The Omaha Henry Doorly Zoo & Aquarium on Tuesday announced the death of a female white rhino.

Marina, a 21-year-old white rhino, was euthanized Monday after the discovery of an inoperable colonic obstruction, zoo officials said in a statement.

Marina was being treated for symptoms similar to colic after zoo staff noticed that she appeared lethargic last Wednesday, a zoo spokeswoman told The World-Herald. When she didn’t respond appropriately to medical treatment, she underwent a surgical procedure, and doctors discovered the obstruction.

Marina lived in the African Grasslands habitat and came to the Omaha zoo in 1999.

Omaha’s staff previously had collaborated with rhino specialists at the Cincinnati Zoo and Botanical Garden to inseminate Marina, but she didn’t become pregnant.

Currently, one male and one female live in the zoo.

White rhinos are listed as near threatened by the International Union for Conservation of Nature’s Red List.

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