A Japanese zoo spent four years trying to mate two spotted hyenas - before realising they were both male.
Maruyama Zoo, in Sapporo, said they were told Kami and Kamutori were a "male and female couple" by the South Korean zoo that sold them the spotted hyenas in 2010.
According to AFP, the zoo released a statement saying: "We have attempted to house them together for breeding many times but they often fought against each other and never engaged themselves in breeding behaviour.
"We have determined that the two animals are both male after conducting ultrasound imaging and hormone tests on them under anaesthesia."
According to the Guardian, the zoo explained that is very difficult to determine the sex of a spotted hyena from the appearance of its external genitalia.
The animals underwent 'sex tests' after Kami, the supposedly female hyena, showed no estrus symptoms after reaching sexual maturity.
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