“On the problem of the origin of the domesticated dog and the incipient (aboriginal) formation of breeds”
By Alexander Vlasenko (Moscow, Russia)
Translation By Vladimir Beregovoy [ www.laikabreeds.com ]
In search for an answer to the question about the ancestors of the domesticated dog and where and when it originated, it is not enough to use an approach from the standpoint of one branch of biological science, such as genetics, morphology, comparative anatomy or ethology. Controversial results of genetic investigations and paleontological findings require the use of a complex analysis of obtained data.
Manwell and Baker (1983) wrote that the investigation of the origin of the dog is hampered by “disciplinary dogmatism” and that the origin of the dog out of the wolf is still a hypothesis. Coler-Matznick (2002) thinks that dogs originated from small Pleistocene wolf similar to the Dingo. Not sharing completely the traditional or the innovative point of view, I will try to explain and substantiate my position. To start with, I will discuss aspects of the biology of the wolf (Canis lupus) as a primary candidate to be an ancestor of the domesticated dog, which is supported by phenotypical (including behavior) similarities, the results of investigations of DNA and the fact that easy interbreeding between dog and wolf results in fertile offspring.
The contemporary species of wolf is subdivided into 25 subspecies, one of which, the Indian wolf, is separated by some researchers as a different species, based on DNA studies. All subspecies of wolf are capable of breeding with dogs and, moreover, different data indicated that from 5% to 40% of wolves of European populations actually are wolf/dog hybrids. There are breeds of dogs obtained by deliberate interbreeding with wolves: Czech Vlach, Saarloss Wolf-dog, Italian Lupo and currently developing in Russia the Volkosob. Certain wolf subspecies of North America have certainly resulted from interbreeding with dogs. Besides, based on DNA analysis, it is well known that the American Red Wolf (Canis rufus) is a result of the natural interbreeding of gray wolf with coyote, which took place during the last 12,500 years (most likely during the last 2,500 years). In captivity, jackal/wolf, jackal/dog and even coyote/jackal hybrids were obtained, although the natural ranges of jackal and coyote do not overlap. Thus, the fertility of mixed offspring alone cannot be considered as evidence of the origin of dog from wolf.
Furthermore, we should take into account that the contemporary gray wolf evolved under powerful anthropogenic pressure. The wolf became a permanent foe of humans and was subject to extermination since the emergence of livestock. This pressure influenced the wolf’s behavior. We know from publications by I. A. Arshavsky (1982) and G. Kh. Shaposhnikov (1966) that the environment with intensive functional stress results in changes of both phenotype and genotype. In other words, environmental stress is a driving force of evolution. Another side of the environmental pressure is the systematic extermination not only of the most bold and trusting wolves in the population, but also whole subspecies and, possibly, species. Besides, a considerable decline of wolf populations often results in the appearance of feral wolf/dog mixes, which become absorbed by the rebounding wolf population. Therefore, it is highly probable that the contemporary wolf is different in many qualities from that wolf, which existed at the time of the origin of the domesticated dog and the results of DNA analysis of their affinity should be treated with caution.
During recent years, international group of researchers led by P. Savolainen has shown that all dogs originated during a short time period between 5,400 and 16,300 years ago in the southern part of eastern Asia (south of the Yangtze River) out of small group of a few hundreds of individuals of small Chinese wolves. Based on archeological data this time range is between 11,500 and 16, 300 years. The idea of the east Asian origin of the dog out of an extinct Asian subspecies of wolf, Canis lupus variabilis Pei, 1934, is shared by other researchers (Olsen and Olsen, 1977) based on studies of morphology of skulls of wolves and dogs of different populations. However, Tsuda and co-authors (1997) came to conclusion that the dog was domesticated multiple times in different geographic regions.
This point of view is corroborated by the fact that mt- DNA extracted out of skeletal remains of pre-Columbian dogs revealed sequences not detected in samples taken from more then 350 contemporary dogs (Leonard et al. 2002).
Parker et al. (2004) distinguished four groups of dog breeds with definitely different fragments of mt-DNA. They described the process of the evolution of dogs of east Asian origin like the sequential divergence from a primordial wolflike ancestor, first, the Australian Dingo, then the Basenji and, finally, Laika-like northern dogs. They consider Asian sighthounds the last group diverged from the dog’s ancestral species.
