ROYAL OAK, Mich. (AP) — A male polar bear killed a female polar bear at the Detroit Zoo when he attempted to mate, officials said.

“This was completely unexpected and the Detroit Zoo staff is devastated by the loss of Anana in this sudden and tragic event,” said Scott Carter, the zoo’s chief life sciences officer.

Nuka killed 20-year-old Anana on Monday after being apart for several months.

“When bears and other large carnivores breed it is sometimes described in nature as an aggressive encounter,” Carter told WDIV-TV. “It usually involves physical control of the female by the male, which usually includes him holding her by the back of the neck.”

Nuka, who is 16 years old, has lived at the zoo since 2011 and bred with other bears without harmful behavior, Carter said.

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

“They lived together fine up until” Monday, Carter said.