Attack of the Giant Hyenas

Dinocrocuta

I suspect that most non-specialists would assume that hyenas are fairly closely related to dogs. They certainly look more like dogs than they do anything else, so, absent any further information, that seems reasonable enough. But, in reality, hyenas belong to the cat-like, not the dog-like, branch of the carnivoran family tree.

This isn't some new discovery on the basis of molecular evidence, like the splitting off of the skunks from the weasel family; it's been known for a long time. This is because, when you start looking at the structural details of the skull, especially the area around the ear, everything fits with a cat-like ancestry. This much was already obvious when Miklós Kretzoi formally named the two carnivoran branches while he was working at the National Museum of Hungary during World War II. Modern evidence has merely confirmed the view, showing more precisely that the closest living relatives of the hyenas are the mongooses.

In fact, the earliest known fossil "hyenas" do, in fact, look a lot like mongooses in both size and shape, and probably had a similar diet. The shift towards the modern hyena lifestyle is one that took millions of years, with the animals becoming much bulkier in the process. While they have been regarded as a distinct family of mammals for over two centuries now, when it comes to fossils, however, there is naturally some question as to what exactly belongs in the family and where to draw the boundaries. Opinions on this have changed over the years.

For example, take the case of the genus Percrocuta. Fossils of this hyena-like animal were first discovered in Europe in 1913, although we now know that various species assigned to the genus lived across Asia and into at least central Africa. Originally placed with the striped hyenas, even when Kretzoi recognised the genus of those original fossils as being distinct in 1938, he considered them to at least belong to the hyena family. 

Over the following decades, they were slowly shifted further and further away, partly because better and more complete fossils were discovered, including some that could be assigned to different species within the genus. The break finally came in 1991, when researchers split off Percrocuta and its close relatives and placed them in their own family: the Percrocutidae, or "false hyenas". This was on the basis of the shape of some of their teeth which, it was argued, showed that percrocutids were more closely related to cats than to mongooses and true hyenas.

Then the pendulum swung back again. First, most of the other genera in the newly named group were shifted back to the true hyenas again, then it was decided that, no, actually, the remaining "false hyenas" really were closely related to the regular sort. Today, the general opinion seems to be they are a subfamily within the true hyenas, although the issue is not fully settled, and there are still some who would consider them merely the closest known relatives of the living animals and thus allowed to keep their own family.

Regardless of what they really are, and what they are most related to, we can at least say that, as currently defined, the "false hyenas" (or whatever we want to call them) are a genuine group of animals, and they're certainly as worthy of study as anything else. It's not an exceptionally big group, consisting of just two genera, although with over a dozen species between them. Percrocuta, which lived in the Middle Miocene, is, of course, one of those genera, while the other its descendant - the giant hyena, Dinocrocuta, which died out, without descendants of its own, some time around 5 million years ago.

It's not called the "giant hyena" without good reason. One estimate is that the largest, D. gigantea, weighed somewhere around 200 kg (440 lbs), roughly comparable to an adult male brown bear. Since, in other respects, it looked quite like a modern hyena, we can safely assume that it wasn't something you'd want to get too close to if it was alive today.

But that was the largest one. Just how fearsome were the others, including the smaller and more primitive Percrocuta species? Even if they really were members of the hyena family, that doesn't mean that they lived like the typical modern species do. That's because, when we look at the full range of fossil hyenas, it seems that they had a pretty wide range of lifestyles. Some of the most primitive were probably omnivores and insectivores, much like the mongooses to which they were related. While even Percrocuta isn't small enough for that to be likely, there are other possibilities. Some prehistoric hyenas were relatively predatory, chasing down their prey like a wolf or cheetah (although obviously not quite as fast as the latter) while others were, like the modern species, specialised for cracking open bone, often from scavenged carcasses rather than anything they had killed themselves.

And, of course, we're ignoring the one living species of the hyena family that eats almost nothing but termites. We'd probably have noticed if Percrocuta was that strange.

Among the regular hyenas, this variation has been catalogued and refined by a series of studies since the 1990s. However, because of the complicated taxonomic history of the group, when the studies were conducted, Percrocuta and Dinocrocuta were considered to belong to a different family, and so were not included. That has now changed, with a 2023 study comparing these two genera with various others that are undeniably "true" hyenas.

The analysis, like the ones before it, looked specifically at the teeth at the back of the mouth. These are often more distinctive than those at the front, and more specifically adapted to the type of diet - in this case, we want to know whether they are better adapted for slicing through tough meat like a knife or for crushing bones like a hammer. 

The analysis showed that species of the genus Percrocuta followed a similar evolutionary pattern to those of the ancestors of living hyenas, becoming better at cracking bone as they evolved through time. But there was a crucial difference: they did it earlier. For example, the first hyena capable of limited bone-cracking was, under the older classification system, Metahyaena, which lived in Turkey around 11 million years ago. Species such as P. abessalomi were doing the same thing 15 million years ago, and by 12 million years ago had already evolved to the specialised form we see in modern spotted hyenas, which their own ancestors didn't reach until around 8 million years ago.

Whether Percrocuta counts as a "true" hyena or not, it was living in the same way 4 million years before the modern kind got started, only later being supplanted by what we have now.

The giant hyenas were a different matter. Appearing later, and probably having evolved from some form of Percrocuta, the early ones were already bone-eating scavengers. These include D. algeriensis, which lived in Algeria at the same time as the (presumed) first pure scavenger in the rest of the family, Allohyaena, lived in Ukraine. From there, however, as the animals grew in size, the shape of the teeth at the back of the upper jaw began to shift, not only becoming larger, but sharper. By the time we reach D. gigantea - the bear-sized one - these teeth look to be even better at slicing through fresh meat than those of the most specialised of the earlier hunting hyenas, such as the "running hyena" Chasmaporthetes.

This makes it hard to escape the idea that these, the largest hyenas ever to have lived, were not mere scavengers, but active predators. This fits with the discovery of a fossil rhinoceros in China that showed what looked like the bite marks of a giant hyena on its skull. Modern spotted hyenas are certainly predatory, rather than living purely on carrion, but this new evidence suggests that giant hyenas may have been even more so, making them even more dangerous than their size alone would suggest.

At the same time, they retained some of the heavy bone-cracking teeth in the lower jaw, so there probably wouldn't have been much left of any carcass by the time they had finished with it.

[Photo by "Laikayiu", from Wikimedia Commons.]