TOKYO , Japan ( CNN ) -- Puzzled zookeepers in northern Japan have discovered the reason why their attempts to mate two polar bears kept failing : Both are female . A 4-year-old polar bear sent to impregnate a female polar bear at a zoo in Kushiro was found to be female as well . The municipal zoo in the city of Kushiro in Hokkaido brought in a polar bear cub three years ago . They named it Tsuyoshi , after the popular baseball outfielder Tsuyoshi Shinjo , and waited until it reached reproductive age . In June , the zoo introduced Tsuyoshi to its resident bear , an 11-year-old female named Kurumi , and waited for sparks to fly . But much to the disappointment of zookeepers , Tsuyoshi never made any amorous advances toward Kurumi . Earlier this month , zookeepers put Tsuyoshi under anesthesia to get to the bottom of the matter . That 's when they made their discovery : Tsuyoshi is a female . Still , the Kushiro zoo plans to keep Tsuyoshi because he -- or rather , she -- has become immensely popular with visitors . `` I have rather mixed feelings , given the need for breeding , but Tsuyoshi is an idol for Kushiro , '' Yoshio Yamaguchi , head of the Kushiro zoo , told Japan 's Kyodo news agency . Tsuyoshi will even keep her name . `` We will not be changing it to 'Tsuyoko ' since it is loved by citizens ( by the current name ) , '' Yamaguchi said . `` Ko '' is a common suffix for a Japanese female name . Meanwhile , Tsuyoshi 's `` brother , '' who was adopted by another zoo , has also turned out to be female , Kyodo reported . -- CNN 's Chie Kobayashi contributed to this report .
Zoo introduced polar bear cub to its resident bear and waited for sparks to fly . And it waited and waited : Tsuyoshi never made amorous advances toward Kurumi . Months later , the zoo makes key discovery : both bears are female .