Hybridization with dogs and other Canis

Although dogs and gray wolves are genetically very close, and have shared vast portions of their ranges for millennia, the two generally do not voluntarily interbreed in the wild, though they can produce viable offspring, with all subsequent generations being fertile. In North America, black colored wolves acquired their coloration from wolf-dog hybridization, which occurred 10,000–15,000 years ago. Although wolf-dog hybridization in Europe has raised concern among conservation groups fearing for the gray wolf's purity, genetic tests show that introgression of dog genes into European gray wolf populations does not pose a significant threat. Also, as wolf and dog mating seasons do not fully coincide, the likelihood of wild wolves and dogs mating and producing surviving offspring is small. Like pure wolves, hybrids breed once annually, though their mating season occurs three months earlier, with pups mostly being born in the winter period, thus lessening their chances of survival. However, one genetic study undertaken in the Caucasus Mountains showed that as many as 10% of dogs in the area, including livestock guardian dogs, are first generation hybrids. The captive breeding of wolf-dog hybrids has proliferated in the USA, with 300,000 such animals being present there.

F1 hybrid coyote-gray wolf hybrid, conceived in captivity
The gray wolf has interbred extensively with the eastern wolf across northern Ontario, into Manitoba and Quebec, as well as into the western Great Lakes states of Minnesota, Wisconsin, and Michigan, producing a hybrid population termed Great Lakes boreal wolves. The boreal wolf is 25% larger than a pure eastern wolf, and typically has a similarly colored gray-fawn coat but, unlike the eastern wolf, can also be black, cream, or white. It also specalises on larger prey such as moose and caribou rather than white-tailed deer. Unlike pure eastern wolves, Great Lakes boreal wolves primarily inhabit boreal rather than deciduous forests.

Unlike the red and eastern wolf, the gray wolf does not readily interbreed with coyotes. Nevertheless, coyote genetic markers have been found in some wild isolated gray wolf populations in the southern United States. Gray wolf Y-chromosomes have also been found in Texan coyote haplotypes. In tests performed on a putative chupacabra carcass, mtDNA analysis showed that it was a coyote, though subsequent tests revealed that it was a coyote–gray wolf hybrid sired by a male Mexican gray wolf. In 2013, a captive breeding experiment in Utah between gray wolves and western coyotes produced six hybrids through artificial insemination, making this the very first hybridization case between pure coyotes and northwestern gray wolves. At six months of age, the hybrids were closely monitored and were shown to display both physical and behaviourial characteristics from both species.

Although hybridization between wolves and golden jackals has never been observed, evidence of such occurrences was discovered through mtDNA analysis on jackals in Senegal and Bulgaria. Although there is no genetic evidence of gray wolf-jackal hybridization in the Caucasus Mountains, there have been cases where otherwise genetically pure golden jackals have displayed remarkably gray wolf-like phenotypes, to the point of being mistaken for wolves by trained biologists.