On Monday, Oct. 15, zookeepers at the Indianapolis Zoo witnessed a murder.

The staff arrived at work that morning to a thunder of roars coming from the zoo's outdoor lion exhibit. By the time the keepers reached the enclosure, Zuri — a 12-year-old lioness — was driving her teeth into the neck of Nyack — a 10-year old male lion, and the father of Zuri's three cubs — while their three-year-old daughter Sukari watched.

Keepers attempted to separate the warring lions, but Zuri could not be calmed. According to zoo officials quoted in the Associated Press, the lioness kept her jaws clamped around her mate's neck until he stopped moving. Nyack was publicly pronounced dead on Friday (Oct. 19). The cause of death: suffocation resulting from neck injuries. [The Top 10 Deadliest Animals (Photos)]

It's a sad and startling case of leonine violence that has baffled zoo personnel (who are investigating the incident) and lion experts alike. Why, after eight years of peaceful cohabitation and three years of child rearing, did Zuri suddenly turn on her mate and maul him to death in the morning?

According to Dr. Paul Funston, Southern Africa Regional Director for Panthera, the global wild cat conservation organization, it's not uncommon for lions to attack and even kill each other in the wild — however, in his 26 years of field work he has never heard of a case where a lone female lion attacked and killed a lone male lion in one-on-one combat.

"This is very unusual," Funston told Live Science. "But lions have a very wide behavioral repertoire and occasionally they will do things that surprise us. [This incident] might be the result of an animal being in captivity for a long period of time and choosing to behave in an unusual way."

In the wild, lions kill each other in two main contexts, Funston said. The first is territorial struggle — violence as the result of two prides living or hunting too close to one another. According to a 2009 study in the journal Animal Behaviour of 46 prides living in Serengeti National Park in Tanzania, territorial skirmishes most often turned lethal only when male lions were involved — and the casualties were almost always lionesses.

The second context in which lions kill each other is sexual struggle. When a strange male enters a new territory populated by new potential mates, he might show his interest in less than gentlemanly ways. Females who resist may suffer for it.

"When males are approaching females to mate with them and the females won’t, males can get very aggressive, beat the females up and occasionally kill them," Funston said.

While it's impossible to know exactly what transpired between Zuri and Nyack in the hours before the fatal attack, a sexual encounter gone wrong could be a strong possibility, Funston said.

One big clue, he said, is the fact that all three of the couple's cubs were born in 2015, about three years ago. In the wild, three years old is a crucial age when young lions begin to take care of themselves. Males disperse from their family to find new territories and mating opportunities (earlier this year, Nyack and Zuri's two male sons were moved to a separate enclosure). Females tend to enter their mother's pride to become potential breeding lionesses themselves.

In either case, Funston said, mom no longer has to spend her day taking care of the cubs — and that means she can mate again.

 "I think this is what the background of the Indianapolis incident was," Funston said.  "The cubs were now big and the male wanted to mate again."

Maybe Nyack approached Zuri too aggressively to mate with her one too many times. Maybe Zuri felt threatened, or maybe she was on a contraceptive administered by her keepers (as is common practice in some zoos) and physically could not mate with Nyack. Maybe tension had been growing between the couple for a while, and finally came to a head on Monday morning when one or both lions snapped, and Zuri was able to seize a final, firm hold on her mate.

"Fighting got intense and, the next thing, she killed him," Funston said.

While Funston has never observed a scenario precisely like this play out in the wild, he said he sees no reason why a female lion shouldn't be able to best an attacking male at least some of the time.

Whether or not Zuri killed Nyack in self-defense, friends of the Indianapolis zoo are mourning the loss of a good lion. As to whether Nyack was a good husband? Only Zuri knows for sure.

Originally published on Live Science.