DOMESTIC DOG
Canis lupus familiaris Linnaeus, 1758
Jiayi_000.png
Pet dog (Image Source: Lim Jia Yi)



“Dogs are a man’s best friend”
is a common saying about domestic dogs, describing the close relationship between them and humans. Learn more about them below!
1. RELATIONSHIP WITH HUMANS | 2. ORIGIN OF DOMESTIC DOGS | 3. DOG BREEDING | 4. DOGS IN SINGAPORE | 5. BIOLOGY OF DOMESTIC DOGS | 6. TAXONOMY

1. RELATIONSHIP WITH HUMANS

Have you ever wondered why many people love dogs and keep them as pets? Researchers have found out that humans and domestic dogs share a dynamic relationship and this relationship is mutually beneficial.[1] Studies have shown that therapy dogs can have a positive effect on human’s pain level[2] , help autistic children increase their social behaviors[3] , lower stress levels and risks of heart diseases[4] [5] . A study in China also found out that dog owners visited the doctor less frequently, took fewer sick leaves from work, and exercised more often as compared to non-dog owners.[6]
Dogs are the only non-primate animals that look at humans in the eyes.[7] Researchers in Japan found out that when a human and a dog look into each other’s eyes, both release a type of hormone called oxytocin.[8] When a human mother looks into her baby’s eyes, the same type of hormone is released in both of them and results in a positive feedback loop, creating a strong emotional bond.[9] This bond is likely to be formed between a human and a dog too, hence the saying “Dogs are a man’s best friend”. A recent study showed that almost half of the American households owning at least one dog and two-thirds of them consider their dogs to be family members.[10] While we know that humans love dogs, scientists have also found out that dogs depend on humans more than their own kind for protection and affection.[7]

This deep relationship with dogs could be traced to several thousands of years ago. Ancient remains of dogs discovered at Lake Baikal, Siberia, dating as far back as 8,000 years ago, have been found buried alongside that of humans in cemeteries.[11] Analysis of the remains also revealed that the dogs shared the same diet as prehistoric humans, which includes meat and fish.[11] Some dogs were even buried like humans, and had decorative collars or objects such as spoons in their graves, suggesting that the prehistoric humans believed that dogs had souls in their afterlives.[12] [13] In other parts of the world such as Egypt, the ancient Egyptians would shave off the eyebrows of their pet dog when they die, smear mud in their fur and mourn aloud for several days.[14] Researchers also found out that there were more dog burials in the past than any other animals, suggesting that they occupy a very special place in the human communities.[13] So how exactly did domestic dogs come about?





2. ORIGIN OF DOMESTIC DOGS

Dogs were the first and only animals domesticated before human started agriculture.[15] Although dogs are of great importance to humans, we have not yet figured out the exact geographic and temporal origins of dogs. It is widely believed that dogs are domesticated descendants of wolves. Analysis from the whole genome sequences suggested that domestic dogs and the gray wolves have diverged from Taimyr wolves (now extinct) around 27,000 to 40,000 years ago.[16] The fossil fragments of Taimyr wolves were discovered on the Taimyr Peninsula in Siberia a few years ago.[17]

However, some have claimed that dogs only appeared in Europe[15] and Far East Asia[18] around 15,000 and 12,000 years ago respectively, while others believed that dogs could have originated from more than one geographic area[19] [20] . Past genetic studies have supported the hypothesis that dogs were domesticated only once[21] , but disagreeing on where this occurred[22] [23] [24] , leading to several hypotheses of how dogs migrated around the world. According to Gregar Larson, these conflicting hypotheses is due to the messy dog genetics caused by “the giant whirlwind blender of European crazy Victorian dog-breeding frenzy”, interbreeding with wolves, and random breeding by dogs themselves.[25]


Jiayi_002.png
One of the hypotheses that depicts migration of dogs around the world, originating from southern East Asia. (Image Source: Wang et al. 2015 [24], under a CC BY-NC-SA 4.0 license).

However, recent genetic studies suggested that dogs were domesticated more than once, indicating a dual origin[26] . Frantz et al.[26] have came up with a new hypothesis suggesting that they could have been domesticated separately in Western and Eastern Eurasia, from different populations of wolves. The people who moved to Europe could have brought the east Eurasian dogs over, which somewhat took over the now-extinct European dogs.[26]

Domestication of dogs
Scientists took almost a century to figure out the process of dog domestication. Domestication, according to Merriam Webster, is defined as “to breed or train (an animal) to need and accept the care of human beings”. In the early 20th century, Francis Galton, an English scientist, proposed that the early humans domesticated them by capturing some wolf pups and raising them as pets.[27] However, the wolf pups could be raised as pets but grow up into wild wolves.[27]

Over the years, scientists found out that domestication takes place over hundreds or thousands of years, and could not possibly have happened in a once-off event, hence arriving with a new hypothesis.[27] It is now believed that dogs first domesticated themselves – early humans could have discarded piles of leftover food at the edges of their campsites and the more daring wolves approached the campsites to feast on the leftover food, which allowed them to survive longer and produce more pups.[28] [29] Natural selection then selected for bolder ones over many generations until they were comfortable enough to interact with humans.[28] The early humans could then have discovered the services that these canines could provide, thus initiating the second phase of domestication, breeding them to be better herders and hunters.[29] Gradually these canines also adapted to a starchy diet from the carnivorous diet, signifying the domestication of dogs.[30]



3. DOG BREEDING

Dog breeding refers to the intentional mating of artificially selected dogs by their owners, to retain or produce specific qualities or characteristics.[31] Humans have bred dogs for the past milleniums for various functions such as hunting or herding. This artificial selection of dogs has altered their shape, size and behavior.[32] The World Canine Organization, also known as the Federation Cynologique Internationale (FCI), recognizes 340 breeds of dogs. However, the standards for breed recognition differs among various countries. The American Kennel Club (AKC) only recognizes 167 breeds. Breeders that produce new dog breeds will have write Breed Standards for them to be registered and recognized as a new breed. Breeds are often categorized by its functional type. AKC recognizes several dog breed groups shown in Table 1 below.[33]

Table 1. Various functional groups of dog breeds and its characteristics.

Group
Function/ Characteristics
Examples of Dog Breed
Herding
Able to control movement of other animals.
Australian Shepherd,
Border Collie
Hound
Used for hunting in the past.
Some are able to follow trials using their acute scenting powers while others have strong staminas that enable them to run long distances.
Beagle,
Greyhound
Non-sporting
Have varied personalities and appearances.
Bulldog,
Chow Chow,
Dalmatian,
Finnish Spitz,
Poodle
Sporting
Naturally active and alert, require regular and intense exercise.
Able to hunt and take part in other field activities.
Golden Retriever,
Pointer
Terrier
Energetic, have little tolerance for other animals, including other dogs.
Australian Terrier,
Bull Terrier,
Miniature Schnauzer
Toy
Small sized, cute appearances to appeal to humans as pets.
Chihuahua,
Maltese,
Shih Tzu,
Yorkshire Terrier
Working
Able to perform jobs e.g. pulling sleds, guarding properties, performing water rescues.
Typically strong and large-sized.
Alaskan Malamute,
Samoyed,
Siberian Husky

Breeding conflictsTypically, natural selection would allow a species to improve in traits over many generations. Artificial selection supposedly speeds up this process to give the traits that humans desire, but has also resulted in many diseases, especially those carried by the autosomal recessive genes in purebred dogs.[34] Shrinking genetic pool for purebred dogs and inbreeding have led to health problems of many breeds. Caen Elegans compared several breeds of dogs after 100 years of breed “improvement” and commented that “if “improvement” in looks (or traits) imposes a health burden, then it is not a breed improvement”.[35] Below are some examples of breed “improvement” condemned by Caen Elegans.

Bull Terrier

Image Source: C. Elegans, under a CC BY 4.0 license
Image Source: C. Elegans, under a CC BY 4.0 license

The Bull Terrier was bred by crossing a Bulldog and a Terrier. The Bulldog was bred to be used in animal-based blood sports to fight bulls and bears tied to a post.[36] Then, humans started crossing them with terriers, arguing that it increases the fight quality in them. However, this cross breeding has not only affected their appearances, but also their health. The Bull Terrier has a mutated skull that is almost flat on the top, and is the only breed of dog that has triangular eyes. About 20% of the pure white Bull Terriers are deaf[37] and many of them tend to develop skin allergies[38] .

Basset Hound

Image Source: C. Elegans, under a CC BY 4.0 license
Image Source: C. Elegans, under a CC BY 4.0 license

Over the years, the hind leg structure of the Basset Hound has changed, causing its body to be nearer the ground.[35] It now also has excessive skin, disproportionately huge ears, vertebra problems and droopy eyes that are prone to entropion or ectropion – a painful condition when the eyelids turn outwards.[35]

English Bulldog

Image Source: C. Elegans, under a CC BY 4.0 license
Image Source: C. Elegans, under a CC BY 4.0 license

The English Bulldog now has a much shorter face, excessive skin and much more storky than its counterpart from 100 years ago.[35] It has been bred out of its natural proportions such that it is unable to mate or give birth without human intervention.[35] The English Bulldog also has a much reduced lifespan of about 6.25 years due to the wide range of diseases it has.[35]

Dachshund

Image Source: C. Elegans, under a CC BY 4.0 license
Image Source: C. Elegans, under a CC BY 4.0 license

The Dachshunds were initially bred in Germany to fight badgers in their dens.[39] Subsequently, it got bred out of its natural proportions for its appearance. Its neck and back have became significantly longer, while its legs became shorter, with its chest jutting forward, leaving almost no space between its chest and the floor.[35] It has the highest risk amongst all the dog breeds for intervertebral disc diseases (which could lead to paralysis), and is susceptible to many other problems with their out-of-proportion body size.[35]

Tips!
Want to get a dog? Adopt one instead! (See Section 4 below for more details). To be a responsible dog owner, it is essential to read up on the breed of dog that you will be getting, to understand the needs of the dog. Breeding dogs also require knowledge of their genetics and health, hence it is important to not let pet dogs breed randomly. Avoiding purebred dogs would be a wise choice if you want a healthy dog free of diseases!



(Video Source: CollegeHumour, under a CC BY 4.0 license)
Functions of Dogs[40]
Dogs are known to aid humans in herding and hunting in the past, but they are also useful in many other areas as shown below.

Guide Dogs[41] : Guide Dogs help to enhance the quality of life for the visually impared and the blind, through better mobility around their environment. In Singapore, no pets are allowed on the Mass Rapid Transport System (MRT) except Guide Dogs. Not all dogs are suitable to be Guide Dogs. They are specially selected for their gentle temprement and hardworking traits. The two most popular breeds of Guide Dogs are the Labrador Retriever and the Golden Retriever.

Search & Rescue Dogs: These dogs have been saving people’s lives from all over the world for many decades by tracking human scent to locate them under debris during a disaster. The most common breed used is the Labrador Retriever. Other breeds such as Dobermans and Bloodhounds are usually trained to differentiate between scents in order to track only one (e.g. an escaped convict or a missing child).

Truffle-hunting Dogs: The Lagotto Romagnolo is a breed of dog that originated from Italy. As they were born with the instinct to hunt, swim and retrieve, they were used to assist hunters in finding and retrieving birds in the past. However, in recent years, they have been bred and trained to look for truffles, a highly priced ingredient for food, as their highly developed nose also made them excellent search dogs.

Therapy Dogs: These dogs are trained to provide affection and comfort for people, especially those in hospitals, hospices and nursing homes etc. In Singapore, Therapy Dogs Singapore (TDS), a non-profit voluntary welfare group, is committed to reach out to the disadvantaged through pet-assisted therapy, using therapy dogs.



(Video Source: atdianopra, under a CC BY 4.0 license)



4. DOGS IN SINGAPORE

People in Singapore often see dogs as “more affectionate and loyal” as compared to other pet animals. According to the Agri-Food and Veterinary Authority of Singapore (AVA), it is estimated that there are 62,000 pet dogs in Singapore in 2015, a 32% increase from the last decade.[42] The increase in dog population has also resulted in the increase of vet clinics and dog-related services (see below).[42] The most popular breeds of dogs in Singapore in 2015 are Toy Poodle, Shih Tzu, Minature Schnauzer and Maltese.[42] These dogs are cute looking, especially with their short muzzles and small faces. Small dogs are more popular in Singapore because most people live in the flats of the Housing and Development Board (HDB).[42]

License
According to the Housing and Development Board (Animals) Rules 1989, only 62 breeds of dogs listed here are allowed in Singapore’s Housing and Development Board (HDB) residential units, and only one dog is allowed in each residential unit. Those who do not comply with the rules can get a maximum fine of SGD$4,000. A maximum of three dogs can be kept on non-HDB premises, unless special permission is obtained from AVA.
To instil a sense of responsibility and ownership in pet owners and discourage pet abandonment, AVA ruled that dogs above three months old must have licenses, which need to be renewed yearly. The license cost $15 for a sterilised dog and $90 for an unsterilised dog. Click here to manage your dog license(s) online!


To prevent potential human-dog conflicts that could arise in public areas, AVA also stated that all dogs must be leashed in public places. Dog owners are also encouraged to microchip and register their pet dogs with Pet-Call. The small chip is usually implanted between the shoulder blades of the dog, and it carries a unique identification number so that the dog can be traced back to the owner, in case it wanders off and get lost.

