She was outgoing, a quick learner and a "protective mother, the matriarch of the group." And for 26 years, she captivated children of all ages like few others at the Minnesota Zoo.

Rio, the zoo's oldest female dolphin, died Monday night of causes yet to be determined. She was 35.

"Everybody's taking this rather hard," said Jim Rasmussen, the zoo's senior veterinarian. "We're really mourning this one. This was rather unexpected."

Bottlenose dolphins often live 30 to 50 years, and the nearly 9-foot-long, 450-pound Rio seemed in fine health.

But Rio, who could be temperamental, stopped interacting with the zoo's four other dolphins a few days ago, Rasmussen said. When she stopped eating Monday, an exam was scheduled. By 9 p.m. Monday, Rio had taken a turn for the worse.

The zoo in Apple Valley has continued to run tests to determine the cause of her death.

Rio is the second dolphin the zoo has lost this year. One of her calves, Harley, died in January in a freak accident. The 5½-foot-long, 120-pound male had been learning to swim between two black pools with his mother, when he jumped out of the water and hit his head, fracturing his skull, according to Kevin Willis, director of biological programs at the zoo. Harley was seven months old.

"She has been a protective mother," Willis said of Rio after Harley's death.

Rio's three other calves are Shadow, a male born in 1992 and now at the National Aquarium in Baltimore; another male, D.J., born in 1996 and now at the Brookfield Zoo in Chicago, and Spree, a female born in August 2002 and still at the Minnesota Zoo.

Other dolphins at the zoo's exhibit are Semo, a 42-year-old male; Chinook, a 23-year-old male, and Ayla, a 13-year-old female.

Rio arrived at the Minnesota Zoo in 1980 from the New York Aquarium.

She had digestive problems 20 years ago, because of a diet that was too rich for her liking. She apparently dined on pennies tossed into her tank by visitors. The mild case of copper poisoning was not life threatening, but enough to force zoo officials to temporarily take her off display.

Paul Levy • 612-673-4419