The first known hybrid offspring of domestic dogs and golden jackals have been identified.
The dog-jackal hybrids, reported in the latest issue of Royal Society Open Science, add to the growing body of evidence -- mating between dogs and coyotes, grey wolves, and Ethiopian wolves has previously been documented -- that dogs can breed with just about any type of wild canine and produce fertile offspring.
Because dogs can successfully breed with wild canines, the line separating the various species of canines becomes ever more blurry.
"If we refer to the biological species concept, meaning that species are (defined as) groups of interbreeding natural populations that are reproductively isolated from other such groups, then indeed they cannot represent separate species," the study's lead author, Ana Galov, an assistant professor at the University of Zagreb's division of biology, told Discovery News.
She added, though, that biologists have proposed a range of other species definitions and concepts, such as separating animals by ecological niches and/or physical characteristics. When these are considered, dogs and different types of wild canines are thought to represent unique species.
The jackal-dog hybrid investigations began after hunters in Croatia legally killed what they thought were ordinary golden jackals. Jackals are considered pests in Croatia because they target game calves, such as those from wild boar and roe deer, Galov explained.
Upon closer examination, though, the animals the hunters had killed displayed some dog features. DNA, chromosome, and other analysis confirmed that the canines were indeed the hybrid offspring of golden jackals and domestic dogs.
In the two cases under study, both originally involved a female jackal mating with a male dog, pairings that resulted in pups.
Galov said that could be because male dogs may "be sexually stimulated at any time during the year, while golden jackals have a well-defined mating season, from December until February." She added that dog owners in Croatia tend to let their male dogs roam more freely while restraining their female dogs, especially during in-heat cycles.
Such free-roaming, free-breeding dogs were also the subject of another new paper, published in the journal Proceedings of the Royal Society B. For this research, senior author Wieslaw Bogdanowicz, of the Polish Academy of Sciences' Museum and Institute of Zoology, and his team used DNA to trace the evolutionary history of such dogs in Europe and Asia.
Bogdanowicz and his colleagues agree that dogs interbreed with wild canines. They even found that free-breeding dog populations are genetically distinct from purebred dogs.
The researchers also determined that a large, early, domesticated dog population from East Asia gradually expanded westward over time.
"It is very likely that migrating populations of early farmers were followed by dogs, but these dogs were not feral, although they were likely free-breeding," Bogdanowicz told Discovery News.
Bogdanowicz and his team further determined that the first wave of migrating dogs was later replaced by a second, even more expansive wave that nearly wiped out the earlier breeds. Native American dogs, for example, were later replaced by dogs of European origin.
A similar phenomenon is thought to have affected humans in Europe and Asia, with migrating humans from Africa gradually absorbing and replacing pre-existing Eurasian populations and cultures such as Neanderthals.
Interbreeding among canines has already affected wild canine populations within the northeastern United States and in the Arabian Peninsula, where dog hybrids seem to be more common. In these regions, "the species boundaries become more fluid," Bogdanowicz said.
Now, however, there is concern for the conservation of golden jackals. As Galov said: "Domestic dogs might affect the genetic composition of golden jackals [ and ] that could have harmful effects on the fitness of jackal populations. Another risk for jackals might be transmission of canine diseases from dogs."
Galov believes it's too early, however, to take steps to prevent such hybridization. She said further studies are needed to determine what effect such matings have on populations of jackals and other wild canines.