ROYAL OAK, Mich. (AP) — Officials say a male polar bear killed a 20-year-old female polar bear at the Detroit Zoo when he attempted to mate.

The zoo says 16-year-old Nuka killed Anana on Monday after being apart for several months.

Scott Carter, the zoo’s life sciences officer, says the death was “completely unexpected.”

The zoo says Nuka has lived at the zoo since 2011 and bred with other female bears without harmful behavior.

Anana arrived at the zoo in suburban Detroit in January 2020 and was introduced to Nuka a few months later.

The zoo says the mating program “is vital to sustaining this endangered species.”