Lindblad-Toh with co-authors (2005) determines the age of the domesticated dog within range of 15,000 to 100, 000 years and proposed multiple domestications. Finally, the group of researchers of R. Wain (2010) published their results suggesting that the haplotypes of dogs are the closest to haplotypes of the Middle Eastern wolf and that wolves of the Middle East were the source of the genetic diversity of the domesticated dog. How can we understand these controversial results, if it is well known that the radiocarbon method determined the age of the oldest fossil skulls of domesticated dogs as 26,000 years (Cauvet Cave, France), 33,000 years (Razboinichya Cave, Altay) and Predmosti (Czech Republic) and even 36,000 years (Goye Cave, Belgium)?
There is no clear answer to this question so far. One possible and most likely cause of the discrepancy in the data is the extinction of several ancient populations of dogs caused by epidemics, such as distemper. In the past century, in the vast territories of the Russian north and in the Far East distemper almost wiped out nearly all dogs. If dogs originated in southern Asia and then they were first to pass selection for resistance to a virus caused disease, then these dogs migrated to other countries, where their chances of survival would be considerably higher than those of local dogs. It is also possible that the mechanism of “the molecular clock” works not quite as well as modern geneticists believe and the process of domestication and evolution of the dog in different regions of the Earth might be accompanied by a parallel channeled process of recombination of DNA, and likewise the recombination of DNA might take place in the wolf. As a result, the comparative analysis of DNA produced an error.
The results of the Russian-Vietnamese Tropical Center expedition, 2006-2009, the purpose of which was to run a survey of the variation of the aboriginal dogs of Vietnam, allowed the analysis of the origins and evolution of the domesticated dog from a different perspective.
In 2008, in the Ma River Valley (Khanh Hoa Province) I discovered a very small population of wolf-like dogs, extremely similar to Indian and Arabian subspecies of wolves. The distribution of this variety of dog is limited in a small territory populated by Man people, one of the tribes of Miao (Hmong). According to available data, these people are one of the first ethnic groups to have settled in contemporary Vietnam, which took place about 1000 years ago, near the once large principality of Champa. A high percentage of the people of this principality originally came from India. If 150 years ago Ch. Darwin had reliable information, one of his sources reported that in the upper Gung River the local people had similar dogs. We can believe that this was one hypothetical way of populating Indochina with wolf-like dogs. According to paleontological data, the gray wolf never existed here. However, Dinesh K. Sharma, Jesus E. Maldonado, Yadrendradev V. Jhala and Robert C. Fleischner (2003) are sure that the Indian wolf (from India and Pakistan), which they consider a separate species and part of Himalayan wolves, did not leave a trace in the mt-DNA of the dogs of Vietnam and, therefore, could not originate from their populations. This does not matter. The fact of the survival of an aboriginal population of dogs with considerably greater similarity to wolves than to the breeds of wolf hybrids created by breeders, who were trying to maximize their similarity to wolf by selection, is most interesting. This indicates that the presence of morphological characteristics of domestication is not a necessary feature of domesticated dogs. The very fact of the existence of relic wolflike dogs tells us that at the first stages of the domestication of the wolf morphological (phenotypical) changes did not exist. It was simply a socialization of wolf to life with humans. This is done by Australian aborigines with the Dingo until present time. Probably in the past, the behavioral makeup of the wolf allowed this to be done without many difficulties. Thus, a wolf could become a dog and at the same time remain a wolf and even continue interbreeding with free feral wolves.
Based on the description above, I can say that some variety of wolf was one of ancestors of dog, but it was not enough to claim that only one wolf subspecies was the dog’s ancestor. Fossil remains of wolves cannot be considered a priori as belonging to the same species, because identical skeletons could belong to animals with different soft parts, systems and behavior, including way of life in general.
The development of morphological characteristics typical of domesticated dogs could appear at a considerably later time, after the wolf became domesticated. Perhaps, the traits of domestication in skeleton appeared last. Comparative morphology shows that in domesticated dogs of different breeds the same muscles can be attached not only to bone, but also with an expanding area of attachment, involving other muscles, tendons and skin and differ in the degree of differentiation and integration without changes in the shape of the bone.
