Sunday, 17 June 2012

Dire Wolves and Direr Dogs

Reconstruction of a dire wolf
We are all familiar with the idea that, for much of the Age of Mammals, there have been large, scary cats with enormous teeth. They died out relatively recently, and as a result, we know a fair bit about them, and they remain one of the most popular prehistoric mammals with the general public - although mammoths do pretty well, too.

But, while the sabre-tooth cats stalked the world, what were the dogs like?

Perhaps the best known prehistoric dog is the dire wolf (Canis dirus). Do a google image search for "dire wolf" and you'll find far more images than you will searching for "sabre tooth", let alone something more specific like "Smilodon". But a look through those images gives you some idea why, and it doesn't have much to do with real-world fossils. The term "dire wolf" has been co-opted by fantasy fiction and games, and many of the pictures show the animals as they appear in Dungeons & Dragons, World of Warcraft, or Game of Thrones. They are generally portrayed as immense wolves, at least the size of a small horse, and sometimes with sabre teeth of their own, or bizarre spikes on their bodies.

Dire wolves were real animals, but, even leaving aside the spikes, they bore little resemblance to the modern fantasy image. They lived during the Ice Ages - the Pleistocene epoch - and were larger relatives of modern wolves. But not that much larger. Modern grey wolves vary in size according to Bergmann's Rule, but they're generally about as large as a German Shepherd, or possibly a little bigger. Dire wolves were larger, approaching the size and build of an English Mastiff, the largest of all modern dog breeds by weight. Pretty impressive, but hardly the size of a horse.

No dog has ever really been that size. It's just, well... fantasy. However, dire wolves were not the largest dogs ever to have lived, or necessarily the most fearsome. In fact, to look at, dire wolves probably wouldn't have seemed much different to the wolves of today. They were larger, of course, and with a stockier build, but otherwise very wolf-like. Indeed, they were very close relatives of living wolves, and hence, of domestic dogs, although they were probably even closer to coyotes - which makes sense, given that they only lived in North America.

But, of course, wolves and coyotes are just one branch of the dog evolutionary family tree. Taking the broad, palaeontological view, we can divide the dog family into three subfamilies. Only one of these, the canines, survives today, and it includes everything from the largest of wolves down to the smallest of foxes. The canines became the dominant kind of dog during the Pliocene, the cool epoch leading up to the Ice Ages, and by the time the ice sheets began to advance in earnest, they were the only kind left.

But, if we go back further, into the much warmer Miocene epoch that preceded the Pliocene, we find that it was instead the borophagines that were the dominant subfamily. The borophagines were not the direct ancestors of today's canines, and, in fact, lived alongside the animals that were - small fox-like animals, of which the best known is Leptocyon. For millions of years, the borophagines were the most successful group of dogs, so what happened to them?

Wolf-like     Fox-like    Borophagines
  Dogs          Dogs
    ^            ^             ^
    |            |             |     Hesperocyonines
    |            |             |            ^
    --------------             |            |
          |                    |            |
       Canines                 |            |      Bears, etc.
          |                    |            |           ^
          ----------------------            |           |
                     |                      |           |
                     |                      |           |
                     ------------------------           |
                                 |                      |
                              CANIDS                    |
                                 |                      |

Initially, the borophagines were not particularly impressive. They were small, fairly generalist animals, much like Leptocyon and living foxes. Living in the shadow of larger and more effective predators, the initial path in their evolution was away from pure carnivory, and the evidence suggests that for much of the early Miocene, the most common species of borophagine were basically omnivores, perhaps eating a similar diet to living raccoons. As the climate warmed during the mid Miocene, many of their competitors suffered, and the borophagines took advantage of the new opportunity, and switched back to pure meat-eating. At first, they were were wolf-like, although some of them were larger than modern species.

But, while the later canines could never quite become top predators, due to the presence of large sabretooths, and, later, humans, the borophagines continued to grow. In the late Miocene, the climate began to cool again, and, seemingly, the borophagines began to struggle with the changes in climate, just as their own predecessors had. It was at this time that the canines began to diversify, and the first true foxes began to appear. Indeed, so successful were the canines that they spread beyond the North American continent, where they had first evolved, to enter Asia and South America, giving rise to the great diversity of dogs and foxes that we see today.