Dog adoption
There are several non-profit organizations that advocates to improve the welfare of stray and abandoned dogs in Singapore. They usually resuce, foster and rehome the stray and abandoned dogs, and also promote sterilization to control the population of stray dogs.
Check out the following non-profit organizations if you wish to adopt a dog!
- Action for Singapore Dogs
- Adopt A Dog
- Animals Lovers League
- Causes for Animals Singapore (CAS)
- Homeless Dogs Rehomers
- HOPE Dog Rescue
- LostPaws
- Mdm Wong’s Shelter Dog Adoption
- Pet Central Dog Adoption
- Pets Channel Dog Adoption
- Purely Adoption
- Save Our Street Dogs (SOSD)
- Society for the Prevention of Cruelty to Animals, Singapore (SPCA)
- Voices For Animals (VFA)


Dog-related services
With the increase in pet dogs in Singapore, dog-related services became more popular over the years. Dog owners can leave their dogs with Super Cuddles, a dog boarding home in Tanjong Katong, when they go overseas. Pet owners who work and could not accompany their dogs during the day can also send their dogs to Doggie Retreat, a dog daycare center in Marine Parade. It could be difficult to bring pet dogs around Singapore without private transport as dogs (except Guide Dogs) are not allowed on the trains. Here is a list of Pet Taxi services that would be useful for pet owners!


5. BIOLOGY OF DOMESTIC DOGS

Domestic dogs, like other canids, have a head, a body with a pair of front and hind legs and a tail. Over the millennium, humans have bred dogs to exhibit a diverse range of fur coats which vary in texture, colour and markings.[43]
Anatomically, it is similar to other predatory mammals; domestic dogs have strong muscles and a cardiovascular system that allows them to run fast and long distances. Dogs have small feet and walk on their toes. Their hind legs are somewhat stiff and sturdy while their front legs are only attached to their body by the muscles, thus being loose and flexible. This enables dogs to jump and leap, which is essential when chasing after preys. All dogs also have a ligament that connects the chest bone to the neck bone. This allows them to run long distances and follow scent trials with their nose close to the ground without spending much energy to support the weight of their head.[44]

Size and weight
Domestic dogs are extremely variable in size and weight, depending on the breeds. Table 2 below shows most extreme sizes of dogs in the world.[45] [46] [47]


Table 2. Extreme sizes of dogs
Dog Breed
Height
Length
Weight
Chihuahua
9.63 cm
-
-
Chihuahua
-
15.2 cm
-
English Mastiff
-
250 cm
155.6 kg
Great Dane
106.7 cm
-
-




(Video Source: Guinness World Records, under a CC BY 4.0 license)

Senses
Dogs, like humans, have the same five senses as us – sight, sense of smell, sense of hearing, sense of touch and hearing. On top of that, research has also shown that dogs are sensitive to earth’s magnetic field and are able to see them.[48] Studies have even shown that dogs have a preference to align their bodies along on the north-south axis before they urinate or defecate when they are not leashed and when the Earth’s magnetic field is calm.[49]

  • Sight[50] Like most mammals, dogs are dichromats as they only have two types of cone photoreceptors (humans have three types). Their vision is similar to humans who are red-green colour blind. Despite that, dog’s vision has evolved to aid hunting. They are able detect moving objects from as far as 900 meters, and also have superior night vision.[51] Most breeds of dogs have good vision except for some which tend to be myopic (such as Rottweilers).
  • Hearing[50][52] [53] Dogs are able to detect sounds beyond the upper limit of the human auditory spectrum, with a hearing frequency range between 16-40 Hz and up to 45-60 kHz, while humans only have a hearing frequency range between 20-70 Hz and up to 13-20 kHz. Their ears are extremely sensitive and can hear sounds from as far as four times the distance compared to a human. Their ears consist of at least 18 muscles, enabling them to move their ears quickly and identify the exact location of a sound.
Anatomy of dog's ear (Image Source: R. Lawson, under a CC BY 3.0 license)
Anatomy of dog's ear (Image Source: R. Lawson, under a CC BY 3.0 license)
  • Smell[50][54] [55] [56] A large portion of the dog’s brain is occupied by the olfactory cortex, which is part of the sensory system. A dog has around 40 times more smell-sensitive receptors than humans, hence having a more sensitive sense of smell. The dog’s mobile nostrils help to determine where the scent originate from. The dog has a bony shelf structure inside its nose, which allows scent molecules to stick on it when the air passes over it, which intensifies when the scent molecules accumulate as the molecules do not get blown away when the dog breathes normally. Hence it only needs several sniffs to “remember” a scent.


