In such a vast area, it was an encounter of extraordinary good fortune. “Cinderella’s prince showed up and they lived happily ever after,” said Dale Miquelle, lead tiger scientist for the Wildlife Conservation Society and an author of the study. Zolushka and the male also produced a litter of cubs, the first known cubs to be born in that area since the 1970s.

In all, Russian scientists raised 13 orphaned Siberian tiger cubs in captivity, avoiding any contact between the growing cubs and their human caretakers to prepare them for life in the wild. The team gradually introduced the cubs to live prey so they could learn how to hunt.

Also critical to the success was the timing of the cubs’ release: during spring when prey was plentiful.

One male cub failed the test of freedom. He wandered into China and preyed on domestic animals, including 13 goats in one shed in a single night. Russian scientists recaptured the young male and sent him to a captive-breeding program at a zoo.

But the remaining 12 proved they were able to hunt wild prey and to survive as well as wild tigers that had never spent time in captivity.

As the Pri-Amur population grows, the Russian-American team hopes it can join up with other tigers, including across the border in China. “The grand vision is that this whole area would be connected,” said Luke Hunter, executive director of the Big Cats Program at the Wildlife Conservation Society. “There’s lots of habitat that could be recolonised by tigers.”

With so much potential habitat across Asia – a 2023 study found there was about 700,000 square kilometres of potentially suitable habitat across Asia where tigers remained absent – the implications of this success are wide-ranging.

“These results indicate that it is possible to care for young cubs in a semi-captive environment, teach them how to hunt and to release them back into the wild,” said Viatcheslav Rozhnov, former director of the Severtsov Institute of Ecology and Evolution of the Russian Academy of Sciences and leader of the reintroduction project.

“These findings provide a pathway for returning tigers to large parts of Asia where habitat still exists but where tigers have been lost.”

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And just as Boris and Svetlaya’s unlikely partnership has proved critical to the project’s success, the Russian and American scientists hope their efforts may be a model for international conservation co-operation.

“It’s a testimony to how really good things can happen when you start working collaboratively irrespective of nationality and politics,” Miquelle said.