A baby dolphin was born over the weekend at a Chicago zoo. The calf appeared healthy up until its abrupt death Wednesday evening.

The male calf was just four days old. He was born Saturday to 37-year-old Allie, an experienced mom of five.

The male calf had appeared healthy and was meeting milestones up until its abrupt death, Brookfield Zoo said in a news release. A necropsy is pending to determine the cause of death.

“The first few days after birth are a vulnerable, critical timeframe for dolphin calves,” said Dr. Rita Stacey, Brookfield Zoo Chicago senior vice president of programs and impact. “While the calf was progressing well, dolphin neonates face immense biological challenges. We are deeply saddened by his sudden passing.”

A dolphin calf’s first year of life, both in the wild and under professional care, is filled with challenges and milestones it must reach. Newborn dolphins must quickly master a range of complex behaviors, from nursing underwater to navigating the aquatic environment and learning to rest while pulled along in the mother’s wake. Even slight developmental challenges can become critical in a short time. In the wild, one in five calves born to first-time mothers do not survive their first year.

This calf was meeting all milestones, including healthy levels of nursing and other activities, and Allie was providing appropriate maternal care, Stacey said. The pair was under 24-hour observation.

Wednesday evening, staff members noticed a rapid, unexpected change in the calf’s behavior. Within minutes, he was unresponsive and died.

Animal care and veterinary staff are monitoring Allie and the dolphin group closely following the death.

“We know our guests and supporters share this sadness with us,” Stacey said. “This calf’s birth brought hope and joy, and his sudden loss reminds us of the delicate balance of life.”