Analyzing incipient breed formation (performed by the methods of primitive peoples’ selection), general trends in changes of dogs’ phenotype should be taken into account. Such markers of domestication as piebald color, lop ears, changes in shape of tail and structure of coat, etc. appear spontaneously as a result of systematic selection for behavior. These changes cause a shift in the chain reactions of regulatory mechanisms of homeostasis (D. K. Belyaev and L. N. Trut, 1989). The emergence of these characters in the offspring is not necessarily accompanied by their fixation in the subsequent generations, unless they are deliberately selected for breeding. The direction of selection depends primarily on the practical needs of life and economic activity of a particular ethnic group. Therefore, I see two basic types of selection for visible phenotype, deliberately or not, used in the process of breed formation, depending on the purpose of keeping dogs.
The first type includes cases, when dogs were needed for guarding or hunting. These dogs should retain a body size for optimal performance and have new conspicuous characteristics of the appearance, allowing them to be quickly distinguished from other animals. In this instance, the choice of traits for selection is determined by the benefits for working qualities and survival.
The second type of selection includes cases, when dogs are considered as food. Then, the best result is achieved by selecting for juvenile traits. Infantile behavior is displayed as dependence on a higher ranking individual in a social group, in this case on the human owner, and a reduction in the size of the territory in which it roams. In physiology this selection favors neoteny, the acceleration of development and the ability to breed at a younger age. In the body structure, preferential traits are a shorter head with a dish face, a stronger pronounced stop, a domed skull and a smaller body size. A smaller dog requires a smaller amount of valuable protein rich food and it shortens the period of susceptibility to diseases caused by malnutrition. Besides, a small dog can be eaten by the family at one meal time, which is also important in an environment with a hot climate and without refrigerators. Such dogs become better adapted to survive on poor quality food, refuse and even coprophagy (Coppinger, R, and L. Coppinger, 2001).
Because the ancestor of domesticated dogs with its characteristic “wild type” met the requirements of humans, it means that at that time there was no need to distinguish domesticated dog from its wild relatives. This is because they did not present any danger and did not cause any harm to humans. Therefore, the most likely reason for breeding dogs with a different appearance emerged with the appearance of livestock and poultry, subjects to depredation by wild Canidae. At the same time, there was an additional reason for selecting in favor of different dogs. Under conditions of a shortage of hunting grounds and conflicts between adjacent tribes, it was important that hunters could tell apart their own dogs from dogs of neighbors and conspicuous variation in dogs became beneficial. However, in either case, the cause-effect link is traced between population increase, transition to a settled way of life, deficit of food resources obtained by hunting and gathering and the establishment of certain complexes of domestication traits in local populations of dogs.
This is well illustrated by comparing the pattern of the distribution of dogs of certain aboriginal breeds with the dispersion of different peoples in North Vietnam. The aboriginal dogs of Vietnam represent a unique model for study of microevolution, the mechanisms of domestication and the primitive formation of breeds.
In general, the dogs of North Vietnam are represented by a population with very diverse composition, in which all morphological markers of domestication are present. Persistently inherited morphological types are characterized by their own peculiar and limited complexes of characters. Many of Vietnamese breeds are absolutely identical to some old well known breeds, or their original types, of the world in their appearance and body structure. The regions of the world, where these breeds are traditionally bred, are separated by thousands of kilometers from the region of our study. The majority of the breeds we found in North Vietnam can be united in groups of close origin, in which transitional types and ancestral forms can be clearly observed, and they are represented by a series of forming variation.
The dogs I named “Viet Dingo” are smallish dogs with infantile traits. These dogs are associated with the culture of rice paddies and typological traits of local populations allowing the tracing of directions of development of land for this form of agriculture. Thus, the maximal number of typological variants of Viet-Dingo is found in the eastern provinces of Lang Son and Tainguen, where Tai people live. Dogs of Viet-Dingo type are most numerous in North Vietnam, but their share in the local population declines from east to west and to north-west. The frequency of Viet-Dingo is highest mainly in the fertile valleys cultivated for rice production, but also in other regions with conditions unfavorable for agriculture, where people are poor and do not keep big dogs, because of food shortages. Moving along the river valleys to the west and north-west, the typological diversity of Viet-Dingo dogs declines and local populations are represented only by one-two types instead of the six types found in the eastern provinces. Judging by the variation series, this group of dogs originated as a result of the interbreeding of small Dingo-like dogs with Laika-like dogs either in north-eastern Vietnam or in the border territories with China.
