Many members of the dog family are also capable of interbreeding and producing fertile offspring. This depends on how closely they are related. Molecular analysis divides the dog family into 4 divisions:

1. Wolf-like canids including the domestic dog, dingo, grey wolf, coyote, and jackal
2. South American canids
3. Old and New World red-fox-like canids, e.g. red fox and kit fox
4. Monotypic species, e.g. bat-eared fox, raccoon dog

The wolf, coyote, jackal, and domestic dog (including the dingo) have 78 chromosomes arranged in 39 pairs. They are interfertile (unless size or behavioural differences obstruct mating) and produce fertile offspring. The wolf, coyote, and golden jackal diverged around 3 to 4 million years ago. The African Wild Dog also has 78 chromosomes, but is considered distinct enough (with different physical features) to be placed in its own genus. Other members of the dog family diverged 7 to 10 million years ago and are less closely related. These cannot hybridise with the wolf-like canids and they have different numbers of chromosomes: the yellow Jackal has 74 chromosomes, the red fox has 38 chromosomes, the raccoon dog has 42 chromosomes, the Fennec fox has 64 chromosomes. Differences in chromosome number are not always a barrier to producing viable hybrids; it depends on the gene combinations in the hybrid and whether these allow an embryo to develop. However, large differences in chromosome number make female hybrids poorly fertile and male hybrids sterile. In the case of dogs and foxes, it appears that viable embryos are not formed.

People wanting to improve domestic dogs have sometimes bred them back to wolves; as well as compensating for genetic problems in domestic dogs, the wolf-dog hybrids tend to be dominant and less domesticable. Grey wolves have been crossed with wolf-like dogs e.g. German Shepherds, Malamutes. Many become a problem in adulthood, usually because the owner expects them to act like a domestic dog and is not prepared for their more wolf-like behaviour. The Saarloos breed is derived from such crosses. Parts of Europe, the wolf naturally hybridises with stray domestic dogs resulting in mongrel populations. The Australian Dingo (a feral, rather than wild, species) also hybridises freely with domestic dogs. Harmsworth Natural History (1910, main authors R Lydekker, Sir H Johnston & Prof JR Ainsworth-Davis) notes The dingo breeds freely with the various European dogs introduced by the colonists” and classes the dingo as a descendent of Asian domesticated dogs rather than a distinct species.

An unconfirmed female fox-dog hybrid (terrier/fox) has been reported in the UK, unfortunately it was euthanized when the owner died because no suitable home could be found for it. Although gamekeeper folklore claims that Terrier bitches can produce offspring with dog Foxes, there are no authenticated hybrids between domestic dogs and Red Foxes. According to anecdote, "dox" (fox/bitch) hybrids are stronger and more vigourous than either parent and can be bred to other doxes. Thus far, all reported dox hybrids have turned out to be natural variation in the domestic dog. The great genetic mismatch between red foxes and domestic dogs makes the dox hybrid no more real than a cabbit (cat-rabbit) hybrid.

There has been a reported cross between a domestic dog and a South American fox, but the latter was most likely a fox-like wolf and not a true fox.

Harmsworth Natural History (1910, main authors R Lydekker, Sir H Johnston & Prof JR Ainsworth-Davis) notes the inability of the dog to form hybrids with the fox: ”Additional testimony that foxes have nothing to do with the origin of domesticated dogs is afforded by the fact that hybrids between the fox and the dog, notwithstanding numerous specimens of supposed hybrids of this sort which are from time to time brought to notice, are of extreme rarity, if not altogether unknown.

There was a report of a dox in Saskatchewan, Canada. This was believed to be the result of a miniature sheltie bitch with a wild fox (fox species not identified). One out of the three offspring survived, this being a sterile female that resembled a fox. However, the variability of dogs in appearance make it impossible to determine whether an animal is hybrid based on looks alone. I have not found genetic evidence to support the hybrid identity.

Coy-dogs (male coyote/female dog) have occurred e.g. a Coyote/Irish Setter mix (resembling a liver-coloured, spaniel-like dog) has been reported. Coydogs were once believed to be present in large numbers in Pennsylvania due to Coyotes being in decline and domestic dogs being available as mates. Most were probably naturally occurring red or blond Coyotes or feral dogs. The breeding cycles of dogs and coyotes are not synchronized and studies indicate that Coyote-Dog interbreeding is uncommon. If it such interbreeding was common, the Coyote population would acquire more and more dog-like traits.

The converse mating results in a Dogote and there is currently one known Dogote which arose from a male German Shepherd/female coyote mating in the wild. Hybrid pups were found after a female coyote was shot. The adult Dogote resembled a German Shepherd in colour.

Coyotes have also been crossed with Australian dingoes. This cross was made at a time the dingo was considered to be different species to the domestic dog.

The dingo has also been crossed with the wolf. According to Vol 1 of "Animal Life and the World of Nature" (1902-1903), Mr HC Brooke owned a female wolf which had become quite tame and was an excellent mother. This wolf was the mother of the wolf-dingo hybrid owned by Lord Walter Rothschild. The book also depicted a tame dingo owned and exhibited by Mrs HC Brooke, so it's likely that one of those dingoes was the sire of Lord Rothschild's hybrid.

