Wolves, the ancestors of today's domestic dog, are social creatures who cooperate with each other to hunt down prey. These social and hunting skills of course proved useful for eventual cohabitation with humans. Canis familiaris, the domestic dog, has been living with humans for a long time: longer than cats, longer than horses, longer than any other animal. While some might argue that the dog is no longer man's best friend, having been supplanted in popularity by the cat, it's a safe bet that dogs are man's oldest friends.
But just when and where were dogs first domesticated? A series of three articles published in Science (November 2002) shed a great deal of light on the domestication of dogs. One of these studies, by Peter Savolainen and his Swedish and Chinese coworkers, suggest that dogs were first domesticated somewhere in east Asia about 15,000 years ago. Savolainen and his co-researchers compared mitochondrial DNA samples taken from over 600 domestic dogs throughout the world. It takes time for variability to develop in DNA samples. Therefore, the greater the DNA variability, the longer that type of animal has existed. Although all of the sampled dogs shared a common gene pool indicating a common ancestry, East Asian dogs exhibited the greatest variability in DNA, suggesting that dogs have lived there longer than anywhere else in the world.
By estimating how long it would take for these changes in DNA to occur, Savolainen theorized that dogs became domesticated about 15,000 years ago. Although this seems like quite a long time, other researchers had estimated that domestication had occurred as far back as 135,000 years ago. Savolainen admits that a different interpretation of his data could lead to the conclusion that domestication of dogs in east Asia occurred 40,000 years ago, a much longer time frame, but still much less than 135,000 years.
A second study focused on DNA samples taken from domestic dogs who lived in the Old and New World, including samples taken from dog bones of canids that had died before the arrival of Europeans in the Western hemisphere. Based on their results, Jennifer Leonard and her colleagues concluded that New World dogs are not the descendants of local wild species but are instead related to Old World wolves and arrived in the Western Hemisphere 10,000 to 15,000 years ago, when they traveled together with humans over the Bering Strait land bridge.
Interestingly, DNA taken from contemporary New World species, such as the Mexican hairless, show that modern New World dogs are descended from canines that traveled over from Europe, not from pre-Columbian New World dogs. Leonard did not find why the descendants of the dogs that crossed the Bering Strait apparently died out within the last 500 years and were supplanted by the descendants of more recent immigrant European dogs.
The third of the studies reveals that domestic dogs have evolved in their abilities to understand human cues. Brian Hare and his colleagues compared the ability of adult domestic dogs, domestic puppies, adult wolves, and chimpanzees to interpret signs given by humans to communicate the location of food. In one experiment, for example, a human would indicate which one of two containers had food by reaching for, looking at, or putting a wooden block on the full container. (It was ensured that the dogs were not tipped off by scent.) Nine of the 11 dogs picked up the hint, but only two of the eleven chimpanzees.
Another experiment focused on the ability of domestic puppies who were nine to 26 weeks old to read human cues. Even those puppies who had been raised in litters and had only been exposed to humans for a few minutes daily were able to pick up on human cues as to where food had been hidden.
Other research from Siberia suggests that the transformation from wild canid to domestic dog may have taken far less time than originally thought. Since 1959, researchers have selectively bred Vulpes vulpes, the silver fox, to produce tame animals. Although it has been conducted for less than 50 years, this study has already produced impressive results. The foxes not only are tamer, but 70-80% lick and smell the human experimenters just like domesticated dogs, and will even whimper for attention.
The behavior of these foxes is not the only thing that has changed. They also are starting to develop different physical characteristics. Their tails are shorter, their ears more droopy, and they have white splotches of fur. These types of physical
variations have also occurred in domestic dogs. Another interesting characteristic has been noted in these tamed foxes that may be related to change in behavior: their brains have higher levels of serotonin, a neurotransmitter which may be linked to reduced aggression.
Some of these fox pups have been taken out of the study and raised in the experimenters' homes. One describes these pets as being "good-tempered creatures, as devoted as dogs but as independent as cats, capable of forming deep-rooted pair bonds with human beings."