Lifespan[57] [58] [59]
The average lifespan of a domestic dog is about 10 to 13 years although it varies widely among different breeds. Mixed breeds are found to live about 1.2 years longer than pure breeds, and the heavier dogs tend to have a shorter lifespan. Dogue de Bordeaux are said to have the shortest lifespan (about 5.2 years) while Toy Poodles, Japanese Spitz, Tibetan Spaniels and Border Terriers are the longest-lived breeds, living up to about 14 years on average.


Reproduction
Male and female domestic dogs are usually sexually mature between six to twelve months.[60] The female dogs experience an estrous cycle once every six months, and they will be receptive to copulation at the peak of the cycle. This period lasts for seven days or more and she can mate with several dogs (of different breeds) during this period of time. The litter of puppies produced could then look significantly different as they are produced by different fathers. Although different breeds of dogs are able to mate with each other, it can be dangerous for smaller breeds of female dogs to mate with larger breeds of male dogs, especially those with larger skulls, as the female dogs may experience difficulties when giving birth to the puppies with larger skulls. The average gestation period is about 63 days and depending on the breed of dog, each litter consist of about six puppies.[61] Toy dogs usually have one to four puppies each time while larger breeds may have as many as twelve in each litter.
Neutering
Dogs can be neutered by removing the testicles of males or ovaries and uterus of females. This can not only stop them from reproducing, but also reduce their sex drive and chances of developing cancers related to the reproductive organs.[62] [63] In Singapore, the Society for the Prevention of Cruelty to Animals (SPCA) also has a sterilization programme to control the population of stray dogs humanely and effectively.[64]




6. TAXONOMY


Under the Biological Species Concept (BSC), the domestic dog is classified as Canis lupus familiaris, but under the Evolutionary Species Concept (ESC), it is classified under Canis familiaris.[44]
According to BSC, biological species are groups of interbreeding natural populations that are isolated from other such groups.[65] Domestic dogs and wolves are able to interbreed and produce viable, fertile offsprings, hence is considered as one species. However, wolves and domestic dogs can still be differentiated based on their behaviour (wolves are extremely shy while dogs are very friendly towards humans), hence the domestic dogs are classified as a sub-species, Canis lupus familiaris, under the same species as the wolves, Canis lupus.
According to ESC, an evolutionary species is an entity composed of organisms that maintains its identity from other such entities through time and over space that has its own independent eolutionary fate and historical tendencies.[65] Domestic dogs and wolves have sufficient differences between them to be considered to have their own “independent evolutionary fate”, and hence domestic dogs are named as Canis familiaris while wolves are named as Canis lupus under ESC.


Etymology
In Latin, “canis” means “dog”, “lupus” means “wolf” and “familiaris” means “domestic”.

Original Description
In 1758, Carl Linnaeus, a well-known Swedish taxonomist who is regarded as the father of taxonomy, published a list of species in 10th edition of Systema Naturae, which included the Canis species. The dog was recorded as Canis familiaris while the wolf was recorded as Canis lupus. He did not designate any Type Specimen for Canis familiaris but he designated it as the Type Species for the genus Canis. In 1978, a review of Canid classification by Van Gelder stated that “Canis dingo is now generally regarded as a distinctive feral domestic dog. Canis familiaris is used for domestic dogs, although taxonomically it should probably be synonymous with Canis lupus.”
In 1982, Canis familiaris was listed under Canis lupus in the first edition of Mammal Species of the World, with the comment “Probably ancestor of and conspecific with the domestic dog, familiaris. Canis familiaris has page priority over Canis lupus, but both were published simultaneously in Linnaeus (1758), and Canis lupus has been universally used for this species” for Canis lupus, thus separating the wolf from domestic dogs. Then, the dog is listed as a subspecies of Canis lupus, as Canis lupus familiaris.