Coy-wolves (Coyote/Wolf) have occurred in captivity or, rarely, in the wild where the choice of same-species mates has been limited. Coyote/Red Wolf hybrids have been found. Some consider the American Red Wolf is not to a true species because it can hybridize with both the Grey Wolf and the Coyote; however it is now known that hybridization between species (in general) happens more often than previously thought. Some consider it a Grey Wolf/Coyote hybrid and use this argument to prevent conservation of the Red Wolf. Some hybridization occurred when pure Red Wolves were in decline and interbred with more numerous Coyotes. More recent studies in many mammals shows that true species can and do hybridize and that the species boundary is preserved by geographic or behavioural separation (wolves will kill coyotes), not by genetic separation.

The Wolf and Jackal (an African wild dog) can interbreed and produce fertile hybrid offspring. In Russia, Dog/Jackal hybrids were bred as sniffer dogs because Jackals have a superior sense of smell and Huskies are good cold climate dogs. Male Jackal pups had to be fostered on a Husky bitch in order to imprint the Jackals on dogs. Female Jackals accepted male Huskies more easily. The half-bred Jackal-Dogs were hard to train and were bred back to Huskies to produce quarter-bred hybrids (quadroons). These hybrids were small, agile, trainable and had excellent noses. They are called Sulimov Dogs after their creator and may one day be registered as a working breed of dog.

This extract from the "Book of The Dog" by William Youatt (1846) demonstrates nineteenth century beliefs about hybridisation between different species. It was known that a dog and wolf could be crossed and that the union would produce vicious mongrels, but there were claims of hybrids now known to be impossible: "There are some naturalists that even go so far as to state that the different varieties of dogs are sprung from, or compounded of, various animals, as the hyaena, jackal, wolf, and fox. The philosophic John Hunter commenced a series of experiments upon this interesting subject, and was forced to acknowledge that "the dog may be the wolf tamed, and the jackal may probably be the dog returned to his wild state. The ancient Cynegetical [dog] writers were not only acquainted with the cross between the wolf and dog, but also boasted the possession of breeds of animals, supposed to have been derived from a connection with the lion and tiger. The Hyrcanian dog, although savage and powerful beast, was rendered much more formidable in battle, or in conflict with other animals, by his fabled cross with the tiger. In corroboration of this singular, but not less fabulous belief, Pliny states that the inhabitants of India take pleasure in having dog bitches lined by the wild tigers, and to facilitate this union, they are in the habit of tieing them when in heat out in the woods, so that the male tigers may visit them."

In "Origin of Species" (1859) Charles Darwin wrote The German Spitz dog unites more easily than other dogs with foxes [...] certain South American indigenous domestic dogs do not readily cross with European dogs ..." and in "Darwinism An Exposition Of The Theory Of Natural Selection With Some Of Its Applications" (1889), Alfred Russel Wallace commented: "Dogs have been frequently crossed with wolves and with jackals, and their hybrid offspring have been found to be fertile _inter se_ to the third or fourth generation, and then usually to show some signs of sterility or of deterioration." (The deterioration was due to inbreeding among the hybrid offspring)

In "The Variation Of Animals And Plants Under Domestication" Charles Darwin wrote: "Buffon got four successive generations from the wolf and dog, and the mongrels were perfectly fertile together. [...] M. Flourens states positively as the result of his numerous experiments that hybrids from the wolf and dog, crossed inter se, become sterile at the third generation, and those from the jackal and dog at the fourth generation [attributed to inbreeding]. ('De la Longevite Humaine' par M. Flourens 1855 page 143. Mr. Blyth says ('Indian Sporting Review' volume 2 page 137) that he has seen in India several hybrids from the pariah-dog and jackal; and between one of these hybrids and a terrier. The experiments of Hunter on the jackal are well-known. See also Isid. Geoffroy St.-Hilaire, 'Hist. Nat. Gen.' tome 3 page 217, who speaks of the hybrid offspring of the jackal as perfectly fertile for three generations.) [...] Mr. Philip P. King, after ample opportunities of observation, informs me that the Dingo and European dogs often cross in Australia. [...] Several years ago I saw confined in the Zoological Gardens of London a female hybrid from an English dog and jackal, which even in this the first generation was so sterile that, as I was assured by her keeper, she did not fully exhibit her proper periods; but this case was certainly exceptional, as numerous instances have occurred of fertile hybrids from these two animals. [...] We have already seen how often savages cross their dogs with wild native species; and Pennant gives a curious account ('History of Quadrupeds' 1793 volume 1 page 238.) of the manner in which Fochabers, in Scotland, was stocked "with a multitude of curs of a most wolfish aspect" from a single hybrid-wolf brought into that district. [...] the jackal is prepotent over the dog, as is stated by Flourens, who made many crosses between these animals; and this was likewise the case with a hybrid which I once saw between a jackal and a terrier. "

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