One African lion killed another one at the Indianapolis Zoo in an attack not fully understood by staff.

In a statement, the zoo said staff were alerted to an unusual amount roaring from the lion pen Monday last week.

They arrived to find a 12-year-old lioness named Zuri in a fight with a 10-year-old male named Nyack.

The zoo said Zuri held Nyack by the neck until he stopped moving.

The two animals had been held together at the zoo for eight years, producing three cubs.

Zookeepers never noticed any aggression between the two.

The zoo curator calling the death 'extremely devastating' to the staff.