Image copyright Reuters Image caption Lioness Zuri had lived with Nyack for eight years and the zoo said the two had never shown aggression before

A lioness at a US zoo has killed the father of her three cubs in their pen - an incident experts say is shocking and unprecedented.

The pair had lived in the same enclosure at Indianapolis Zoo for eight years.

According to the zoo, there had never been any unusual aggression between the pair before the attack last week.

The BBC asked lion researchers for their theories on what could have sparked the attack.

What happened?

Zuri, 12, attacked Nyack, 10, and staff could not separate the pair. Nyack died of suffocation, while Zuri was uninjured.

The zoo said it is conducting a "thorough review".

A personality clash?

Prof Craig Packer, director of the University of Minnesota's Lion Research Center, told the BBC this sort of attack is "unprecedented".

"We've seen examples of males killing females, and groups of females chasing away males, but a single female killing a male? Never heard of it."

He suspects the individual lions' personalities played a role in the killing.

Image copyright Reuters Image caption The zoo called 10-year-old Nyack a "magnificent" lion

In the wild, male lions "totally dominate" lionesses. Nyack had been hand-reared, which possibly made him more vulnerable, Prof Packer said. In contrast, Zuri was more dominant than a typical female.

Zuri was only 25lbs (11kg) lighter than her male counterpart, weighing 325lbs. Male lions typically weigh 330lbs-573lbs and females 265-397lbs, according to San Diego Zoo.

"If it was a result of discordant personalities, maybe that's a risk factor that should be considered in other captive situations," he said.

He said extra mystery came from the fact that Zuri attacked Nyack twice: the first time he ran off and returned behaving very submissively, only for her to attack him again.

"Until we see a number of cases, there's no way we can say what caused this to happen," he said.

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How unusual is this behaviour for lions?

Paul Funston, Southern Africa Regional Director for Panthera, a global wild cat conservation organisation, also agreed that this incident is surprising.

In the wild, groups of lionesses do attack lions, typically in defence of their cubs or territory, and such incidents have been filmed at safari parks. However Mr Funston said he has never seen an instance that has ended in a death.

Media playback is unsupported on your device Media caption Moment lionesses attack lion at West Midland Safari Park

Wild male lions will also typically chase off any male cubs when they grow up to ensure they are alone with the pride lionesses. Sometimes the lions will kill cubs - usually when they take over new territory from another pride - to stake their claim on the females.

Male lions have also been known to get aggressive with females and can kill lionesses who refuse to mate.

One possibility, Mr Funston said, is that Zuri - who is described by the zoo as "an attentive and protective mother" - became fearful of Nyack, which led to the fight.

Zuri's natural instincts could have taken over at that point, he explained, and so she ended up killing him.

"Even if animals are calm or seem to be calm, it doesn't mean that there aren't underlying tensions," Mr Funston said.

Image copyright Courtesy Bruce Patterson, Field Museum Image caption Lionesses have been known to attack male lions who upset them

Bruce Patterson, a researcher at the Field Museum in Chicago, said he knows cases of wild lionesses that have attacked - and injured - male lions who upset them. "[But] unlike the zoo case, no one went for the throat!" he said.

Mr Funston, who has studied lions for 25 years, acknowledges this is "an unusual" and "rare" incident - but that does not mean it is is necessarily strange.

"We see a typical model and we tend to think we know it all. But this is a highly socially complex species."

"That's one thing I love about lions," Mr Funston added. "You don't quite know exactly what's going to happen in a particular scenario, and that makes them really interesting animals to observe and want to protect and conserve."