The pregnant rhino cow Marcita from the Thuringian Zoopark in Erfurt died of severe pneumonia.
That was the result of the pathological examination of the animal in Berlin, said zoo director Sabine Merz on Monday in Erfurt.
The examination of the little rhino bull Tayo, who suddenly died a few days before his mother, has so far not provided any information about the cause of death.
Both white rhinos, which are critically endangered in the wild, had shown no signs of serious illness.
"We continue to grope in the dark as to what the causes of this could be," said Merz.
The investigations would continue.
The cub and its mother, who was pregnant again, were found dead in their boxes within two days at the beginning of February.
Their cadavers were pathologically examined to clarify the cause.
Marcita was pregnant with a girl.
According to Merz, a Covid 19 infection has already been ruled out for Tayo.
Marcita's pneumonia and peritonitis were probably triggered by a pathogen that spread through her body at great speed.
A blood test the day before her death showed only low levels of inflammation.
There is a "tendency towards a viral infection," said the zoo director.
But poisoning has not yet been completely ruled out.