Relatively little is known about the spread of dogs throughout the world although, as mentioned above, many researchers now theorize that dogs spread through the New World by following humans over the Bering Strait land bridge and then across North and South America. The earliest physical indication that dogs lived with humans has been found in Israel, where the 12,000-year-old remains of a woman holding a puppy in her hands has been found. Whether it's a dog or wolf pup is not known, but it does suggest that some sort of relationship existed between humans and canines.
In spite of their usefulness as work animals, dogs have not been universally valued to the same degree. Ancient Greeks and Romans sometimes sacrificed dogs, and dogs are still used as a food source in the Far East. The Bible mentions dogs about thirty times, but only two of these references are not derogatory. Islam also takes a dim view of them as being unclean. Work dogs do exist in Arabic countries but are typically viewed as animals to be used for a specific function, not as pets or companions.
Hinduism, on the other hand, perhaps because reincarnation is an integral component of its core beliefs, offers a more positive view of dogs. A delightful story in the Mahabharata, a 2000-year-old classic of Indian spirituality, relates the attempted entry of Dharmaputra into heaven. Dharmaputra, one of the main characters in this epic, wants to take his dog with him. Heaven's gatekeeper refuses to let the dog enter into heaven. Dharmaputra then refuses to go in without his dog. At this point, the dog turns into the Lord Krishna who had only been pretending to be an animal, an ending with which many dog lovers could identify.
Although most of the breeds currently in existence are relatively new, dating back a few centuries at most, the initial differentiation into breeds evidently occurred thousands of years ago. Fossil remains indicate five different types of dog dating back to approximately 4500 B.C.: hunting dogs, sheepdogs, wolfish guard dogs, mastiff-type draft and guard dogs, and greyhound-type sight dogs.
Today, the variety of dogs is staggering. A Yorkshire terrier weighs only about 4-7 lb (1.8-3.2 kg), is 9 in (23 cm) tall, and is smaller than many cats. Mastiffs, however, typically weigh up to 190 lb (86 kg) and are 30 in (76 cm) tall.
Interestingly, both breeds originated as work dogs. The Yorkshire terrier was developed by English miners in Yorkshire who wanted a dog that would attack rats but was small enough to be carried in a pocket. Mastiffs, on other hand, go back about 2,000 years and were used by militaries.
Today, of course, dogs are extremely popular throughout much of the world. The increasing number of dog breeds is a reflection of this popularity. Out of an estimated 400 breeds of dog, the American Kennel Club recognizes about 150, forming eight groups of dog breeds: sporting, hunting, working, terrier, toy, nonsporting, herding, and miscellaneous.
Some of these groups are related to the roles that dogs have played in human society. Sporting dogs, such as pointers and retrievers, have been used to help in hunting and are still used for this purpose. These dogs are energetic and need regular exercise. Hounds have also been used for hunting. Some breeds have been used for their sense of smell in following the trail of their quarry, others for their ability to run down prey, and all have a unique vocalization (baying). Beagles and Afghans are two types of hounds.
Herding dogs, as their name indicates, have been used to herd animals. Border collies and German shepherds are two
popular breeds. Working dogs tend to be large animals and have been used to help humans by performing tasks other than hunting. Some are guard dogs, others have pulled sleds. Doberman pinschers, great Danes, and Siberian huskies are working dogs.
Toys are at the other end of the size continuum. Pekinese, poodles, and Yorkshire terriers all belong to the toy group. While some of these animals have been work animals, such as ratters, others have been bred as companion animals. Terriers were bred to control rodents, somewhat like a canine version of the cat. There are about two dozen or so recognized breeds.
The nonsporting group includes recognized breeds that do not fit into any of the above groups. They range from the Bichon frise, which weighs about 10 lb (4.5 kg), to the Dalmatian, which weighs in at about 50 lb (23 kg). Their backgrounds are similarly varied. The bichon frise started out as a pet of European royalty and became a circus performer after it fell out of favor, while Dalmatians have been used for everything from guarding and shepherding to being the mascot of fire fighters. The miscellaneous group currently con sists of seven breeds which do not quite as yet meet the American Kennel Club's requirements for fully recognized breeds.
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