What was supposed to be an ordinary day for staff at a zoo in the United States turned out to be anything but when they discovered a gorilla they thought to be male had given birth to a baby.

The surprise development took place at the Columbus Zoo and Aquarium in the US midwestern state of Ohio on July 20.

Sully, an eight-year-old western lowland gorilla, was believed to be a male, until her keepers went to her enclosure and found her cradling a baby gorilla.

Explaining in a blog post why they did not know Sully, who has been at the zoo since 2019, was female and pregnant, the zoo said it is hard to tell the sex of younger gorillas. Gorillas do not have prominent sex organs, and males and females are about the same size and look until around the age of eight.

“Males don’t develop their characteristic large size, silver backs, and large head bumps (called sagittal crests) until age 12 or later,” the zoo said.

Veterinarians at the facility where Sully was born had maintained a hands-off approach in their care, as she was a healthy baby whose mother cared for her well.

When Sully arrived at the Columbus zoo, she was young and healthy, and did not need any procedures requiring immobilisation that would have led to this discovery sooner, the zoo added.

The pregnancy was also missed because gorillas, which naturally have large abdomens, rarely show any outward signs of having a baby.

Sully lives with a troop of gorillas that includes three males and five females. The zoo estimated that Sully got pregnant last autumn, and a DNA test will be performed later to determine the newborn’s father.

For now, mother and daughter are fine, and “first-time mum Sully is taking good care of her”, the zoo said, adding that visitors will be able to view the two at the gorilla habitat from July 28.

The yet-to-be-named baby is the 34th gorilla born in Columbus zoo since 1956, part of the zoo’s effort to conserve the species.

The western lowland gorillas are categorised as critically endangered, with habitat loss, deforestation and hunters targeting them for bushmeat, reducing their numbers to about 100,000 in the wild in Central Africa.