A lioness killed her longtime mate at the Indianapolis Zoo last week, leaving officials puzzled as to what prompted the attack.
The female lion, Zuri, attacked and killed Nyack, the father of her three cubs, in an outdoor yard where they were kept last Monday, the zoo said.
Zookeepers heard roaring and found Zuri with her teeth clamped around Nyack's throat, according to The Associated Press. Officials tried to split the two up, but couldn't get Zuri to let go of Nyack in time.
A necropsy showed the lion died of suffocation due to injuries to his neck.
“It’s extremely devastating to the animal care team,” curator David Hagan said in a statement. “They work with the lions on a daily basis so not only are they very sad for this tragic loss, but I know our zoo guests, visitors and members will also be sad."
The pair had been kept together for eight years without incident, according to the zoo.
The zoo is investigating the incident.