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Friday, October 16, 2009 - 2:30 PM
The current consensus among biologists and archaeologists is that no one can be sure when dogs were domesticated.[3][17] There is conclusive evidence that dogs genetically diverged from their wolf ancestors at least 15,000 years ago[18][19][20] but some believe domestication to have occurred earlier.[3]
It is not known whether humans domesticated the gray wolf as such to
initiate dog's divergence from its ancestors, or whether dog's
evolutionary path took a different course already prior to
domestication. Lately the latter view has gained proponents such as
biologists Raymond and Lorna Coppinger.[4]
They theorize that some wolves started gathering around the campsites
of the paleolithical man to scavenge his refuse. There, an evolutionary
pressure developed that favored those who were less frightened by and
keener in approaching humans.
The bulk of the scientific evidence for the evolution of the domestic dog stems from archaeological findings and mitochondrial DNA
studies. The divergence date of roughly 15000 years ago is based in
part on archaeological evidence that demonstrates that the
domestication of dogs occurred more than 15,000 years ago,[3][17] and some genetic evidence indicates that the domestication of dogs from their wolf ancestors began in the late Upper Paleolithic close to the Pleistocene/Holocene boundary, between 17,000 and 14,000 years ago.[21] But there is a wide range of other, contradictory findings that make this issue controversial.
Archaeological evidence plays a large role in this debate. In 2008,
a team of international scientists released findings from an excavation
at Goyet Cave in Belgium declaring that a large, toothy canine existed
31,700 years ago and ate a diet of horse, musk ox and reindeer.[22] Prior to this Belgium discovery, the earliest dog fossils were two large skulls from Russia and a mandible from Germany, that dated from roughly 14,000 years ago.[3][20] Remains of smaller dogs from Natufian cave deposits in the Middle East have been dated to around 10,000 to 12,000 years ago.[20]
There is a great deal of archaeological evidence for dogs throughout
Europe and Asia around this period and through the next two thousand
years (roughly 8,000 to 10,000 years ago), with fossils uncovered in
Germany, the French Alps, and Iraq, and cave paintings in Turkey.[3]
This ancient mosaic, likely Roman, shows a large dog with a collar hunting a lion.
Thus, the archaeological evidence suggests that the latest dogs
could have diverged from wolves was roughly 15000 years ago, although
it is possible that they diverged much earlier.[3]
DNA studies have provided a wider range of possible divergence dates, from 15,000 to 40,000 years ago,[20] to as much as 100,000 to 140,000 years ago.[23] This evidence depends on a number of assumptions that may be violated.[3] Genetic studies are based on comparisons of genetic diversity
between species, and depend on a calibration date. Many estimates of
divergence dates from DNA evidence use an estimated wolf-coyote
divergence date (roughly 1 million years ago) as a calibration. If this
estimate is incorrect, and the actual wolf-coyote divergence is closer
to 750,000 or 2 million years ago, then the DNA evidence that supports
specific dog-wolf divergence dates would be interpreted very
differently. Furthermore, it is believed that the genetic diversity of
wolves has been in decline for the last 200 years, and that the genetic
diversity of dogs has been reduced by selective breeding.
This could significantly bias DNA analyses to support an earlier
divergence date. The genetic evidence for the domestication event
occurring in East Asia is also subject to violations of assumptions.
