A lion killed a lioness in a zoo enclosure after spending years living peacefully together in captivity - an event witnessed by shocked visitors to the zoo.
The incident was captured on video by a visitor at Dallas Zoo in the United States.
A female named Johari died when a lion sank its teeth in to her neck and squeezed until she died, in the enclosure they shared together.
Wildlife experts said the chance of such an attack taking place were "vanishingly small."
The zoo said in a statement that Johari had died very quickly and that the incident left staff "heart broken." Tests revealed puncture wounds to her neck, haemorrhaging under her skin. She died of suffocation, the statement said.
Eye witness Jim Harvey said: "Everyone thought they were playing at first but then they could see that she was struggling."
The zoo's vice president of operations Lynn Kramer said he had never known anything like it. "This is a very rare and unfortunate occurrence. In my 35 years as a veterinarian in zoos, I've never seen this happen."
An investigation has been launched in to a possible motive for the attack by the lion - which shared the enclosure with three females and another male. Both animals involved in the incident had been bred in captivity and had never been in the wild.
Kramer told wfaa.com: "There are male coalitions in the wild of two to five males, and sometimes they come in and displace a male that's been the head of a pride. At that time, it's not unusual for them to kill the cubs and then mate with the lionesses, but it extremely rare that they would attack or kill the females."
"I think this is a very isolated incident and the chances of it occurring again would be very remote," said Kramer.