The Process of Domestication Today, it is generally accepted that dogs are the descendants of Eurasian grey wolves, and there is evidence that they may have originated in east Asia. The dog may be a very young species at only 14 000 years old, although genetic evidence suggests dog may have began to diverge from the wolf as long as 135 000 years ago. However, even though dogs are the recent descendants of wolves and are very similar to wolves in terms of both behaviour and morphology, they are in several ways quite different from wolves. Dogs display a large number of characters not seen in wolves, such as piebald coats, floppy ears, and curly tails. Wolves are also seasonal breeders, with both males and females becoming fertile once per year. Dogs are not seasonal breeders, and most female dogs (with some rare exceptions) come into season twice per year, and male dogs are fertile year round. Dogs also have smaller skulls relative to their overall size than wolves and are able to form social bonds for a longer period of time during their early lives than are wolves.
Interestingly, many of the morphological and physiological differences that exist between dogs and wolves may not have been intentionally selected for by humans, and could have been a result of selection for tameness in dogs. An experiment which involved Siberian foxes (Vulpes vulpes) demonstrates how this could have happened (see Trut, 1999 for a review). The experiment was started in the 1940's by the Russian geneticist Dmitri Belyaev, who studied the process of domestication using a population of fur farm foxes (see Fig. 1). The foxes used in the beginning stages of the experiment were difficult to handle, very afraid of people and generally behaved like wild animals. The experimenters began to selectively breed the foxes for one trait - tameness around people. At the age of one month, an experimenter would offer food to each fox kit while trying to pet and handle it. This was done twice - while the kit was alone and while it was with other fox kits. This routine was repeated monthly until the kit was seven to eight months old and at that point, each kit was assigned to one of three classes based on how tame it was. Class III foxes attempted to flee from experimenters or tried to bite them. Class II foxes were not friendly to the experimenters, but allowed themselves to be touched. Class I foxes were friendly towards the experimenters and would often approach them. After six generations of breeding only tame foxes, a new class, Class IE, ("domesticated elite") had to be added. These foxes were very dog-like and actively sought out human attention and would lick experimenters and wag their tails like dogs. After twenty generations, 35% of the experimental foxes were domesticated elite and today, 70-80% of the foxes are.
-A photo of a captive silver fox, Vulpes vulpes.
Since the foxes in the experiment were being selectively bred for a behavioural trait (tameness), the experimenters hypothesized that physiological changes in the systems governing the fox's hormones and neurotransmitters would also occur, as an animal's behaviour is often mediated by these chemicals. Indeed, that is exactly what happened. As the experiment proceeded, a steady drop in the hormone producing activity of the domestic fox's adrenal glands was measured. For example, after several generations of selective breeding, the basal level of corticosteroids in the blood of the domesticated foxes was far lower than that of the control group of non-domesticated foxes. Changes in the neurochemistry of the domesticated foxes was also noted, as they had higher levels of serotonin in their brains compared to the control group of foxes.
After several generations of selecting for tameness, new traits only rarely seen in wild foxes began to become more common in the domesticated population. For example, after ten generations, several of the domesticated foxes had piebald coloured or brown mottled coats. Later in the experiment, it was noted that several of the tame foxes had floppy ears, short tails or curly tails. Even later, changes in the skull morphology of the foxes was noted as well, as skull measurements showed that the cranial height and width of the domesticated foxes tended to be smaller than those of control group foxes. The domestic foxes also had shorter and wider muzzles than the control group ones.
Many of the differences between the domestic foxes and the wild foxes are similar to the differences seen between domestic dogs and wolves. Wolves do not have floppy ears, curly tails, or piebald coloured coats, but many dogs do. Skull size is also one of the main ways dogs differ from wolves and selecting foxes only for tameness changed their skull size. The results of this experiment seem to suggest that many of the unique characters seen in dogs and not wolves are a result of the selection of dogs for tameness. But, how does selecting animals for a behavioural trait change their overall morphology like this? It has already been noted that selecting animals for a behavioural trait can change the amount of hormones and neurotransmitters produced by the animals because an animal's behaviour is often controlled by such chemicals. The early development of an animal is also, in part, controlled by these chemicals, so a small change in the animal's endocrine and neurochemical systems may result in changes to the early development of the animal.
Several developmental differences in the domestic foxes compared to the wild foxes were noted in the experiment. The domestic fox kits' eyes opened earlier than the control foxes, and they also responded to noise earlier than the wild foxes. The domestic foxes also began to show a fear of unknown stimuli starting later in life than the non-domestic foxes. This means that the domestic foxes had a longer window of socialization than did the wild foxes. In canids, the window of socialization in which the animal can form social bonds begins when when its ears and eyes open and it can explore its environment and closes when it begins to fear novel stimuli. Dogs differ from wolves in the length of their window of socialization. In wolves, this window closes at about three weeks, and it dogs it closes at 8 to 12 weeks, depending on the breed.
Several of the morphological changes seen in the foxes seem to be a result of changes to their early development. Floppy ears and curly tails, for instance, are characteristic of young fox kits and these traits are carried over into adulthood in many of the domestic foxes. The changes noted in the morphology of the fox's skulls may also be a result of early developmental changes, but this conclusion cannot be made since only the skulls of adult foxes were studied. Overall, however, many of the changes to the domestic foxes resemble paedeomorphosis, or the retention of juvenile traits in adults.
Many researchers consider dogs to be paedeomorphic wolves, meaning that they have retained characteristics that are typical of juvenile wolves as adults. For example, the floppy ears that characterize most dog breeds may be paedeomorphic trait, as very young wolf pups have floppy ears, which straighten shortly after birth. Even the erect-eared dogs, such as huskies and German shepherds, have ears which straighten up later than do the ears of wolf pups. The curled sickle tail of most domestic dogs is also a neotenous trait. Adult wolves typically have straight tails that are carried at a downward-pointing angle, whereas wolf pups, like many adult domestic dogs, have tails that are carried up above the back
The bark of domestic dogs is another juvenile trait. Adult wolves can and do bark as an alarm call, but they rarely do. However, wolf pups bark more often than adult wolves, which makes them similar to many domestic dogs. Adult dogs also have skull characteristics that make them rather similar to four-month-old wolf pups (Coppinger and Coppinger, 2001). When a dog reaches four months of age, its head's growth rate slows down relative to the growth rate of the rest of its body. A four month old wolf's head will continue to grow at a more rapid rate relative to a dog's until it is about seven or eight months old. The result is that adult wolf-sized dogs have head sizes that are similar to that of a juvenile wolf's.