"Damaging their canines could be a life-threatening event," said Boyd. Yet fatal nimravid bite marks are found on a surprising 10 percent of nimravid skulls in three species of nimravids over a range of four million years.
"They're still taking into consideration not damaging their canines," said Boyd, noting how the eyes are a common target with the other canine just glancing the skull. But they are definitely taking a bigger chance when they attack their own kind.
Among other things, the discovery suggests that the typical museum mural representation of nimravids facing off in battle is probably dead wrong.
"Upper canines and lower canines can be seen in the (skulls)," he said. "So all the attacks are coming from behind. This was an ambush style attack against a competitor."
The lack of any signs of healing also means that the majority of these attacks were fatal, which rules out another old hypothesis, based on the 1936 specimen (which showed some healing), that the biting might be part of a mating behavior.
"It's very hard to get behavior from fossils," said Kurt Spearing, a researcher at Morningside College in Sioux City, Iowa, who works on fossil cats and their close relatives and was not directly involved in Boyd's work.
But in this case, he agrees that the behavior of nimravids is remarkably clear: "These guys were incredibly aggressive towards each other."