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Staff at an Ohio zoo received a surprise delivery when a gorilla they believed for four years to be a male gave birth.

Columbus Zoo said that Sully, who has been a resident since 2019, was discovered with her baby on Thursday.

Zookeepers said in a blog post that the infant "appears to be healthy" and that Sully was being a excellent mother.

They added that Sully had always been in good health so had never needed examining, leading to the mix-up.

The birth "was unexpected, yet exciting for the care team and important for the conservation of a critically endangered species", the keepers said in a blog post.

Zookeepers suspected nothing until they saw the "male" nursing her child. "It's hard to tell the sex of younger gorillas," they said.

At eight years Sully was old enough to reproduce but still too young to be showing obvious signs of her sex, they added.

"Until about eight, males and females are about the same size, and they don't have prominent sex organs. Males don't develop... their silver backs until 12 or later."

Gorillas do not typically show signs of pregnancy either due to their large abdomens, and infant gorillas are significantly smaller than human babies.

"We're thrilled by the addition of another birth for this critically endangered species," the zoo said.

"As the 34th gorilla born here since 1956 when the Columbus Zoo became the first zoo in the world to welcome the birth of a baby gorilla, she's an important part of our work to conserve these magnificent animals."

The zoo said it would conduct DNA tests to determine the father. Sully lives in a troop led by silverback Mac, 39, that also includes two younger males Kamoli, 10, and JJ, six.

And despite the hiccup with Sully, Columbus said it was confident of the sex of the new arrival.

"It's a girl! Our team confirmed that visually and with photographs that were also sent to a primate expert at another leading zoological facility," the zoo said.