Hope for a new baby panda grew Monday as officials confirmed that a pair of giant pandas at Ueno Zoo in Tokyo had mated for the first time in four years.

They said female panda Shin Shin and male Ri Ri, both 11, mated after they were put in the same enclosure Monday morning. They normally live in separate pens.

Last week, the zoo removed the pair from public viewing after the female went into heat.

The breeding season for pandas is from February to May, and the window for conceiving is said to be just a few days.

In 2012, a cub was born to the pair but died six days later from pneumonia. In 2013, Shin Shin showed signs of being pregnant, but it was a false alarm.