A large type of aboriginal dog is bred mainly by the Hmong, who until recently made a living by hunting, and the distribution of these dogs is clearly associated with the distribution of the Hmong people.
It is possible to say that most probably the bobtail dogs of the Hmong, which are very similar to the now lost bobtail type of Karelian Bear Dog, called Shong were distributed from the Northern Province of Ha Gyang along the Shonglo River and further to the south into the province of Son La. Dogs that are phenotypicaly similar to Siberian Laikas and Arctic sled dogs (Figs. 1-5) are distributed in the form of a semicircle from the northern part of Lao Cai province, along the Shongda River, across northern part of Yen Bai, Tuyen Quang and Bac Can provinces to the center of Cao Bang province, in other words, from northwest to southeast and further to the east. These dogs are most typical in Lao Cai province and are rather common, but they decline numerically to the east. In Lao Cai, we found the best looking representatives of this breed, very similar to big Japanese Laika-like dogs, and populations of ancestral Chau-Chau (Bakha Dog) are concentrated here. Smallish Laika-like dogs similar to Karelo-Finnish Laikas and Spitzes are most likely the product of mixing with small Dingo-like dogs (Fig. 6).
The series of variation is best represented in the group of hound-like dogs, starting from a primitive type identical to the hounds of Tibet, West China, known in Russian literature as Mahugou (Fig. 7), to a primitive sighthound type (Fig. 8) and to various variants of analogs of ancient Russian scent hounds (Fig. 9) and dogs similar to the modern Foxhound (Fig. 10), Labrador Retriever ( Fig. 11) and to dogs very similar to herding dogs of Scotland (Fig. 12) and also to types of livestock guarding dogs of Tajikistan and Tibet (Figs. 13), Caucasian and Karakachan Dogs, but relatively smaller in size (Figs. 14-16).
Different forms of dogs I included in the scent hound type dogs group are distributed mainly in provinces of Lao Cai, Yen Bai and Tuyen Quang. They were most diverse in Lao Cai province. The uniqueness of this series of variation is in the fact that there is nowhere else in the world a continual transition between types of dogs considered by cynologists as breeds of absolutely different origin and purpose. The theoretical projection of the series of variation in both groups in the direction of diminishing specialization of the appearance brings us to a prototype (archetype) common to both Laika-like and hound-like dogs. I was lucky to find a type of dog that combined in its appearance basic traits of both groups. I think that the pictures alone show that this prototype is one step different from the wolf. However, if we consider the variation series of the scent hound group, selecting only light and medium built dogs, their hypothetical ancestor would be very different. In this case, the variation series will be ascending to dogs that I tentatively named the Big Chinese Dingo.
The North Vietnamese type of Big Dingo (Figs. 17-18) has differences from the South Vietnamese Dingo, which were probably caused by interbreeding with Laika-like and scent hound-like dogs. I can say that this type is closer to the wolf, especially to the Tibetan wolf. The big Indochinese Dingo is not numerous in Vietnam, although it occurs in most of the regions surveyed by the expedition.
Its southern morphological type (Fig. 20) is very similar in the appearance to the basic, not mixed, type of Australian Dingo. However, the Australian Dingo often has domestication markers and pedomorphic traits characteristic of the North Vietnamese “Viet-Dingo”. According to the results of molecular genetic studies, the time of divergence of the Australian Dingo from Dingo-like dogs of Southern China, Taiwan and Polynesia, belonging to the same form as the “Viet- Dingo”, was determined as 5,000 years (Savolainen, 2002). The Big Indo-Chinese Dingo is probably considerably older than the age of populations in the regions surveyed.
The Australian Dingo had been secondarily changed to running wild during several thousands of years until recent times, when a considerable part of its population became mixed with stray dogs of European and American breeds. Nevertheless, it retained and possibly even obtained new, stabile morphological traits identical to the Big Indo-Chinese Dingo. Under conditions of genetic isolation and feral life, the Australian Dingo did not develop traits similar to the wolf or jackal, surpassing those in variation of the Big Indo-Chinese Dingo.
On the other hand, it is even more surprising that in the territory of Indochina a form of the Big Indochina Dingo survived. In the best representatives of it, “wild” traits are even more pronounced than in the Australian Dingo. While being selected for behavior under conditions of domesticated life, the Big Indochinese Dingo should be subject to destabilizing selection and, living among Dingo-like, Laika-like and houndlike dogs, it should be genetically influenced by interbreeding with them. However, for some reason, some of the Big Dingos, living in different corners of Indochina, remain absolutely identical to type and have the same measurements, body structure and appearance. Such stability of morpho-type most likely tells us that other morpho-types, occurring around these dogs, are its descendants.
