Dogs skulls show astounding variety in shape

In a mere century and a half of selective breeding, humans have managed to create a range of creatures within a single species that are more diverse than the members of an entire biological order.

In a new study, a pair of researchers measured the diversity among skull shapes in dogs, from the pushed-in face of a Pekingese to the long slender skull of a collie. And astonishingly, the differences in skull shape between these two breeds is more extreme than the most disparate animals in the order Carnivora (namely the walrus and the falanouc, similar to a mongoose), which has been evolving for about 60 million years.

The authors of the study, biologists Christian Klingenberg, PhD, of the University of Manchester and Abby Drake, PhD, of the College of the Holy Cross, say the study is important because it furthers our understanding of microevolution and selective breeding as an evolutionary force.

A photo comparing some of the diverse skull shapes of dogs.
(Courtesy of Abby Drake, College of the Holy Cross)

“For dogs we have created a whole new set of selection rules based on breed standards,” Drake said. “Finding or hunting food is not something they need to be able to do and most modern breeds probably wouldnt be able to. We also arrange their reproduction and we protect them from anything that might harm them including disease. Instead we select them based on our own (sometimes bizarre) ideas of what they should look like. And this selection pressure does work.”

For the paper, Drake and Klingenberg compared 50 points on the skulls of 106 dog breeds, 122 different species within Carnivora, as well as wild canids (grey wolves, coyotes and golden jackals). One of the most interesting findings in the study was that some of the skull shapes in dogs were unparalleled in any of the other species they looked at.

The skull shapes of the collie, Pekingese, bull terrier, mastiff, etc., do not resemble any of the other canids nor any other carnivore; they are phylogenetically novel,” Drake said. “Most of the variation in dog skull shapes is contrasted between very short and wide brachycephalic skulls like Pekingese and Bulldogs and the elongate and slender dolichocepahlic skulls of Collies and Borzois.”

Klingenberg said that veterinarians might find the study relevant for its findings on the sheer influence of humans on a species.

“For veterinarians, the study illustrates the power of artificial selection -- a power that should be used with caution,” he said. “Breeders generated some really bizarre dogs, and it has not all been to the dogs benefit. Veterinarians, responsible breeders and their organizations have become aware of this and are developing programs for enhancing the welfare of dogs by using genetic means for tackling those problems.”

Klingenberg and Drake also speculated on what might occur if the selective breeding pressures put on dogs were taken away.

“A key feature of purebred dogs is that they are kept as separate breeds, which are effectively separate evolutionary lineages,” Klingenberg said. “If youd take away selection and allow the dogs to reproduce with other members of their breed, you would get a greater or lesser extent of random drift, depending on how many dogs of that breed there are. You would also get some natural selection against genetic conditions that keep dogs from surviving and reproducing (hip dysplasias, etc.). I guess youd see some really strange shapes such as English bulldogs disappear in the mix fairly quickly.”

Drake pointed out that this is actually happening all the time among stray and feral dog populations, such as Australian dingoes and the stray dogs of Moscow.

“Interestingly none of them seem to exhibit the more extreme appearances that we see in dolichocephalic or brachycephalic breeds,” she said. “Probably this is due to the lack of fitness of these breeds in fending for themselves. We certainly dont see packs of French Bulldogs roaming the streets of Paris!”

The study was published in the journal, The American Naturalist.

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