An Ohio zoo staff was shocked when their "male" gorilla gave birth

Zookeepers at an Ohio zoo were dumbfounded when one of their gorillas gave birth, because — somehow — for the last four years they thought the mama gorilla was a male.

How the mix-up could have happened is beyond head-scratching, but the staff at Columbus Zoo says that at only 8 years old, Sully wasn't developed enough to show outward signs of her sex. "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."

Zookeepers also blamed their confusion on Sully's health, which has always been good, giving them no reason for a close-up examination.

In fact, it wasn't until they spotted the new mother nursing that they even realized the infant was hers.

From BBC:

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.

"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. 

To prevent another silly – and perhaps embarrassing – surprise, the zoo immediately examined the newborn gorilla, determining with confidence this time, "It's a girl!"