Based on these observations and considering the origin of dogs diligently, it would be reasonable to suppose that the Big Indochinese Dingo retains an unchanged complex of morphological characters of its ancestor, just as the wolf-like dog of the province of Thanh Hoa retains the traits of the wolf.
Unfortunately, the program of studies of the aboriginal dogs of Indochina was cut short at half way and we did not have enough time to the complete survey of dogs of Southern Vietnam. Now, using fragmentary and scarce data we can restore the general pattern of typological diversity of the dogs of the southern center of formation of breeds in Vietnam. Probably, its center was near the shores of the Siam Gulf or Andaman Sea. If the Australian Dingo carries intermediate traits of southern and northern Vietnamese dogs, then New Guinea Singing Dogs belong to the southern center of breed formation, to which dogs of Southern and Western Indochina also belong, as well as the dogs of the eastern shores of India (Figs. 21).
The result of the comparison of morpho-typical traits of dogs of the southern center of breed formation compels us to accept that the northern morpho-type of Big Indochinese Dingo was primary in relation to the southern one. The northern morpho-type was almost certainly ancestral to the variation series of big Dingo-like dogs. However, with small Dingo-like dogs of the southern center, the picture is not that simple. Among small southern Dingoes, there is one type, which represents the regular result of the reduction of body size, but it is relatively rare. The majority of small dingoes are represented by the variation with too big a hiatus, separating them from the Big Dingo, and, according to known regularities of changes in body structure caused by diminishing size; they cannot be drawn directly out of the Big Dingo.
In the example of the longhaired variation of small Dingo-like dogs of the southern center of breed formation , it seems that the ancestral form of this group had much in common with the morpho-type of red wolf.
The southern morpho-type of Big Indochinese Dingo could be the result of interbreeding the northern morpho-type with an ancestral form of small southern dingoe. Because nothing is known about the existence of hybrids of the Dhole with dogs and nobody ever checked the possibility of such crossing, we can propose that at some time in the past either a sibling species of Dhole was domesticated or the evolution of big and small Dingo-like dogs of the southern center of breed formation was going on independently at a different time and in different regions.
The origin of small Dingo-like dogs out of wild ancestors is corroborated by the unusual physical peculiarities of the New Guinea Singing Dog that are poorly specialized in running but are well adapted to climbing and agility. Dissection of a small Dingo-like dog killed by an accident, purchased during the work of the expedition, showed that the amplitude of rotation movement of the forearm was about 1.5 times that of an ordinary dog. This dog was very similar to the New Guinea Singing Dog. Study of the proximal radius-ulna joint allowed us to discover the presence of a sesamoid bone, which protects the joint on the lateral side. Unfortunately, I could not confirm the presence of this trait in other representatives of this morpho-type, because of the premature closure of the research program. A similar and not constantly present structure in about the same part of body, the sesamoid bone in the tendon of musculus supinator, is found in the coydog (coyote and wolf hybrid, “Miller’s Anatomy of Dog”).
While discussing topics of comparative anatomy, I will make clear the following: we are interested in the possible inheritance of anatomical peculiarities from wolf to dogs of different breeds, because we have chosen the wolf as the “gold standard” of a healthy anatomy. The goals of my work are rather practical, they are usually considered as topics of veterinary medicine. First, until now, I had in my possession corpses of wolves sampled from populations of Tver, Rostov and Voronezh provinces, all territories known as places where in recent times wolves and dogs have interbred. In two cases I found characteristics of possible crossing with dogs. Second, these territories are very close geographically, and, therefore, I cannot extend the obtained results to all subspecies of wolf across its vast geographic range.
Evaluating the direction of morphological changes, I assume that specialization to running (both wolf and dog are specialized runners) always results in the reduction of the complexity of the locomotion apparatus numerically, for example, the structural consolidation of originally separate muscles with the enhancement of their integration.
