A Polish zoo is celebrating the arrival of four Sumatran tiger cubs, announcing their birth on Friday after keeping it a closely guarded secret for weeks because of fears they may not survive.

Officials at the Wroclaw Zoo in the southwestern Polish city of Wroclaw said the tigers — a critically endangered species whose numbers have dwindled to around 400 in the wild — were born on July 22.

But the zoo chose not to go public with the news until they could grow a bit, gain some strength and be vaccinated, zoo spokesperson Weronika Lysek said.

Each cub is now more than double the size of a domestic cat, at around eight kilogrammes (about 18 pounds) each.

“The joy is even greater that all four tigers are developing healthily, are active, eager to eat and play with each other and with their parents,” said Sergiusz Kmiecik, acting president of the zoo, which has been breeding Sumatran tigers since the 1960s.

The four cubs are the offspring of mother Nuri and father Tengah, who four years earlier produced a female, Surya. She currently lives in a zoo in Cottbus, Germany.

The zoo proudly hailed the births as “a breeding success on a global scale”.

Pawel Sroka, the head of the zoo’s department of predatory mammals, noted that Tengah had stayed with the cubs from the beginning, which is rare for tigers.

“He was calm, gentle, and even helped Nuri with care, learning how to ‘handle’ the cubs,” he said.

The zoo is also celebrating the fact that the litter included three males and one female. Females predominate in European zoos, so the birth of three males can help balance their population in zoos.

It is not yet clear where the four cubs will end up going, and they still have growing to do before that is decided, Lysek said. They also have not been named yet, and the zoo is considering letting the zoo’s visitors and fans take part in their naming.

Sumatran tigers – the most critically endangered tiger subspecies – are under increasing pressure because of poaching and a shrinking jungle habitat.

There are only about 400 of them left in the wild, according to the Wroclaw Zoo, which also has a foundation that works to support their protection.