The borophagines never managed that. They too, had first appeared in North America, but they never left, and were known nowhere else in the world. Even on their home continent, they were apparently unable to compete with the canines, perhaps because they were, by this time, already too specialised, and the canines out-performed them in the "small generalist" role. Evolutionarily speaking, they responded by becoming even larger and more ferocious. On that level, the canines simply couldn't respond, and it was they, as the borophagines had before them, that now had to stand aside for the true top dogs.

By this stage, during the late Miocene, the borophagines were well past their peak. Like all large predators, they needed a lot of space to hunt, and there was no longer room on the continent for many different kinds. At their height, there had been about twice as many species of dog in North America as there are know, and almost all of them had been borophagines. As the Miocene transitioned into the cooler Pliocene, and the Arctic Ocean began to freeze over, very few borophagines were left.

But those that did survive were truly spectacular. They include Epicyon haydeni, the largest dog that has ever lived. Standing around 90 cm (3 feet) at the shoulder, they were taller than many Great Danes, but with a much more muscular build, and they probably weighed over 75 kg (165 pounds). Unlike most wild dogs, they had a high, domed, forehead, making their muzzle appear short in comparison. They needed that large head, not for an exceptional brain, but so that the skull could provide suitable attachment for huge jaw muscles.

Epicyon was a pure carnivore, and its teeth were modified to rapidly crack bone, splintering it into fragments so that they could easily extract the juicy marrow. For this reason, they are often called "hyena-dogs", although, physically, their build was much closer to that of other dogs than to the short-legged sloping backed bodies of true hyenas. As the climate cooled further, prey apparently became harder to find, and the borophagines that came after Epicyon were not quite as large.

Borophagus sp.
The very last borophagine is the one for which the subfamily is named, Borophagus. Although they were smaller than their predecessors, perhaps only 45 kg (100 pounds) at most, that's still a fair size for a dog, and noticeably larger than modern wolves. However, they were even more specialised, with highly modified bone-cracking teeth, allowing them to scavenge on carcasses and extract every morsel of nutrition from the remains. Like most modern scavengers, they probably hunted as well, and it's quite plausible that most of the carcasses they consumed were of animals they had killed themselves, perhaps hunting them down in packs.

Once the ice sheets began to advance, it was all over. The animals that Borophagus preyed on began to die out, and this, the last of the hyena dogs, was simply too specialised to switch to any other diet. They died out, but the more generalist and adaptable canines has no such problem. With the borophagines out of the way, however, some of the surviving canines became larger, at least partly taking their place; it was at this time that a primitive wolf entered North America from eastern Asia, and evolved, with time, into the more famous dire wolf.

It wasn't even the first time this had happened. Both borophagines and canines probably arose from a third, more primitive, subfamily of dogs, the hesperocyonines. First appearing in the late Eocene, with the earliest known true dog, Prohesperocyon, these had generally been small animals, still with five toes on each foot (unlike later dogs), and feeding on animals such as rabbits and rodents. The best known is Hesperocyon itself, which has left numerous fossils, and, judging from its limb proportions, spent a fair portion of its time climbing about in trees.

For much of the Oligocene, hesperocyonines had been the only kind of dog there was, for all that they might have looked slightly un-dog-like to modern eyes. Once the warmer Miocene began, they proved less able to compete with their relatives, the borophagines, and (to a lesser extent) canines. Part of the reason that the first borophagines were relatively inoffensive omnivores was because the hesperocyonines were taking all the meat - just as the borophagines would later on, they had become larger and more carnivorous, and some even developed bone-cracking teeth.

The hesperocyonines were never as large or as specialised as the later hyena-dogs, and the very largest, Enhydrocyon, looked, at most, like a muscular jackal. But they were among the best that was going at the time, and were among the top North American predators of their day (like the borophagines, they never left the continent). It was only when they died out, in the middle Miocene, that the borophagines could take over their niche, returning to pure carnivory, and setting themselves on what would prove to be an ultimately fateful trajectory.

[Images by Sergio Larosa and Jay Matternes, from Wikimedia Commons. Cladogram adapted from Wesley-Hunt & Flynn, 2005]

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