The Indianapolis Zoo announced Friday that its male lion was killed earlier this week by a lioness in their outdoor yard.

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Nyack, a 10-year-old male African lion, died Monday when he was attacked by Zuri, an adult female, before the zoo opened for the day.

Nyack and Zuri had been housed together for eight years and produced a litter of three cubs in 2015. Staff said there were no prior instances of unusual aggression, WTTV reported.

Staff said they made every effort to separate the lions, but Zuri held Nyack by the neck until he stopped moving.

A necropsy, or animal version of an autopsy, showed Nyack’s cause of death was suffocation, the station reported.