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DETROIT – A female polar bear has died after officials at the Detroit Zoo said she was breeding with a male polar bear that also lived at the zoo.

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Anana, who was 20 years old, had lived with Nuka “without incident” last year, officials at the Detroit Zoo said.

The pair had been reintroduced to each other last week as part of a program to help sustain the endangered species. They had been apart from one another for several months, officials said in a news release. They were reunited after two polar bear cubs, both fathered by Nuka but whose mother was another female polar bear at the zoo, were recently born. He had bred with several other female polar bears without showing dangerous behavior.

There are 55 polar bears in 25 zoos and aquariums that have been accredited by the Association of Zoos and Aquariums.

Officials didn’t say exactly what happened between Anana and Nuka.

The last time an animal killed another animal at the Detroit Zoo happened in 1988. The incident also involved polar bears, officials said.