Image caption Male lions are known to turn on rivals in the wild, but rarely females

Officials at a zoo in the US state of Texas are investigating why a male lion attacked and killed a lioness in full view of visitors.

The male seized five-year-old Johari by the neck on Sunday at the Dallas Zoo - she was later found to have died from neck wounds and haemorrhaging.

Zoo officials said the lions had lived together peacefully for years.

They said they had no idea why the male had turned on Johari, but they had no plans to have him put down.

Video taken by a zoo visitor showed the apparently calm male lion with its jaws clamped around the female's neck.

In my 35 years as a veterinarian in zoos, I've never seen this happen Lynn Kramer, Dallas Zoo

"Everyone thought they were playing at first but then they could see that she was struggling," witness Jim Harvey told local news station WFAA.

'Very rare occurrence'

Security personnel closed the exhibit to the public and the male was removed.

The remaining four lions appeared unaffected by the incident, said officials.

"Johari was a remarkable animal, as are all of our lions," said Lynn Kramer, vice-president of animal operations and welfare at the zoo, in a statement.

"This is a very rare and unfortunate occurrence. In my 35 years as a veterinarian in zoos, I've never seen this happen."

The statement said Johari, known as Jo-Jo, was a staff favourite who was "sweet and loving with her sisters, and often could be found grooming them".

Mr Kramer told WFAA that male lions do sometimes kill other males in the wild, and sometimes cubs, but that attacking a female was extremely rare.

The chances of such an attack happening again were "very remote", said Mr Kramer.

However, the zoo has said it will keep its two males lion apart from the surviving two females while they try to establish what happened, and would "absolutely not" put down the killer.