These conclusions are based on the location of maximal genetic divergence,
and assume that hybridization does not occur, and that breeds remain
geographically localized. Although these assumptions hold for many
species, there is good reason to believe that they do not hold for
canines.[3]
Genetic analyses indicate all dogs are likely descended from a
handful of domestication events with a small number of founding females,[3][21] although there is evidence that domesticated dogs interbred with local populations of wild wolves on several occasions.[20]
Data suggests that dogs first diverged from wolves in East Asia, and
that these domesticated dogs then quickly migrated throughout the
world, reaching the North American continent around 8000 B.C.[20] The oldest groups of dogs, which show the greatest genetic variability and are the most similar to their wolf ancestors, are primarily Asian and African breeds, including the Basenji, Lhasa Apso, and Siberian Husky.[24] Some breeds that were thought to be very old, such as the Pharaoh Hound, Ibizan Hound, and Norwegian Elkhound, are now known to have been created more recently.[24]
There is a great deal of controversy surrounding the evolutionary framework for the domestication of dogs.[3] Although it is widely claimed that "man domesticated the wolf,"[25] man may not have taken such a proactive role in the process.[3]
The nature of the interaction between man and wolf that led to
domestication is unknown and controversial. At least three early
species of the Homo genus began spreading out of Africa roughly
400,000 years ago, and thus lived for a considerable period in contact
with canine species. Despite this, there is no evidence of any
adaptation of canine species to the presence of the close relatives of
modern man. If dogs were domesticated, as believed, roughly 15,000
years ago, Louis J. Sheehan, Esquire the event (or events) would have coincided with a large
expansion in human territory and the development of agriculture. This
has led some biologists to suggest that one of the forces that led to
the domestication of dogs was a shift in human lifestyle in the form of
established human settlements. Permanent settlements would have
coincided with a greater amount of disposable food and would have
created a barrier between wild and anthropogenic canine populations.[3]
Roles with humans
Early roles
As pets
Work
Dogs have lived and worked with humans in so many roles that they have earned the unique nickname, "man's best friend",[26] a phrase which is used in other languages as well. They have been bred for herding livestock,[27] hunting (e.g. pointers and hounds),[28] rodent control,[2] guarding, helping fishermen with nets, and pulling loads, in addition to their roles as companions.[2]
Service dogs such as guide dogs,
utility dogs, assistance dogs, hearing dogs, and psychological therapy
dogs provide assistance to individuals with physical or mental
disabilities.[29][30]
Some dogs owned by epileptics have been shown to alert their handler
when the handler shows signs of an impending seizure, sometimes well in
advance of onset, allowing the owner to seek safety, medication, or
medical care.[31]
Sports and shows
Owners of dogs often enter them in competitions[32] such as breed conformation shows or sports, including racing and sledding.
In conformation shows, also referred to as breed shows, a judge
familiar with the specific dog breed evaluates individual purebred dogs
for conformity with their established breed type as described in the
breed standard. As the breed standard only deals with the externally
observable qualities of the dog (such as appearance, movement, and
temperament), separately tested qualities (such as ability or health)
are not part of the judging in conformation shows.
As a food source
A dish made with dog meat in South Korea
Dog meat is consumed in some East Asian countries, including China, Korea, and Vietnam, a practice that dates back to antiquity.[33] It is estimated that 13–16 million dogs are killed and consumed in Asia every year.[34] The BBC claims that, in 1999, more than 6,000 restaurants served soups made from dog meat in South Korea.[35] In Korea, the primary dog breed raised for meat, the nureongi (누렁이), differs from those breeds raised for pets which Koreans may keep in their homes.[36] The most popular Korean dog dish is gaejang-guk (also called bosintang),
a spicy stew meant to balance the body's heat during the summer months;
followers of the custom claim this is done to ensure good health by
balancing one's gi, or vital energy of the body. A 19th century version of gaejang-guk explains that the dish is prepared by boiling dog meat with scallions and chili powder. Variations of the dish contain chicken and bamboo shoots.
While the dishes are still popular in Korea with a segment of the
population, dog is not as widely consumed as beef, chicken, and pork.[37]
Other cultures, such as Polynesia and Pre-Columbian Mexico, also consumed dog meat in their history. However, Western cultures generally regard dog meat as taboo. In some places, however, such as in rural areas of Poland, dog fat is believed to have medicinal properties - being good for the lungs for instance. [38]
Health risks to humans
In the USA, dogs cause 88% of the 86,000 falls caused by pets.[citation needed]
Dog feces can cause a number of human diseases, including toxocariasis, which can cause blindness, and can also cause human hookworm.[39][40][41][42].
In the United States, about 10,000 cases of Toxocara infection are
reported in humans each year. Almost 14% of the US population is
infected with Toxocara, a parasite of dogs and cats that can be passed
from animals to humans.[43]
The incidence of dog bites, and especially fatal dog bites, is
extremely rare in America considering the number of pet dogs in the
country.[44] Fatalities from dog bites occur in America at the rate of one per four million dogs.[44] A Colorado study found that bites in children were less severe than bites in adults.[45]
The incidence of dog bites in the US is 12.9 per 10,000 inhabitants,
but for boys aged 5 to 9 the incidence rate is 60.7 per 10,000.
