Monthly Archives: April 2016
Modern dogs differ from other carnivores in one interesting anatomical feature: they have a “nuchal ligament”, a string-like structure that runs along the dorsal part of their necks and allows them to support the weight of the head with little muscular effort. Dogs share this feature with ungulates, but there are differences in detail: while in ungulates the ligament extends from the spines of the thoracic vertebrae to the back of the head (hence the name “nuchal”, meaning “attaching to the nape”), in dogs it only runs as far ahead as the second cervical vertebra (the axis), so the term nuchal ligament is somehing of a misnomer here. But, at any rate, what would a carnivore want a nuchal ligament for? Such an adaptation makes sense in a cow or a sheep, who spend many hours grazing with their heads down, an activity that calls for some passive mechanism to save the muscular effort of supporting and then lifting the weight of the head. But carnivores don’t graze, do they?
Well, what dogs do is track scent trails. They walk and trot for long distances, nose close to the ground, as they search and follow their prey’s smelly paths. And they also happen to be relatively long-legged carnivores, which implies they need a long neck for their snout to reach the ground as the animal trots. So the neck of a wolf, jackal or coyote is proportionally very long, but some of the muscles that turn it to the sides and pull it up are relatively reduced, compared to other carnivores, partly because their role is taken by the nuchal ligament.
Here is a drawing of the neck of a wolf (Canis lupus), showing the skull and vertebrae (top) selected deep muscles (middle) and more superficial muscles. See how the “nuchal ligament” actually doesn´t reach the nape, just the back of the axis vertebra.
But, have the necks of dogs always been like that? Several years ago, while working on the reconstructions of fossil dogs for our book “Dogs: their fossil relatives and evolutionary history”, Xioaming Wang, the late Dick Tedford and myself looked in detail at the anatomy of the fossil dog Aelurodon, from the American Miocene. Fortunately there is an amazing collection of fossils of these animals at the American Museum of Natural History in NYC, so we had all the information we could hope for. While studying the cervical vertebrae I found something strange about them: they somehow resembled the vertebrae of a big cat, such as a leopard, more than they did the same elements in a wolf. Concretely, the vertebrae were relatively short, and the processes for muscle attachment were proportionally larger, projecting farther away from the vertebral body. When I assembled the bones to create a reconstruction, the neck looked suprisingly short, and when I reconstructed the musculature of the neck on the basis of the shape and position of attachment areas, it was evident that this animal had a more powerful neck than a wolf of comparable size. There is no obvious evidence for the presence or absence of a nuchal ligament, but the morphology of the back of the axis, where the ligament would attach, is rather different from that of modern dogs. Also, the short neck and its powerful muscles would make the function of such a ligament rather irrelevant.
Here is a reconstruction of the head and neck of Aelurodon. The morphology of the cervical vertebrae (top) implies a relatively short neck, while the shape of muscle attachment areas speaks of very strong musculature (center). Both features resemble the necks of modern big cats. When external layers are added, we see that the animal’s head and neck woudl look powerful and socky (bottom)
These anatomical differences must have implied differences in behavior, but it is not clear what differences those would be. Given the simmilarities with a cat’s neck, it is tempting to assume a more cat-like hunting style for Aelurodon, implying that the predator was more able to handle its prey individually, using its paws to restrict its struggles, and using its neck as a base for delivering a more precise killing bite, a bit like big cats do. Also it is possible that trotting for miles in search of scent trails as some modern dogs do was a less important part of its behavioral repertoire. But it is also possible that the predatory behaviour of Aelurodon was essentially similar to that of modern wolves and it simply had not evolved some of their anatomical refinements.
A broad comparison of neck morphology in fossil dogs revealed to us that the first taxon to clearly show a modern wolf-like neck anatomy was the late Miocene and Pliocene genus Eucyon. This animal also developed proportionally longer forelimbs than any of its earlier relatives, probably reflecting an adaptation to drier, more open environments, and wider foraging areas. So it would make sense to think that a modern foraging stlye in these members of the dog subfamily caninae was accompanied by the development of a modern forelimb and neck anatomy.
Only additional research will take us closer to solve these riddles, but one thing is certain: just like in the cat family, fossil dogs reveal a diversity of adaptations that we could hardly suspect by looking only at the living species. And the dramatic difference that we see today between dogs and cats could have been a bit more blurred in the distant past!
To learn much more about dog evoution and fossil record, check our book: