Doing study on population ecology of wolves, we faced with the phenomenon of hybridization beetwen wolf and domestic (actually stray) dog. Such hybridization of wolves and dogs is a question worthwhile to consider in the aspect of the wolf reproduction biology. Firstly, it relates to breeding of wolves; secondly, such a hybridization may be considered as the extreme effort of the last wolf individuals to maintain the declined population with such an abnormal reproduction.
Some deliberate hybridization between a wolf and a hunting dog was known in Belarus from the Middle Ages (at the time the Great Lithuania). For instance, this was mentioned, for several times in the Radzivils’ metrics for the magnate game economy. Not once they mentioned that such a hybridization was revealed somehow in the wild. However, still sometimes people (mainly game-related) try to produce wolf-dog hybrids in Belarus without a strong reason for that. As to such hybridization in the wild terrain, a few cases of wolf and domestic dog interbreeding were known in the 1980s, 1990s and early 2000s in different districts in Belarus. We assume that it used to happen, but only quite rare. Therefore, when we sampled wolves in Belarus in the early 2000s for a genetic study, none of these wolves showed dog ancestry (Stronen et al., 2013)
Since 2010, cases of wolf-dog hybridization were registered markedly more often. Personally, we registered seven such cases, for which we saw a lot of details or we found the dens ourselves. For four cases the prey supply was rich or very rich, and the wolf population density was low. In one case the prey supply was rich, and the wolf population density was medium locally, but population-wide it was low. In two cases the prey supply was moderate, and the wolf population density was medium locally, but population-wide it was low.
From the above material, which is not extended, but anyway, it looks like that in Belarus mostly two possible factors determine wolf-dog hybridization: rich prey supply and low density of the wolf population. Nevertheless, wolf and domestic dog interbreeding may happen in another situation, when the wolf population density is not so low, and it is possible for a sexually matured wolf to find a mate. Also, food base may be not so rich. Two cases in Naliboki Forest (June 2014 and April 2016), which were investigated by us in detail, suggest the above statements.
A very interesting question, is how such a wolf-dog pair is created, because in Belarus it is so common that wolves kill domestic dogs as much as possible even on a peasants’ yard and in mating season (e.g. Sidorovich, 2011). We will say that the killing of domestic dogs by wolves mostly happened during January-March, i.e. the mating season in wolves, and before and after mating. Fairly often wolves eat such victim-dogs. Interestingly, what induces the behavioural switch of wolves from such an aggression towards dogs to a courtship and copulation with them.
Concerning that we propose the following hypothesis. An adult male wolf may pair with an adult female dog, which lives as a wolf in a wild terrain by providing food for itself (hunting and or scavenging) and it keeps a territory with intensive marking. So, indeed such a stray dog lives like a wolf. In the ambient conditions of Belarus with a harsh winter this is only feasible for a smart dog, if there are many preys and carrion from the dead prey. Therefore, the factor of a rich prey supply is so important. Such a stray dog naturalizes in dog pairs quite often. In such a case, the male wolf kills the male dog and pairs up with the female dog.
Indeed, among the seven studied cases mentioned above, there were four cases, in which it was known that the female dogs stayed in the wild terrain for long (longer than half a year). In two of these four cases, the female dogs stayed with a male dog, and those male dogs disappeared after pairing of the female dog with a male wolf. In all seven cases only female dogs paired with male wolves, but we did not reveal any opposite case, where a male dog paired with a female wolf.
As to captive hybridization of dogs and wolves in Belarus, both ways are used to get hybrids, i.e. male wolf-female dog and female wolf-male dog.
The last curious question is why the wolf-dog hybridization was so amplified since 2010. We may only make a comment on this. In Belarus in the beginning of the 2010s, there appeared many commercial game husbandries that breed a lot of wild ungulates with feeding stations and fields. These wild ungulates include not only native, but even exotic species like a mouflon or fallow deer. These non-native ungulate species are not good survivors during quite harsh winters, which commonly happen in Belarus. So, carrion from ungulate carcasses appeared more common in the territory of game husbandries. Moreover, these commercial husbandries are well equipped, have enough finance and wardens to eliminate wolves, sometimes, for several times per year. Moreover, it is usual that hunters avoid killing of stray dogs, because they may belong to another hunter or local peasants. They avoid problems that may appear, if they kill such a dog. So, in such commercial game husbandries, there are all the suitable conditions for stray dog naturalizations, their survival may last long enough, and there are a few wolves hardly searching for a mate.
In Naliboki Forest we had the possibility to also trace the breeding behavior and raising of the pups by two wolf-stray dog pairs. These stories are worthwhile to tell.
The first story is as follows. During late autumn at the boundary of Naliboki Forest in the surroundings of a cattle farm and village we detected a pair of stray dogs. They mostly hunted small rodents and scavenged around the cattle farm and in the rubbish dump of the village. Mainly they avoided interactions with people, were mostly active at night, so it was hard to observe them. Quite often they rested in the forest on the distance up to one km from the village. The dogs marked three terrain roads that coming from the village to the forest. The stray dogs lived there between the cattle farm and village rubbish dump till the end of January, when the male dog was killed by a wolf or a pack of wolves. One of the workers of the cattle farm looked through the spot of the kill. There were remains of the male dog only, whereas the female dog had disappeared. In April a few locals from the village observed a strange pair: a big wolf and a medium-sized dog on the road not far away from the cattle farm. In early June we noticed that there was like a wolf and dog pathway coming from an abandoned peatory to road. Searching for a den of the possible wolf-dog pair, we discovered the den in a few hours. The den was a burrow with several entrances (enlarged former red fox burrow), which was situated in a big peat mound that was densely overgrown with raspberry and blackberry bushes. On the distance of about seven meters from the den, we were horribly attacked by the mother dog. It was the same female dog as the one that lived with the killed male dog. The mother dog chased us for about 200 meters, but we were not wounded defensing ourselves by axe and a big wooden stick. There were three repeated attacks during these 200 meters that we were gradually retreating. At the den we only noticed one rather big pup (about 2 kg) and there were a lot of feathers from poultry (mainly from chicken and goose) around. Later we learnt that in the village chicken and goose disappeared one by one; perhaps, the mother dog visited easily the village and killed poultry. Here it is worthwhile to notice that in Belarus it is common that in villages dogs that are freely walking on streets are common; nobody knows all the dogs in a village and nobody chases them. In August-September the wolf-dog pair began to attack cattle in the daytime, mainly young horses and adult caws to get an udder. Hunters tried to kill the wolf-dog pair, but only three other domestic dogs were randomly killed in the village surroundings. In December, while censusing wolves, we faced with a pack trail consisted of one trail of a big wolf and four dog trails. It was on the distance of 4 km from the cattle farm. There was no doubt that the trails belonged to the hybrid family. They stayed there till February on the area of about 220 km2. They marked the area a lot, may be even more that an average wolf pack. Just before mating season in wolves they had disappeared. We heard something about that hunters killed some dogs in forest in that area, but we are not sure that it actually happened. Even if that was the case, we are not sure that all the individuals from the hybrid pack were actually killed. That is all we know about the wolf-dog couple. We would like to add that the food base for wolves was rich and the wolf density was rather high in Naliboki Forest that winter – 2.3 inds per 100 km2.
Another story of a wolf and stray dog pair took place in the center of Naliboki Forest. Prey supply for wolves was moderate, and the wolf population density was 1.4 inds per 100 km2 (so, wolves were not rare). In a hamlet only one house was occupied by a man, who was ill on alcoholism. The man has a female dog that looks like a Siberian lajka. The dog was ranging free in the hamlet proximity for most of the time. It was smart to hunt something, and according to the owner’s words sometimes it brought a roe deer, beaver or hare on the yard. Once this dog killed our domestic red fox and ate it entirely; merely the fox tail and radiocollar were left on the spot of the kill in the hamlet surroundings. Nevertheless, fairly often the dog was keeping on the chain leash at its cabin. Once in the late autumn the man started drinking alcohol too much and continuously. The female dog was not fed anymore for long. The man was so kind that released the dog from the leash before he went on the hard booze. In a month, the man was taken to a hospital. The female dog survived and even succeeded to support with food its two male pups of the year, which mostly continued to stay on the abandoned yard. In mid-January the female dog was registered living with a big male wolf; in the hamlet surroundings we found many wolf and dog paired trails and the pair was observed for several times. They mainly hunted roe deer and beavers and lived together till May. The pair visited the hamlet and brought some food for the two dog pups. They looked not starved. The female dog gave the next birth somewhere in the forest in mid-April and on the 22nd of April it relocated the new pups to the owners yard. There, it dug a deep and broad burrow under the shed base close to its cabin. Also, the pups were placed in several open couches at the shed. The male wolf regularly visited the abandoned hamlet at night and fed the mother. During that time the yearling dogs were chased from the hamlet, and they ranged in the neighboring village on a distance of 13 km apart. Suddenly the owner came back to the house; he accepted the pups and they were taken by other locals as well. The female dog still walked free, and not seldom we registered their paired trails of the male wolf and female dog on sand roads in the proximity of the hamlet. Till March of the next year, the female dog ranged with the male wolf. They produced a lot of territorial marking and regularly visited the former owners yard. At that time, the man sold the house and moved to the neighboring village. In the late March the man captured the female dog on the yard and moved it to the village. Being on the chain leash, it gave birth again in the new cabin of the new house of the owner. The male wolf seemingly disappeared. We did not follow the story anymore.
Zoology by Vadim Sidorovich 
In fact, this is a very important issue and we must take it very seriously if we are to make any difference in the world. In this post here lobo-guará I saw more important information about it
A very good and weel written comment. It reminds me of the way that Sabanev (1876) write about wolvess.