A lioness has killed the father of her three offspring by suffocating her mate at an Indianapolis Zoo, officials have said.
The lions had been held together at the zoo for eight years, with zookeepers having never noticed any aggression before between the two.
In an attack not fully understood by zoo staff, the lioness locked her jaws on to her mate's neck until he lost consciousness and died.
The death devastated zookeepers and the cause may never be understood, a zoo official said.
David Hagan, curator of the Indianapolis Zoo, said: "They build strong bonds with the animals so any loss affects us all greatly. For a lot of us, it's just like a family member."
Staff were alerted to the incident after "an unusual amount of roaring" coming from the lion pen.
They arrived to find the 12-year-old female lion Zuri in a fight with the 10-year-old male lion Nyack, the zoo said in the statement.
"She had Nyack by the neck. Keepers came up and saw what was occurring and made an attempt to separate the two.
"But she continued to hang on to Nyack by the neck until he stopped moving," Mr Hagan said.
A necropsy found that Nyack died of suffocation from injuries to the neck.
The incident was under review but there were no plans to change how the lions would be managed, the zoo said.
The lions produced three cubs in 2015, with one of their offspring, a female named Sukari, being in the outdoor lion pen at the time.
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The two male offspring were kept in a separate part of the exhibit.
The zoo's Facebook page posted a message saying that Nyack "was a magnificent lion and he will be greatly missed".