Giant panda Mao Er rests on a tree trunk at Copenhagen Zoo in Copenhagen, Denmark, Dec. 29, 2019. The Chinese giant panda couple Xing Er and Mao Er arrived at Copenhagen Zoo on April 4, 2019 under a 15-year Sino-Danish Panda International Research Cooperation Project. Photo:Xinhua

Giant panda Mao Er rests on a tree trunk at Copenhagen Zoo in Copenhagen, Denmark, Dec. 29, 2019. The Chinese giant panda couple Xing Er and Mao Er arrived at Copenhagen Zoo on April 4, 2019 under a 15-year Sino-Danish Panda International Research Cooperation Project. Photo:Xinhua

Tourists look at giant panda Xing Er from a panda-themed restaurant at Copenhagen Zoo in Copenhagen, Denmark, Dec. 29, 2019. The Chinese giant panda couple Xing Er and Mao Er arrived at Copenhagen Zoo on April 4, 2019 under a 15-year Sino-Danish Panda International Research Cooperation Project. Photo:Xinhua

Giant panda Xing Er rests on a tree trunk at Copenhagen Zoo in Copenhagen, Denmark, Dec. 29, 2019. The Chinese giant panda couple Xing Er and Mao Er arrived at Copenhagen Zoo on April 4, 2019 under a 15-year Sino-Danish Panda International Research Cooperation Project. Photo:Xinhua