Anatomically, dogs of different breeds conspicuously differ from each other. It is not quite a joke, when we say that the anatomy of dog does not exist; there is only morphology, because the range of variation is too big for one species. When we compare dog with wolf, supposedly dog’s ancestor, sometimes we find a mismatch. Some of the traits may be inherited and some may be lost. There are known cases of the loss of muscles as a result of dwarfism or specialization and some muscles can become hypertrophic. Finally, a new trait can emerge, not existing in the ancestral form. However, if we find an archaic trait in some breed of dog, which is absent in the supposedly ancestral form, but is typical of a higher taxonomical rank, then there is reason to doubt whether the modern wolf was ancestor of the dog.
Speaking of hereditary succession, not all found discrepancies can be easily explained. Thus, in sighthounds, whose locomotion apparatus is more similar to that of the wolf than that of other dogs, peculiarities of the integration and differentiation of the muscle-tendon system could be inherited from the wolf or obtained independently in the process of specialization. Therefore, comparing wolf and dog anatomically, I offer only one example, which in my opinion is vulnerable to criticism.
As I mentioned above, some scientists draw a line between wolf and dog based on craniological indexes. I paid more attention to the structure of the frontal bone. As a result of research of skeletal material in our collection, I found the following:
– The posterior chamber of the frontal cavity of the wolf (Figs. 22-23) has a complex architecture, which is relatively low at the medial margin, curved and higher positioned at the entrance and opening into the anterior chamber, which is connected with the dorsal nasal passage.
– In dogs (except the Hortaya, German Shepherd Dog and East European Shepherd Dog, which are similar in these features to the wolf) the posterior chamber of the frontal cavity (Figs. 24-26) is large and high, with a simpler architecture and has a wide, straight and low positioned entrance opening immediately into the nasal passage.-In the wolf, the area of the perforated plate is approximately 1.5-2 times as large as that in the dog, proportionally to body size.
The differences described above are easy to explain by functional anatomy. Frontal cavities, besides their function of a protective cushioning of the frontal bone, serve as an important part of the thermo-regulation of the body. One major problem of marathon runners is the disposal of excess heat generated in the muscles (Coppinger, L&R. Coppinger, 2002). In the process, maintaining the optimal temperature of the brain is most important (Schmidt-Nielsen, 1972). The main way of cooling the body of the dog and the wolf is by evaporation from the surfaces of the tongue, mouth and nose. However, the perforated plate, bordering the hard wall of the brain and with the rostral epidural net, serves as an effective radiator of heat as well. This way of cooling the brain is important to such an extent that in puppies, from birth to about one month, when heat loss is dangerously high, the perforated plate is separated from the brain wall with pads of fat (Fig. 27). Under conditions when the air temperature is significantly lower than body temperature, the role of the frontal cavities is to protect the brain from hypothermia.
A thin layer of warm air with its low circulation becomes beneficial and this is what we observe in the wolf. However, in the tropics, when high air temperature is combined with high humidity, conditioning the body temperature with this mechanism does not work. In the thermo-regulation of the aboriginal dogs of Vietnam, compared with breeds of European origin, perspiration from the abdominal skin and the reduction of heat generation in the body become very important. In the tropical environment, there is a high danger of overheating shock caused by strong insulation even when weather is cloudy and foggy. Short hair on the forehead of the dog cannot protect the brain from overheating. The surface of the body exposed to the sun can heat up to 10-20° C higher than the ambient air temperature. In such a case, to prevent the transmission of heat from the surface of the forehead to the brain, the frontal large cavities with fast air circulation coming from the nasal passages through a wide entrance opening is very helpful. Thus, the analysis of the structure of the frontal cavities in dogs and wolves shows their original adaptation to different environments, which required different mechanisms of thermoregulation. In my view, this is a strong argument in favor of J. Koler-Matznik’s view that Dingo-like dogs did not originate from the wolf.
Summarizing the results of our work, I think that most likely the domesticated dog originated from several subspecies of wolf and a now extinct wild form of Dingo, not excluding other ancestral forms.
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2 Replies to “On the problem of the origin of the domesticated dog and the incipient (aboriginal) formation of breeds – PADS Journals #36”
I already had some basic knowledge about the wolves are part of the domesticate bred dog; however, I do believe that the inbreeding they are doing now is resulting in dogs that are unpredictable. I believe that there are certain dogs that should not bred with other aggressive breed. One good example is the Stafford Shire Terrier. They now have the pitbull of several different breeds…Out crossing with other dogs that have an agrresive nature should not used.