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.