Marine specialists at the National Aquarium in Baltimore and Johns Hopkins University veterinary pathologists are looking into the death this week of the aquarium's youngest dolphin.
According to aquarium officials, Bridgit -- a 4-month-old Atlantic bottlenose dolphin -- was the target of aggressive sexual behavior by two older male dolphins while other dolphins performed nearby in a late-morning show Wednesday.
Toward the end of the 11 a.m. presentation, the young dolphin showed signs of fatigue after the encounter with the two males, according to Sue Hunter, the aquarium's manager of animal programs. The young dolphin died later in the afternoon.
Tests performed shortly after the dolphin's death showed that she suffered from a serious bacterial infection that could have impaired her ability to tolerate the normal rough play, said Brent Whitaker, the aquarium's director of animal health.
"Dolphins tend to put themselves in a social hierarchy through aggressive behavior," Whitaker said. He said it is common for dolphins to establish social dominance through aggressive acts.
"Had [trainers] known [the dolphin was sick], they would have separated the baby," Whitaker said.
Whitaker said an initial post-mortem examination revealed no obvious evidence of trauma, such as broken bones and bruises.
But Naomi Rose, a marine mammal scientist at the Humane Society of the United States, said the aggressive encounter between the baby dolphin and the two males should not have occurred.
"It's not unusual for male dolphins to do what they did. They do it in the wild, that's why [mother dolphins] segregate themselves," Rose said. "In captivity, they don't have that option."
However, aquarium officials said allowing older and younger dolphins to mingle was appropriate.
"It's proven technique to socialize calves with the group," said Molly Foyle, an aquarium spokeswoman. If segregated too long, calves will eventually be "outcast" by the group, she said.
"We seek to balance their social order where dolphins are exposed to males and females," Foyle said.
As Bridgit's mother performed in the show, the two male dolphins pushed the calf against the pool's glass and submersed her under water, Hunter said. But dolphin bullying is common, she said, so trainers running the show did not intervene.
Rose said the 13-year-old mother should have been allowed to concentrate on protecting her calf from the males.
"A female with that young of a calf should not perform," she said. "She should have been a mother."
Aquarium officials said Bridgit's mother had given birth three years ago, had performed then and that calf thrived.
"She has been a successful mother ... and that [3-year-old] dolphin is a successful member of the dolphin community," Foyle said.
Bridgit's death is the second calf death this year. An unnamed male calf died of a respiratory infection in April.