A hippo is seen at the Schonbrunn Zoo in Vienna, Austria, July 23, 2020. Tourists gradually returned to the Schonbrunn Zoo, which was closed due to the COVID-19 pandemic and reopened in mid-May. (Xinhua/Guo Chen)

Tourists view giraffes at the Schonbrunn Zoo in Vienna, Austria, July 23, 2020. Tourists gradually returned to the Schonbrunn Zoo, which was closed due to the COVID-19 pandemic and reopened in mid-May. (Xinhua/Guo Chen)

Humboldt penguins are seen at the Schonbrunn Zoo in Vienna, Austria, July 23, 2020. Tourists gradually returned to the Schonbrunn Zoo, which was closed due to the COVID-19 pandemic and reopened in mid-May. (Xinhua/Guo Chen)

Tourists view flamingos at the Schonbrunn Zoo in Vienna, Austria, July 23, 2020. Tourists gradually returned to the Schonbrunn Zoo, which was closed due to the COVID-19 pandemic and reopened in mid-May. (Xinhua/Guo Chen)

Tourists view a giraffe at the Schonbrunn Zoo in Vienna, Austria, July 23, 2020. Tourists gradually returned to the Schonbrunn Zoo, which was closed due to the COVID-19 pandemic and reopened in mid-May. (Xinhua/Guo Chen)