Phylogeny

Wayne et al. (1989) used DNA hybridization data to determine the evolutionary relationships of carnivores.[66] Order Carnivora is divided into two superfamilies, Canoidea and Feloidea.[66] With the assumption that the rate of sequence evolution is constant, the canids diverged from other carnivores at the beginning of the Eocene, about 50 million years ago.[66] The canids were the earliest to diverge in the superfamily Canoidea.[67]
Wayne et al. (1997) then used DNA sequences of the mitochondrial protein coding genes cytochrome b, cytochrome c oxidase I, and cytochrome c oxidase II, which are 2001 basepairs long, to reconstruct the phylogeny of the family Canidae.[68] Their results showed that all species from the genus Canis formed a monophyletic group.[68] Although all species of the genus Canis are known to be able to hybridize[69] , the phylogenetic analysis of the cytochrome b gene showed that only the gray wolves were direct ancestors of the domestic dogs[68], thus suggesting that dogs originated from gray wolves.
Vila et al. (1997) then constructed a neighbour-joining tree using the mitochondrial DNA control region I sequences of the wolf and dog.[70] The results showed that the haplotypes of the dog could be grouped into four clades, and this suggested that either there was only one domestication episode of wolves but had a few events of admixture between wolves and dogs, or that there were several domestication episodes of wolves in different regions at various times.[70] Recent genetic studies done by Frantz et al. (2016) have supported the latter hypothesis, and suggested a dual origin of domestic dogs (Refer to Section 2 above).[26]

Jiayi_007.jpeg
Phylogeny of mitochondrial DNA of Canids. Nodes marked with asterisks have posterior probability of >0.9 while stars indicate ancient speciments that are morphologically described as “dog like”. (Image Source: Skoglund et al. 2015 [16], under a CC BY-NC-SA 4.0 license).


Skoglund et al. sequenced the DNA of the Taimyr wolf and found that it belonged to a population that diverged from the common ancestor of present-day dogs and wolves.[16] The Taimyr wolf is presumed to be the intermediate to gray wolves and dogs, with extra genetic drift that occurred in those lineages when the Taimyr wolf went extinct. Their study also supported the hypothesis that there was more than one domestication episodes of wolves.

In conclusion, there has been many studies done on the phylogeny of wolves and dogs. The phylogenetic tree of Canids has been ever-changing with every new fossil found. This could signify that the actual phylogenetic relationship of Canids is not yet established and thus more work needs to be done on this group of animals.



