When I found the article linked below, I thought it was about the Birch Creek that separates the Blackfeet reservation from the state of Montana, But this "Birch Creek" was in Idaho. It turns out that there are hundreds of "Birch Creeks," "Willow Creeks," "Deer Creeks," and the like, all named for what was found there. In this case, the Birch Creek that I know is the southern boundary of the rez. It turns out that the Birch Creek in the title is from the "Intermountain West," which is the area on the west side of the Rockies as far as the Cascades, but some of the study is in the Dakotas and Nebraska. The study is not based on living dogs but on fossils.
The study merely refers to the whole canid family. "The biological family Canidae /ˈkænɪdiː/ (from Latin, canis, “dog”) is a lineage of carnivorans that includes domestic dogs, wolves, coyotes, foxes, jackals, dingoes, and many other extant and extinct dog-like mammals." (wiki) Most of us easily think of dogs, wolves, coyotes and foxes, the best known canids of our continent, but don't know as well the jackals, dingoes, and various canids evolved to fit their location, like fennecs or singing dogs. https://en.wikipedia.org/wiki/List_of_Canis_species_and_subspecies
The scientists who studied these fossils were looking to find out how much weight they could carry. Pulling a travois seemed to max out at about the dog's weight (approaching a hundred pounds) while the weight of carried paniers, backpacks, were about thirty pounds if much distance was involved. Their assumption was that dogs come from wolves, which has been challenged in recent times.
No one, so far as I know, has identified the characteristic of plastic genes in domestic dogs that allows them to look so different in size, kind of coat, and even temperament. If one defines "species" as the ability to interbreed, all domestic dogs seem to be fertile with each other in spite of attempts at control by humans, and also with related animals like coyotes, wolves, and foxes. Coy-dogs and wolf-dogs exist. Fox dogs may be possible but sometimes fertility is prevented by things like size or habits. That is, a fox may have such different habits, ranges and diurnal timing that mating is impossible or unlikely. A wolf cannot fall in love with a fox and follow it down its hole. (Good material for a mythic story.)
Controversy about what happened to make dogs domestic and from which basic canids is unresolved. It appears that some genomic change is powered by the discovery of a new means of subsistence -- attaching to humans and living their lives with them. This may have happened repeatedly to several sub-species in paleo-times. Wolves are popular candidates. Domesticity is somehow linked to the plastic genes. Some domestic dogs revert to being wild, feral, and that behavior is always present just under the surface, potentially a danger or perhaps attached to other dogs rather than humans. They seem naturally to be group animals, which are the only kind I know of that become domesticated like horses, cows or sheep, including humans.
Scientists were startled to realize that domestic dogs actually share molecular genes with humans. We hadn't known that genes could cross from one entity to another. The Blackfeet, who knew nothing about genes, thought of dogs as a separate tribe that chose to join the human tribe, not as dependents but as separately cooperating beings who chose to share the fates of people, becoming attached, intimately involved, and empathic.
Robert Hall's seminal video essay about rez dogs evokes this conviction from the idlers hanging around an alley next to a source of alcohol and drugs, which they wait for the same way a dog watches for a dropped hot dog. The drunks go so far as to say that the dogs were once vulnerable disreputables like themselves, but after dying these guys missed the group so much that they came back as dogs and now hang around with them in that form. Mishaps and suffering are so common among the these people that it forms a solidarity with the dogs' bad luck. https://vimeo.com/21163086
The video shows how the dogs have plans for their dog-lives as they travel among homes, and sometimes form packs to kill livestock or breed. Of course, if they are a danger to livestock, they are shot. Dogs that are bred to guard homes and are therefore inclined to bite are more tolerated in a place where human law and order must cover too wide an area to be effective. Humans on the rez learn to be respectful of this. Arriving at an unknown household by car, it is wise to stay in the vehicle and honk the horn until the owner comes out.
Many of these dogs seem to have genes like those of "molosser" dogs "Molosser is a category of solidly built, large dog breeds that all descend from the same common ancestor. The name derives from Molossia, an area of ancient Epirus, where the large shepherd dog was known as a Molossus." These big sturdy dogs don't hesitate to bite, but can be trained to pull or carry loads. Pit bulls, bred to fight, are in this category. Also, in rez dogs there seem to be genes of shepherd dogs, like collies or Australian heelers. The biggest versions are those white dogs who are raised with sheep as one of them capable of fighting off predators. However different from each other domestic dogs seem to be -- poodles, retrievers, Boston Bull -- when they escape from attachment to humans and join the rez dogs, it only takes a few generations for them to look like generic rez dogs: big to medium-sized, shaggy, alert, and always busy when awake.
Stories about beloved dogs becoming human, the way beloved toys can do, show how little appearance counts when an attachment forms. Other stories are about guardian dogs who fight off bears and murderers. But the wild dog underneath can lead to attacks and even murder by dogs. As an animal control officer I took many reports of damage usually, in the case of toddlers, mutilation to the face and in that of infants, death. Dogs have been used by criminals to hurt their victims, so that the courts classify them as "dangerous weapons." Others have become support to police by finding lost people or even bringing down escaping lawbreakers. Some dogs are purported to write stories!
When I write a story about indigenous people on the rez, I always try to get a dog into it somehow, partly because that's just realistic and partly because it always enriches the dimensions of the tale. There are good reasons for keeping dogs in fenced yards, but I always sort of regret the fences and leashes of "civilization."