But on this June day, with Tua nearing her due date, the team grew worried. On the ultrasound, they couldn’t see any movement from the baby.

“It was scary,” said Rachel Metz, the Philadelphia Zoo’s vice president of animal well-being and conservation. Uncertain yet keeping hope, the staff “just kept on … taking the best care of Tua that we possibly could.”

A week later, Tua, 31, surprised her keepers. When they arrived at the zoo in the morning, they found she had given birth overnight. A baby was clinging to her, nursing and cooing, protected by its mother’s arms.

The healthy baby, whose June 26 birth was announced by the zoo last week, adds another member to a species with fewer than 14,000 left in the wild. Keepers have not yet been able to determine its sex, so the infant hasn’t been named — but zoo guests are already getting glimpses of mother and baby as they bond inside their enclosure.

“We’re just thrilled,” Metz told The Washington Post. “Once we realized it was in fact a healthy baby, it was just pride and adoration and love and respect, all kind of simultaneously, for this little infant that looks so remarkably like a human.”

Intelligent, reddish-hued apes, Sumatran orangutans are the world’s largest tree-dwellers and share 97 percent of their DNA with humans. But the primates, threatened by habitat loss, are listed as critically endangered by the International Union for Conservation of Nature (IUCN).

On Sumatra, the Indonesian island where they’re found, the forest has been cut and burned to make way for agricultural plantations, logging, mining and roads, according to IUCN. The Philadelphia Zoo is among 24 in the United States that work to conserve Sumatran orangutans with the Association of Zoos and Aquariums (AZA).

Tua’s pregnancy was part of that species-survival program, which aims to keep a genetically healthy population of orangutans in U.S. zoos. “Essentially really fancy matchmaking,” Metz said, the program creates an “insurance population” that can help repopulate species in the wild if necessary.

“This birth moves orangutan conservation forward, and creates opportunity to engage and inspire us all to do better for this species,” said Dan Ashe, president of AZA.

The baby has “beautiful red hair” that sticks up in an unruly manner, and zoo staff can hear it making noises and suckling. It has big, thoughtful eyes that are “questioning pretty much everything that’s going on around them,” Metz said.

The bond between mother and child was quickly evident to Tua’s keepers.

“It’s clear how much Tua already loves that baby,” Metz said. “She stares at it and she will not let it go, and she’s just constantly being a protective mom.”

The baby’s father is a 28-year-old male named Sugi. This was the first healthy orangutan birth for the Philadelphia Zoo since Tua delivered a girl, Batu, in 2009.

Orangutans reproduce slowly, with females giving birth once every eight or nine years. Babies can nurse for up to eight years, Metz said, and stay with their mothers through adolescence. Females can live into their 50s and 60s, and may have four or five births in their lifetime. The time orangutans take between births is the longest of any land mammal, according to the Sumatran Orangutan Conservation Programme.

Batu stayed with her parents through 2021, then moved to the Woodland Park Zoo in Seattle, where she is expecting a baby in September.

Because of Tua’s past reproductive challenges and other health issues, including a chronic respiratory disease syndrome found in orangutans, zoo veterinarians and staff were extra careful in readying her for a pregnancy.

They trained her for the voluntary ultrasounds and monitored her diet. In partnering with Penn Medicine, the team treated Tua like an at-risk pregnant human during her eight-month gestation, Metz said.

The orangutan has a spirited personality, often seeming to be one step ahead of the humans, Metz said. In the primate reserve hangs a sign: “Number of days since Tua’s last shenanigans.” It usually reads zero.

“This successful outcome is a testament to the animal care, nutrition and veterinary teams that have worked with Tua to greatly improve her health over the last eight years,” said Donna Ialeggio, the zoo’s director of animal health, in a statement.

The zoo was the first in the United States to successfully breed orangutans, beginning in 1928. Nineteen infants have been born there since, the zoo said.

The new baby will be named after its sex is determined, and the zoo will hold a celebration in mid-August. Though fathers don’t typically live with their offspring in the wild, some orangutans live with their mates in zoos, including Tua and Sugi, the zoo said. If they choose to be together, Sugi will be allowed to live with Tua and the baby after the mother-child bonding period ends.

Because the species reproduces slowly, it is difficult for them to recover from population loss. The IUCN lists the population as decreasing, with about 13,800 animals alive in 2016. If orangutans went extinct, several tree species would also disappear, according to the World Wildlife Fund, because the primates play a key role in spreading seeds.

They are the only great ape that lives mainly in trees, spending about 80 percent of their time in the forest canopy, according to the Orangutan Conservancy. They are also the most solitary of the great apes, though they can be social when in groups.

They make nests in the tree canopy to sleep in each night, traveling as they seek food and moving on once they’ve depleted a food source. They mainly eat fruit, along with leaves, bark and insects. They communicate with their own “languages,” Metz said, and use tools.

The animal’s similarities with humans make it easy to identify with, she added.

“All you need to do is look into their eyes,” Metz said, “and you can see their intelligence.”