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A zoo in Japan has confirmed that a hippopotamus thought for seven years to be a he is, in fact, a she.

A DNA test was carried out after zookeepers noticed that 12-year-old Gen-chan did not display typical male hippo behaviour.

Gen-chan arrived in Osaka from Mexico in 2017, and customs documents at the time said she was male.

"We will keep doing our best to provide comfortable environment to Gen-chan," the zoo said.

The Osaka Tennoji Zoo confirmed the news about Gen-chan's actual sex in a post published on their website last week.

The post said Gen-chan first arrived at their zoo from the Africam Safari animal park in Mexico when she was five years old, and was declared as a male.

The statement said that because Gen-chan was still a calf at the time, they did not question the documents.

But zookeepers got suspicious as Gen-chan got older and they could not visually identify male reproductive organs.

A spokeswoman for the Osaka Tennoji Zoo told AFP on Tuesday that typical male hippo behaviour that Gen-chan was not displaying included making courtship calls to female hippos, or scattering faeces around while defecating with a propeller-like tail motion in order to mark territory.

According to the Mainichi newspaper, the zoo's vice director Kiyoshi Yasufuku said: "We recognise the importance of confirming the sex, and we want to ensure that such mistakes will not happen again."

The zoo confirmed that Gen-chan will not be getting a name change.