Staffers at the Philadelphia zoo are mourning the death of a newborn orangutan who passed away suddenly just hours after it was born.

The Sumatran orangutan was born Sunday afternoon to 25-year-old Tua and her mate, 22-year-old father Sugi. The infant appeared to be behaving normally throughout the day before dying overnight Sunday into Monday, the zoo said.

“Zoo staff and volunteers are saddened by this unexpected loss,” said Dr. Andy Baker, the zoo's Chief Operating Officer. “With Tua’s parenting skills and the apparently successful delivery, we were optimistic about this birth.”

The Sumatran orangutan is critically endangered. There are less than 15,000 of them living in the wild, according to the World Wildlife Fund, and about 85 Sumatran orangutans currently living in U.S. zoos accredited by the Association of Zoos and Aquariums.

“In an initial postmortem examination we identified severe congestion in the newborn’s lungs and a possible congenital heart defect,” Dr. Keith Hinshaw, the zoo's director of animal health, said. “We will likely be able to confirm the exact cause of death after the full necropsy results are available, which will take approximately four to six weeks.”

Tua and Sugi are parents to an 8-year-old daughter, Batu, who was born in 2009 and lives with her parents in the zoo’s PECO Primate Reserve.

“The keepers who work with the orangutans every day are affected deeply themselves, but will also be watching Tua closely for how she reacts to this loss,” Baker said. “Animals are individuals, and we don’t know how this will affect her.”

“Although this is a sad loss, it is also a reminder that we are all responsible for the fate of orangutans in the wild on Sumatra and Borneo,” Baker said.