By John McDevitt

PHILADELPHIA (CBS) — It is a sad day, as baby Sumatran orangutan born at the Philadelphia Zoo over the weekend has died.

The zoo announced Tuesday that the baby orangutan behaved normally through the afternoon and early evening after it was born on Sunday, but died overnight.

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“In an initial postmortem examination we identified severe congestion in the newborn’s lungs and a possible congenital heart defect,” said Dr. Keith Hinshaw, the zoo’s director of animal health. “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.”

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The baby was born to 25-year-old Tua around 2:45 p.m. on July 9. The zoo says Tua delivered the baby without any need for intervention from staff or the team of consultants who were ready to assist if need be.

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“Zoo staff and volunteers are saddened by this unexpected loss. With Tua’s parenting skills and the apparently successful delivery, we were optimistic about this birth,” said Dr. Andy Baker, the zoo’s chief operation officer. “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. Animals are individuals, and we don’t know how this will affect her.”

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Tua and the infant’s father, 22-year old Sugi, have an 8-year-old daughter who still lives with them in the zoo’s Primate Reserve.

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Sumatran orangutans are critically endangered animals, with only 15,000 left in the world.