A 4-day-old bottlenose dolphin born at Brookfield Zoo Chicago last Saturday died suddenly late Wednesday. The male calf had been the first dolphin born at the west suburban zoo in more than a decade.

“This is a devastating loss for our zoo community, especially the animal care and veterinary teams who dedicate their lives to the well-being of animals in our care,” said Rita Stacey, Brookfield Zoo Chicago senior vice president of programs and impact in an announcement on Thursday.

The zoo said it was performing an animal autopsy to determine the cause of death. While the calf was under 24-hour monitoring from animal care and veterinarian teams, it exhibited a “rapid, unexpected” change in behavior Wednesday evening, becoming unresponsive and dying within minutes.

Prior to its death, the calf had been reaching developmental milestones like “slipstreaming,” when a dolphin gets pulled along by its mother to not have to swim as hard.

In the wild, according to the zoo, one in five calves born to first-time mothers don’t survive their first year.

At birth, the calf weighed between 33 to 37 pounds and measured 115 to 120 centimeters long. Its mother, Allie, is 38 years old with four other children. Her newest calf had been expected to be named later this summer.

Animal care and veterinary staff are monitoring Allie and the dolphin group in the zoo’s Seven Seas dolphin habitat.