Zookeepers at the Columbus Zoo and Aquarium arrived to a pair of unexpected discoveries Thursday morning: a newborn baby gorilla and the news that its mother wasn't a male gorilla. Related video above: Baby gorilla takes first wobbly stepsThe gorilla, Sully, has lived at the facility with her mother since 2019 and was thought to be male until "the gorilla care team discovered her holding the unexpected baby gorilla early Thursday," the zoo announced in a news release.But how could the facility not know 8-year-old Sully was actually a female? And that she was pregnant?Well, gorillas "don't have prominent sex organs" and males and females look mostly alike until around age 8, the zoo said in the release, noting it's only later in life that males develop their large size, silver backs and distinctive head bumps.Along with the hard-to-distinguish features, veterinarians at the zoo where the gorilla was born took a "hands-off approach" with their care and allowed the primate to be cared for by its mother, the Columbus Zoo noted. When Sully arrived in Columbus, she was a "young and healthy animal" and didn't require any medical procedures that would have led to the discovery sooner, the zoo said. The pregnancy was also missed because "gorillas rarely show outward signs" they are carrying because "newborns are smaller than human babies and gorillas naturally have large abdomens," the release notes. With the gestation period for gorillas being eight and a half months, the zoo estimates Sully became pregnant in the fall. The zoo says the adorable infant appears to be a healthy female. "The veterinary and animal care teams have not yet approached the infant, giving them time to bond with one another and with the rest of the troop, but will conduct a wellness exam soon," the facility said in the release. A DNA test will be performed later to determine the newborn's father.The new mother and baby will be on display for guests at the zoo's gorilla habitat starting Friday, according to the release.Western lowland gorillas – the subspecies that live at the Columbus Zoo – are critically endangered, according to the International Union for the Conservation of Nature. There are an estimated 100,000 left in the wild across central Africa, says the Columbus Zoo. Their population has been depleted due to habitat loss, deforestation and hunting for bushmeat.The surprise discovery builds on a history of gorilla conservation at the Columbus Zoo. The facility "was the first zoo in the world to welcome the birth of a baby gorilla" in 1956, according to the release.Sully's yet-to-be-named infant is the 34th gorilla born at the zoo, says the release. "She's an important part of our work to conserve these magnificent animals," the facility wrote.