Striped hyenas (Hyaena hyaena) are solitary hunters. They forage alone but occasionally come together to munch on a kill. They are far less sociable than their better-known cousins, spotted hyenas.
Both species are known to be highly intolerant of other large carnivores, and even of other members outside their immediate social group. They will also kill large aggressive dogs that get in their way.
That is why Vladimir Dinets of the University of Tennessee at Knoxville, US, was surprised to find that a striped hyena had travelled with a pack of grey wolves – another hostile predator usually intolerant of other species.
"It went against everything that was known about wolves and hyenas," Dinets says. He first spotted tracks of both creatures near Eilat, in the Negev desert of Israel in 1994. This included three grey wolves and one striped hyena.
"Remarkably, in many places the hyena tracks were on top of wolf tracks… The tracks of the three wolves also overlapped each other in all possible orders," Dinets and his colleagues write in a new study in the journal Zoology in the Middle East.
This indicates, they say, that these four animals were walking together at the same time.
Although Dinets was confident of his initial observations, he knew the footprints were not enough to convince his colleagues at the time. "Most zoologists today don't get trained in conventional tracking and know little or nothing about the kinds of data you can get from it."
It might be because both groups live in such an inhospitable, arid part of the world
It was only four years later, about 1,300m away, that Dinet's colleague Beniamin Eligulashvili, an Israeli zoologist, had a similar experience.
This time he saw the two species together, first-hand. He observed seven wolves with one hyena.
What was even more surprising was that the hyena was not following the wolves, but moving in the middle of the pack.
While hyenas have been known to scavenge the kills of other large predators, they had never before been known to socialise with other hunters.
No striped hyena has been spotted with a wolf pack since.
Predators are smart, flexible animals
Eligulashvili had hoped to see more instances of this unusual friendship, which is why the team waited so long to publish the results. "But he [Eligulashvili] never saw anything like it again," says Dinets. "He didn't even see another striped hyena. They are very difficult to see in the wild."
The question is, why would the wolves tolerate a hyena among their ranks?
The authors believe it might be because both groups live in such an inhospitable, arid part of the world. There is only 29mm of rain each year in the area.
It is these extreme conditions that may have driven the two species together to form this unlikely alliance, the authors propose.
Wolves are better at tracking down large prey, while hyenas have a superior sense of smell. They can also get into discarded rubbish like tin cans, and are extremely good at scavenging and ripping apart large bones.
It is still unclear how long the hyena spent with the wolves, or how many times similar behaviour has occurred before or since.
What it does show is that "predators are smart, flexible animals", says Dinets. This observation reveals that they certainly do not "stick to roles prescribed to them by biology textbooks".
Hyena expert Kay Holekamp of Michigan State University, US, has never seen or heard of anything of the kind. "Stranger things have happened," she says.