A zoo in Japan has abandoned their hyena breeding programme after realising they have spent four years trying to get two males to mate.

Following years of fruitless attempts, the Maruyama Zoo discovered this week that they were not going to get baby spotted hyenas from the “couple” they received in October 2010, AFP reported.

It is only after conducting several tests that the zoo realized its mistake. They say it's difficult to tell the sex of a spotted hyena by external genitalia.

“We have determined that the two animals are both male after conducting ultrasound imaging and hormone tests on them under anesthesia,” the agency quoted the zoo’s statement as saying.