(Newser) – Hyena sexing by sight alone is "very difficult," says a Japanese zoo that spent years trying to mate a pair of males. The Maruyama Zoo in Sapporo says Kami and Kamutori were obtained in a 2010 exchange with a South Korean zoo that said they were a male and female pair, reports ITV. They put the pair in the same cage for breeding in 2012 but didn't discover they were both males until experts conducted hormone checks while they were under anesthesia.

The two animals never really got along and "remained confrontational, leaving bite marks on each other," a zookeeper tells the Wall Street Journal. The zoo, which explains that in hyenas, "the external genitalia of the male and female are similar," now hopes to find at least one genuine female for its breeding program. (Read more zoo stories.)

