WASHINGTON — Nani, the oldest bottlenose dolphin at the National Aquarium, in Baltimore, died Monday. She was 44 and had lived at the aquarium since 1990.

The aquarium said in a statement on Tuesday that she “began to exhibit unusual behavior” on Monday, and that despite the emergency efforts of the animal care team, she died Monday evening. They’re still trying to determine the cause of her death.

“We all loved Nani dearly,” said National Aquarium Chief Executive Officer John Racanelli. “She was very much a member of the National Aquarium family, and we are heartbroken to lose her.”

She was the mother of Beau and Spirit, two of the aquarium’s seven other bottlenose dolphins, the museum said; she was born in the wild and came to Baltimore in 1990 from another aquarium, which closed.

Nani means “beautiful” in Hawaiian, the museum said.