Natural History

Most feral domestic animals revert to a form that is roughly similar to their wild ancestor. You can see this quite dramatically in feral pigs. They generally evolve into a form that is about the same size and even coat type of the Eurasian wild boar. City pigeons look very much like the rock dove or “rock pigeon” that is their wild ancestor after just a few generations of breeding without human care.

Because village and pariah dogs tend to be mid-sized, it has been assumed that the wild ancestor of these dogs surely would have been on the smaller side as well. Therefore, the gray wolf simply could not have been an ancestor.

What actually drives the size of freely breeding and feral domestic dogs isn’t that they have some ancient alleles that force them into returning to an ancestral form.  The truth of the matter is that ecological niche and caloric restraints have a lot more to do with this phenomenon.

Dogs are unique among domestic animals in that they are the only domesticated form of large carnivoran. We have never domesticated any other species of large predatory mammal except for those Pleistocene Eurasian wolves that are at the base of domestic dogs.

Most domestic dogs are poorly adapted to living as predators, and they really don’t have to be. When dogs go feral in societies with extensive agriculture, they readily scavenge and hunt small prey. They dabble in various levels of omnivory.  Some dogs might be good at hunting deer, but deer are a lot harder to catch than garbage and groundhogs.

There is an extensive literature on mammal predator size and prey choice. The best known researcher looking at these issues is Chris Carbone, and in a 2007 paper called “The Costs of Carnivory,” which was published in POLS Biology, Carbone and colleagues looked at body mass of mammalian predators and their prey choices. If a predator weighed more than 20 kg, it hunted large vertebrates. If it weighed less than that weight, it hunted invertebrates or small vertebrates.

Larger predators get a much higher net energy gain by targeting large prey, and this large prey allows them to maintain their larger body size.

Feral and freely-breeding domestic dogs are not hunting large vertebrates. It is much easier for them to scavenge as mid-sized creatures. Natural selection would favor a moderate size, because any dogs that retained the large dog or large wolf alleles in the population would have a harder time feeding itself efficiently on these resources alone.

There are, of course, exceptions to this rule. In Uruguay, there was a population of feral mastiff-type dogs, which are called Cimarrón Uruguayo. These dogs were introduced by Europeans as working dogs, but some of them went feral. They were able to maintain their large size because they hunted livestock and game, and they were such a problem that the government placed bounties on them.  These dogs were living in a feral existence for at least 250 years, but they were able to retain a large mastiff phenotype. The feral mastiff is now being transformed into a standard breed.

However, the general rule is that village and pariah dogs tend to be significantly smaller than wolves, but this smaller size cannot be used to deny that dogs are derived from gray wolves. This smaller size is just more efficient for the ecological niche of feral and village dogs.

And it is poor reasoning to assume that dogs cannot be wolf derivatives simply because they do not evolve back into a wolfish form once they go feral.  Dogs have been domesticated for a long time, and their domestication is quite unique.  As the only large predator we have domesticated, ecological pressures create a different sort of animal than the original wild ancestor.