In the hours before the Indianapolis Zoo was open to the public, zookeepers heard an “unusual amount of roaring” coming from the lion pen. Normally, this wouldn’t be cause for alarm – the zoo’s 10-year-old lion Nyack is often heard roaring from his exhibit. This time, however, was different.

As staff rushed to the lion pen, they saw the 160-kilogram (350-pound) lion pinned down by his pen-partner, a lioness called Zuri.

“She had Nyack by the neck. Keepers came up and saw what was occurring and made an attempt to separate the two. But she continued to hang onto Nyack by the neck until he stopped moving,” David Hagan, curator of the Indianapolis Zoo, told Reuters in an interview. 

A necropsy revealed Nyack, described as “very vocal and laid back”, had died of suffocation caused by injuries to his neck. But what prompted the “attentive and protective” mother to attack remains a mystery. In the eight years that the two had been housed together, staff say they never noticed any aggression between the pair – even when Zuri had birthed their three cubs in 2015. 

The zookeepers say they are devastated.

“[The staff] build strong bonds with the animals so any loss affects us all greatly. For a lot of us, it’s just like a family member,” said Hagan.

It’s not the first case of lions attacking one another in captivity. In 2013, a lioness at the Dallas Zoo died after she was attacked by a male lion after the two had lived together peacefully for years. It’s a pretty rare occurrence in captivity. The wild, however, is a different story.

When male lions outcompete other males and take over their pride, they will often kill the group's cubs and then mate with the females to ensure the success of their bloodline. Females will gang up on these single males (a quick YouTube search will show you just how common this is), sometimes resulting in death.

A lion expert who spoke with the Indy Star said the way Zuri killed Nyack was vicious, noting suffocation is typically the way lions kill their prey. 

While the zoo says it is reviewing the incident, it has no plans to change how the lions are managed. Sukari, the 3-year-old daughter of the pair, was in the outdoor lion pen at the time. Their two male offspring have been kept in a separate part of the exhibit as young male lions typically leave the pride.

A spokesperson for the Indianapolis Zoo told IFLScience this type of attack appears to be "extremely rare." Staff will be conducting a thorough review to "attempt to understand what might have led to this incident."