  1. ^ Coren, S., 2006. The intelligence of dogs: a guide to thoughts, emotions and inner lives of our canine companions. Retrieved from http://emilkirkegaard.dk/en/wp-content/uploads/The-Intelligence-of-Dogs-A-Guide-to-the-Thoughts-Emotions-and-Inner-Lives-of-Our-Canine-Companions-Stanley-Coren-320p.pdf (last accessed 3 November 2016).
  2. ^ Harper, C. M., Y. Dong, T. S. Thornhill, J. Wright, J. Ready, G. W. Brick & G. Dyer, 2015. Can therapy dogs improve pain and satisfaction after total joint arthroplasty? A randomized controlled trial. Clinical Orthopaedics and Related Research, 473(1): 372-379.
  3. ^ O’Haire, M. E., S. J. McKenzie, A. M. Beck & V. Slaughter, 2013. Social behaviours increase in children with autism in presence of animals compared to toys. PloS ONE,8(2): e57010. Retrieved from http://dx.doi.org/10.1371/journal.pone.0057010 (last accessed 3 November 2016).
  4. ^ Allen, K., J. Blascovich & W. B. Mendes, 2002. Cardiovascular reactivity and the presence of pets, friends, and spouses: the truth about cats and dogs. Psychosomatic Medicine, 64(5): 727-739.
  5. ^ Allen, K., B. E. Shykoff & J. L. Izzo, 2001. Pet ownership, but not ACE inhibitor therapy, blunts home blood pressure responses to mental stress. Hypertension, 38(4): 815-820.
  6. ^ Headey, B., F. Na & R. Zheng, 2008. Pet dogs benefit owner’s health: a ‘natural experiment’ in China. Soc Indic Res, 87: 481-493.
  7. ^ “Brain scans reveal what dogs really think of us,” by T. Fisher. Science.mic, 20 November 2014. Retrieved from https://mic.com/articles/104474/brain-scans-reveal-what-dogs-really-think-of-us - .SWMwE4EN4 (last accessed 4 November 2016).
  8. ^ Miller, G., 2013. The promise and perils of oxytocin. Science, 339(6117): 267-269.
  9. ^ “How dogs stole our hearts,” by D. Grimm. Science, 16 April 2015. Retrieved from http://www.sciencemag.org/news/2015/04/how-dogs-stole-our-hearts (last accessed 3 November 2016).
  10. ^ “Pets by the numbers”. The humane society of the United States. Retrieved from http://www.humanesociety.org/issues/pet_overpopulation/facts/pet_ownership_statistics.html (last accessed 4 November 2016).
  11. ^ Losey, R. J., S. Gravie-Lok, J. A. Leonard, M. Anne Katzenberg, M. Germonpre, T. Nomokonova, et al., 2013. Burying dogs in ancient cis-Baikal, Siberia: temporal trends and relationships with human diet and subsistence practices. PLoS ONE, 8(5): e63740. Retrieved from http://dx.doi.org/10.1371/journal.pone.0063740 (last accessed 3 November 2016).
  12. ^ “Ancient burials reveal dogs have always been man’s best friend: remains show canines were ‘treated like humans’ 8,000 years ago,” by S. Griffiths. MailOnline, 10 March 2016. Retrieved from http://www.dailymail.co.uk/sciencetech/article-3485496/Ancient-burials-reveal-dogs-man-s-best-friend-Canines-treated-like-humans-8-000-years-ago.html (last accessed 4 November 2016).
  13. ^ “UAlberta anthropologist Robert Losey unearths ancient dogs,” by UniversityofAlberta, 26 February 2016. Retrieved from https://www.youtube.com/watch?v=281Il1bTBpY (last accessed 4 November 2016).
  14. ^ Brewer, D., T. Clark & A. Philips, 2001. Dogs in Antiquity: Anubis to Cerebrus the origins of the domestic dog. Warminster, UK: Aris & Phillips.
  15. ^ Larson, G., E. K. Karlsson, A. Perri, M. T. Webster, S. Y. W. Ho, J. Peters, et al. 2012. Rethinking dog domestication by integrating genetics, archeology, and biogeography. PNAS, 109(23): 8878-8883.
  16. ^ Skoglund, P., E. Ersmark, E. Palkopoulou & L. Dalen, 2015. Ancient wolf genome reveals an early divergence of domestic dog ancestors and admixture into high-latitude breeds. Current Biology, 25(11): 1515-1519.
  17. ^ “Family tree of dogs and wolves is found to split earlier than thought,” by J. Gorman. The New York Times, 21 May 2015. Retrieved from http://www.nytimes.com/2015/05/22/science/family-tree-of-dogs-and-wolves-is-found-to-split-earlier-than-thought.html?_r=0 (last accessed 23 November 2016).
  18. ^ Pionnier-Capitan, M., C. Bemilli, P. Bodu & J.-D. Vigne, 2011. New evidence for Upper Paleolithic small domestic dogs in South-Western Europe. Journal of Archaeological Science, 38(9): 2123-2140.
  19. ^ Dyan, T., 1994. Early domesticated dogs of the Near East. Journal of Archaeological Science, 21: 633-640.
  20. ^ Ollivier, M., A. Tresset, C. Hitte, C. Petit, S. Hughes, B. Gillet, et al., 2013. Evidence of coat colour variation sheds new light on ancient canids. PLoS ONE, 8(10): e75110. Retrieved from http://dx.doi.org/10.1371/journal.pone.0075110 (last accessed 6 November 2016).
  21. ^ Freedman, A. H., I. Gronau, R. M. Schweizer, D. O.-D. Vecchyo, E. Han, P. M. Silva, et al., 2014. Genome sequencing highlights the dynamic early history of dogs. PloS Genetics, 10(1): e1004016. Retrieved from http://dx.doi.org/10.1371/journal.pgen.1004016 (last accessed 6 November 2016).
