo veterinarian put her to sleep on Saturday because she was suffering from growing complications of old age.

The 3,500-pound hippo could no longer stand on her own.

She was just a few weeks shy of her 53rd birthday.

Cleo will be sent to a university lab for research and study.