A lioness has killed the father of her three offspring, suffocating her mate by locking her jaws onto his neck at the Indianapolis Zoo in an attack not fully understood by zoo staff.

The lions had been held together at the zoo for eight years, producing three cubs in 2015, and zookeepers had never before noticed any aggression between the two, the zoo said in a statement issued on Friday.

The death devastated zookeepers and the cause may never be understood, the curator of the zoo said.