Tokyo's Ueno Zoo has twice as much to celebrate today: early Wednesday morning, giant panda Shin Shin gave birth to twin cubs.

According to Reuters, the babies – whose sexes have not yet been determined – arrived at 1:03 a.m. and 2:32 a.m. Barely the length of a stick of butter, one cub weighed in at 4.37 oz., while the other's weight was not released.

"All the staff are working together to observe and protect the giant panda mother and children," zoo staff wrote in a statement.

Per BBC, one of the cubs is in an incubator, as giant panda mothers don't often care for two newborns at once. The babies will rotate between time with Shin Shin and time in the incubator so she can breastfeed both independently.

Shin Shin, 15, and the twins' father Ri Ri came to the Ueno Zoo from China in 2011, following the death of the zoo's giant panda Ling Ling in 2008. The two are also parents to female Xiang Xiang, who was born in 2017.

According to its website, Ueno Zoo works with the Beijing Zoo and San Diego Zoo on the breeding of wild giant pandas; successful breeding among the animals in captivity is extremely rare. Additionally, the World Wildlife Fund reports just about 1.800 giant pandas are left in the wild, making the species "vulnerable" by the International Union for Conservation of Nature's ratings.

So the birth of the twins, who for now remain nameless and out of sight from visitors, was particularly joyous, according to Ueno Zoo director Yutaka Fukuda.

As he told reporters, per BBC, "When I heard the news that the second baby was born, I couldn't help but whoop."

This story originally appeared on People.com.

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