A lioness has killed the father of her three cubs by locking her jaws onto his neck and suffocating him.
Indianapolis Zoo said that lioness Zuri, 12, attacked her mate Nyack, 10, in their outdoor enclosure before the zoo opened last Monday.
Zoo staff were alerted to the incident on hearing "an unusual amount of roaring" coming from the lion's shared pen.
There they found Zuri holding the father of her cubs by the neck until he stopped moving.
Post-mortem tests confirmed Nyack died of suffocation as a result of his neck injuries.
The zoo said past daily logs kept by staff showed no unusual aggression, injuries or wounds between the pair.
In a statement on Facebook, Indianapolis Zoo said they would be conducting a "thorough review to attempt to understand what may have led to this".
The two had been housed together for eight years and produced three cubs in 2015, none of which were involved in the fight.
Nyack, who is described on the zoo's website as a " very vocal and laid back" lion, was on loan from the San Diego Zoo as part of a conservation effort managed by Association of Zoos and Aquariums.
“She had Nyack by the neck," David Hagan, curator of Indianapolis Zoo, said of Zuri.
"Keepers came up and saw what was occurring and made an attempt to separate the two. But she continued to hang onto Nyack by the neck until he stopped moving.”
Although the incident is under review, the zoo has said there are no plans to change how the lions would be managed.