Between 450 and 500 Siberian tigers remain in the wild
A full veterinary examination is to be carried out on the body of a Siberian tiger found drowned near a natural pool in her enclosure at a Norfolk zoo.
A post-mortem examination carried out by Banham Zoo, where the animal lived found the cause of death as drowning.
Director Martin Goymour said the reasons for the drowning were still "unexplained".
Further tests are planned to see if Malyshka, an Amur or Siberian tiger aged five, had some kind of seizure.
"She was very important to the European breeding programme and was number three in the hierarchy of genetic diversity for captive tigers," Mr Goymour said.
Malyshka had already given birth to two cubs and was pregnant with more when she died last weekend.
She also liked playing in the pool where she was found last Sunday morning so was well used to water, the zoo said.
Mr Goymour said the zoo needed to get to the bottom of why the tiger died and had already counted out her male partner, who shared the large enclosure, as the cause of death.
"He is a gentleman in the tiger world and would often be seen playing with and caring for the two youngsters, so he is not implicated.
"We are also in contact with European stud book co-ordinators to find a suitable replacement female for our conservation programme involving either the senior male or one of the youngsters.
"Zoos do not own the animals in these programmes as they are pooled for conservation programmes.
"Very few zoos in Europe are able to handle tigers, which is why the loss of Malyshka is such a severe blow not only to her keepers and the 80 staff at Banham but to European breeding programmes.
"Keeping livestock means facing up to animal old age, injuries and unexplained tragedies.
"While the loss is very sad we must investigate to find reasons for her death," Mr Goymour said.