CSI Miocene: The Marks of Hyena Violence

Adcrocuta

Today, there are four species in the hyena family, one of which arguably isn't a hyena. However, not only does the family have a long history but, for most of that history, there were many more species than there are now. One estimate indicated that as many as 60 fossil species had been named as of 2008. We can be fairly confident that some - perhaps most - of those will eventually turn out to be the same species named twice, but there are also others that have been named since.

There is considerable variety among those species, as we might expect given over 20 million years of evolution. There are, for instance, at least three subfamilies, only one of which contains the three species we would consider "true" hyenas today. Even that subfamily includes animals that can be considered to have three basic lifestyles: bone-crackers, "running" hyenas, and those considered to be transitional.

All living hyenas are bone-crackers. Back in the Late Miocene, however, the group was perhaps best represented by Adcrocuta eximia. This lived between about 9 and 6 million years ago, and was found across the whole of Eurasia, from Spain to China. In life, it would have looked similar to today's spotted hyenas, which are the ones most people are familiar with, and it was probably closely related to them. 

One feature that distinguishes spotted hyenas from the other two living species is that, unlike them, it lives in packs, making it a particularly effective predator and one can that drive many other carnivores away from their kills. It's less clear whether the same would have been true of Adcrocuta, with some studies showing that its brain may not have been large enough to indicate a complex social life, and others arguing that its fossils are sufficiently common that it probably lived in groups of some sort.

Adcrocuta died out around 6 million years ago, close to the end of the Miocene, with the three living genera appearing not long afterwards. Perhaps the most successful hyena of this era, however, was the "running hyena" Chasmaporthetes. This was a slimmer animal than any of the bone-cracking forms, and had relatively long legs, suggesting that it chased down prey rather than springing on it from ambush. There are several recognised species, mostly in Eurasia, but including one each in Africa and North America. 

Analysis of the wear patterns on its teeth suggests that it ate only about as much bone as a modern lion. Doubtless it was a deadly predator, perhaps similar in some ways to a cheetah, but it was not a dedicated bone-cracker like Adcrocuta. There is no suggestion it would have been a pack hunter, any more than cheetahs are today. In this, if not its diet, it would have resembled today's brown and striped hyenas rather than the spotted sort that tends to appear most in nature documentaries.

Today's hyenas, especially the spotted species, are known to be highly aggressive. This stands them in good stead when driving off other predators from their kills, and, indeed, lions are the only carnivores on the African savannah large enough to stand their ground against spotted hyenas. But it also extends to fights among themselves, with young siblings often fighting to the death, and older animals competing for territory. (The females, incidentally, are stronger and more violent than the males; the latter will generally only attack females if they can gang up on one, and even that is unusual).

Compared with some other aspects of behaviour, signs of a violent life can be seen even in fossil species. If an animal sustains a severe injury and survives, that can leave a permanent mark in the bone, which is something we can examine. For example, there are reports of injuries consistent with jaguar attacks on Ice Age ground sloths, and of some on the skulls of Smilodon sabretooth cats that may even have been caused by fights with one another.

The trick in these sorts of situations is often trying to demonstrate what could have caused the injury. Prehistoric predators, like modern ones, lived violent lives and would have been at risk from competitors of many kinds, not to mention the possibility of taking on a prey animal that fought back more effectively than expected. Such is the case with a recent paper that examined injuries to the skulls of the two types of fossil hyena I described above.

The Adcrocuta specimen was recovered in northeastern Spain and dated to just over 7 million years ago. Judging from the worn teeth, and certain features of the skull, it likely belonged to an adult somewhat beyond its prime, but still in reasonably good health. The skull has a triangular puncture above the left eye, containing two indentations indicating tooth marks, which show some signs of healing. This healing obviously didn't close up the puncture, but that it occurred at all shows that the hyena must have survived the injury for at least 40 days and probably longer.

Given the strength and size of Adcrocuta, whatever inflicted this wound must have been a powerful animal. Sabretooth cats are known to have lived in the area at the time, but theories about their method of attack on prey animals make it unlikely they would have tried to bite the head of a hyena and that they might well have broken their long and narrow teeth had they done so.

A large bear might be a more plausible attacker, with the most likely candidate for the time and place being Indarctos. Here, the authors argue that, being an omnivore rather than a predator (it's related to pandas), it's hard to see why Indarctos would get in a fight with a hyena. Perhaps more significantly, Indarctos was huge, reaching well over 100 kg (220 lbs), so that if there had been such a fight, the hyena wouldn't have survived long enough to heal.

That leaves the only candidate animal known to live in the area at the time as being another Adcrocuta. We don't, of course, know what the fight was about, but it must have been quite vicious to leave such a wound, even if the victim escaped. One of the toothmarks is clearly due to a canine, but the other is slighter; while the obvious conclusion is that it was caused by the nearest incisor tooth, it could also be the same tooth gripping down twice, for example, if the victim tried to twist its head out of its opponent's mouth and only partially succeeded.

The Chasmaporthetes skull came from a site nearby to the other, but is, as we would expect given the evolutionary history, much younger, at around 2 million years old. This has a relatively large, oval-shaped slash close to its left eye. As with the Adcrocuta, there are signs of healing around the wound, so we know it survived.

It's harder to say what animal could have caused this wound, not least because there are rather more candidates. We can probably rule out sabretooth cats using the same logic as before, but there were some large non-sabretooth cats in the area at the time, including giant cheetahs about twice the size of today's sort, European cougars, and the powerful "European jaguar" (which may actually be a tiger). Big cats don't normally fight with a bite to the head, preferring to swipe with their claws, but it isn't unknown.

Bears are less likely, given that the only local species at the time was the Etruscan bear, which ate very little meat, and probably wouldn't have willingly tackled a hyena. But it remains possible that it was taken by surprise or that it was defending its cubs. 

There is another possibility, besides a fight with another member of its own species. A second type of hyena, Pliocrocuta, lived in the area at the time, and appears to have been more common than Chasmaporthetes. This was about the size of a modern spotted hyena and, like it, was a bone-cracker built for strength, not speed. It's entirely plausible that the two species would have competed for the same food supply, and that at least some such conflicts would have resulted in violence.

It's interesting to note that the wounds on the two animals are similarly placed on their skulls. Similar patterns have been seen on the skulls of modern hyenas, although it's hard to be confident that these are entirely distinctive to attacks by other hyenas. But it seems entirely plausible that, whether they lived in packs or not, hyenas have long been aggressive and violent animals, sometimes even willing to kill their own kind in the quest for dominance or territory.

[Photo by Mariomassone, from Wikimedia Commons.]