Preliminary results from an animal autopsy indicated an ulcer in Taijah's stomach, but zoo officials said they were unsure about what caused it, the St. Paul Pioneer Press reported Wednesday.
Taijah is the fifth Atlantic Bottlenose dolphin to die since 2006 at the zoo in the Minneapolis suburb of Apple Valley.
"There has been a lot of crying," said Kevin Willis, the zoo's director of biological programs. "It's hard."
Taijah's death Monday reduced the zoo's dolphin pod to her parents, Semo, 48, and Allie, 24, who delivered a stillborn calf in 2009.
Willis said Taijah exhibited signs of severe illness early Monday. When she stopped eating in the afternoon, "We knew things were more serious than a behavior problem," Willis said. Veterinarians treated her for a possible ulcer, the Pioneer Press said.
Willis said Semo and Allie will be under constant monitoring until the cause of death is known.
"We have to keep watching to see if there are any signs," Willis said. "Not knowing makes us all very nervous."
The zoo's last live dolphin calf, Harley, died during a training exercise in 2006, the Pioneer Press said. Since then, some dolphins died because of disease or old age, and others were sent to other zoos participating in a consortium of facilities that breed the animals.