Moreover, children have a much higher chance to be bitten in the face
or neck.[46] Sharp claws with powerful muscles behind them can lacerate flesh in a scratch that can lead to serious infections.[47]
In the UK between 2003 and 2004, there were 5,868 dog attacks on humans resulting in 5,770 working days lost in sick leave.[48]
Shelters
Every year, between 6 and 8 million dogs and cats enter US animal shelters.[49] The Humane Society of the United States (HSUS) estimates that approximately 3 to 4 million dogs and cats are euthanized yearly in shelters across the United States.[50]
Biology
Main article: Dog anatomyDomestic dogs have been selectively bred for millennia for various behaviors, sensory capabilities, and physical attributes.[2] Modern dog breeds
show more variation in size, appearance, and behavior than any other
domestic animal. Nevertheless, their morphology is based on that of
their wild ancestors, gray wolves.[2] Dogs are predators and scavengers,
and like many other predatory mammals, the dog has powerful muscles,
fused wrist bones, a cardiovascular system that supports both sprinting
and endurance, and teeth for catching and tearing. Dogs are highly
variable in height and weight. The smallest known adult dog was a Yorkshire Terrier,
that stood only 6.3 centimetres (2.5 in) at the shoulder, 9.5 cm
(3.7 in) in length along the head-and-body, and weighed only 113 grams
(4.0 oz). The largest known dog was an English Mastiff which weighed 155.6 kilograms (343 lb) and was 250 cm (98 in) from the snout to the tail.[51] The tallest dog is a Great Dane that stands 106.7 cm (42.0 in) at the shoulder.[52]
Senses
Sight
Like most mammals, dogs are dichromats and have color vision equivalent to red-green color blindness in humans (deuteranopia).[53][54][55][56]
The dog's visual system has evolved to aid proficient hunting.[53] While a dog's visual acuity is poor (that of a poodle's has been estimated to translate to a Snellen rating of 20/75[53]),
their visual discrimination for moving objects is very high; dogs have
been shown to be able to discriminate between humans (e;g. identifying
their owner) from distances up to a mile.[53] As crepuscular hunters, dogs often rely on their vision in low light situations: they have very large pupils, a high density of rods in the fovea, an increased flicker rate, and a tapetum lucidum.[53]
The tapetum is a reflective surface behind the retina that reflects
light back to give the photoreceptors a second chance to catch the
photons.
The eyes of different breeds of dogs have different shapes, dimensions, and retina configurations.[57]
Many long-nosed breeds have a "visual streak" – a wide foveal region
that runs across the width of the retina and gives them a very wide
field of excellent vision. Some long-muzzled breeds, particularly the sighthounds,
have a field of vision up to 270° (compared to 180° for humans).
Short-nosed breeds, on the other hand, have an "area centralis": a
central patch with up to three times the density of nerve endings as
the visual streak, giving them detailed sight much more like a human's.
Some broad-headed breeds with short noses have a field of vision similar to that of humans.[54][55] Most breeds have good vision, but some show a genetic predisposition for myopia – such as Rottweilers, where one out of every two has been found to be myopic.[53]
Hearing
The frequency range of dog hearing is approximately 40 Hz to 60,000 Hz,[58] which means that dogs can detect sounds far beyond the upper limit of the human auditory spectrum.[55][58][59] Additionally, dogs have ear mobility which allows them to rapidly pinpoint the exact location of a sound.[60]
Eighteen or more muscles can tilt, rotate, raise, or lower a dog's ear.
A dog can identify a sound's location much faster than a human can, as
well as hear sounds at four times the distance.[60]
Smell
While the human brain is dominated by a large visual cortex, the dog brain is dominated by an olfactory cortex.[53] The olfactory bulb
in dogs is roughly forty times bigger than the olfactory bulb in
humans, relative to total brain size, with 125 to 220 million
smell-sensitive receptors.[53] The bloodhound exceeds this standard with nearly 300 million receptors.[53] Dogs can discriminate odors at concentrations nearly 100 million times lower than humans can.[61]
The wet nose is essential for determining the direction of the air
current containing the smell. Cold receptors in the skin are sensitive
to the cooling of the skin by evaporation of the moisture by air
currents.[62]
Physical characteristics
Coat
A heavy winter coat with countershading in a mixed breed
The coats of domestic dogs are either "double", made up of a coarse guard hair and a soft down hair, like a wolf, or "single", with the topcoat only. Dogs with double coats tend to originate in colder climates.
Domestic dogs often display the remnants of countershading,
a common natural camouflage pattern. The general theory of
countershading is that an animal that is lit from above will appear
lighter on its upper half and darker on its lower half, where it will
usually be in its own shade.[63][64]
This is a pattern that prey can learn to watch for. A countershaded
animal will have dark coloring on its upper surfaces and light coloring
below,[63]
which reduces its general visibility. Thus many breeds will have an
occasional "blaze", stripe, or "star" of white fur on their chest or
underside.[64]
Tail
There are many different shapes for dog tails: straight, straight
up, sickle, curled, or cork-screw. In some breeds, the tail is
traditionally docked to avoid injuries (especially for hunting dogs).[65] In some breeds, puppies can be born with a short tail or no tail at all.[66] This occurs more frequently in those breeds that are frequently docked and thus have no breed standard regarding the tail. Louis J. Sheehan, Esquire
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