  22. ^ Thalmann, O., B. Shapiro, P. Cui, V. J. Schuenemann, S. K. Sawyer, D. L. Greenfield, et al., 2013. Complete mitochondrial genomes of ancient canids suggest a European origin of domestic dogs. Science, 342(6160): 871-874.
  23. ^ Shannon, L. M., R. H. Boyko, M. Castelhano, E. Corey, J. J. Hayward, C. McLean, et al., 2015. Genetic structure in village dogs reveals a Central domestication origin. PNAS, 112(44): 13639-13644.
  24. ^ Wang, G.-D., W. Zhai, H.-C. Yang, L. Wang, L. Zhong, Y.-H. Liu, et al., 2016. Out of southern East Asia: the natural history of domestic dogs across the world. Cell Research, 26: 21-33.
  25. ^ “The bif search to find out where dogs come from,” by J. Gorman. The New York Times, 18 January 2016. Retrieved from http://www.nytimes.com/2016/01/19/science/the-big-search-to-find-out-where-dogs-come-from.html?_r=1 (last accessed 6 November 2016).
  26. ^ Frantz, L. A. F., V. E. Mullin, M. Pionnier-Capitan, O. Lebrasseur, M. Ollivier, A. Perri, et al. 2016. Genomic and archaeological evidence suggests a dual origin of domestic dogs. Science, 352(6290): 1228-1231.
  27. ^ Grimm, D., 2015. How the wolf became the dog. Science, 348(6232): 277.
  28. ^ Larson, G. & D. Q. Fuller, 2014. The evolution of animal domestication. Annual Review of Ecology, Evolution and Systematics, 45: 115-136.
  29. ^ “Opinion: We didn’t domesticate dogs. They domesticated us.,” by Hare, B. & V. Woods. National Geographic, 3 March 2013. Retrieved from http://news.nationalgeographic.com/news/2013/03/130302-dog-domestic-evolution-science-wolf-wolves-human/ (last accessed 6 November 2016).
  30. ^ Axelsson, E., A. Ratnakumar, M.-L. Arendt, K. Maqbool, M. T. Webster, M. Perloski, et al., 2013. The genomic signature of dog domestication reveals adaptation to a starch-rich diet. Nature, 495: 360-364.
  31. ^ Seranne, A., 1980. //The joy of breeding your own show dog//. New York, N.Y.: Howell Book House.
  32. ^ Akey, J. M., A. L. Ruhe, D. T. Akey, A. K. Wong, C. F. Connelly, J. Madeoy, et al., 2009. Tracking footprints of artificial selection in the dog genome. PNAS, 107(3): 1160-1165.
  33. ^ “Dog breed groups,” by American Kennel Club. Retrieved from http://www.akc.org/dog-breeds/groups/ (last accessed 6 November 2016).
  34. ^ Ostrander, E. A., F. Galibert & D. F. Patterson, 2000. Canine genetics comes of age. Trends in Genetics, 16(3): 117-124.
  35. ^ “100 years of breed “improvement”,” by C. Elegans. Science and Dogs, 29 September 2009. Retrieved from https://dogbehaviorscience.wordpress.com/2012/09/29/100-years-of-breed-improvement/ (last accessed 6 November 2016).
  36. ^ “Bull and Terrier Breeds,” by C. M. Luca. The Bulldog Information Library. Retrieved from http://www.bulldoginformation.com/bull-and-terrier-breeds.html (last accessed 6 November 2016).
  37. ^ “Breed-specific deafness prevalence in dogs (percent),” by G. M. Strain. Louisiana State University, 19 June 2015. Retrieved from http://www.lsu.edu/deafness/incidenc.htm (last accessed 6 November 2016).
  38. ^ “Genetic welfare problems of companion animals: Bull Terrier,” by Godfrey, R., D. Godfrey & M. Hinds. Universities Federation for Animal Welfare, 2011. Retrieved from http://www.ufaw.org.uk/dogs/bull-terrier-canine-atopic-dermatitis (last accessed 6 November 2016).
  39. ^ Choron, H. & S. Choron, 2005. //Planet Dog: A Doglopedia//. New York, NY: Houghton Mifflin Co.
  40. ^ “20 Jobs dogs have performed”. The Dog Guide. Retrieved from http://www.dogguide.net/20-jobs-dog-have-done.php (last accessed 23 November 2016).
  41. ^ “Guide Dogs Association of the Blind”. Retrieved from http://guidedogs.org.sg (last accessed 23 November 2016).
  42. ^ “Licensed dog up 32% over 10 years; pet numbers set to hit 824,600 this year,” by R. Chia. The Straits Times, 24 March 2016. Retrieved from http://www.straitstimes.com/singapore/puppy-love-breeds-more-pet-services (last accessed 23 November 2016).
  43. ^ “Dog anatomy”. Wikipedia, 17 October 2016. Retrieved from https://en.wikipedia.org/wiki/Dog_anatomy (last accessed 6 November 2016).
  44. ^ Wang, X. & R. H. Tedford, 2008. Dogs: their fossil relatives and evolutionary history. New York: Columbia University Press. Pp. 99-98.
  45. ^ “Tallest dog living”. Guinness World Record 2011, 15 February 2010. Retrieved from https://web.archive.org/web/20110711133443/http://www.guinnessworldrecords.com/records/natural_world/fantastic_pets/tallest_dog_living.aspx (last accessed 6 November 2016).
  46. ^ “Smallest dog living (length)”. Guinness World Records. Retrieved from http://www.guinnessworldrecords.com/world-records/smallest-dog-living-(length) (last accessed 23 November 2016).
  47. ^ “Smallest dog living (height)”. Guinness World Records. Retrieved from http://www.guinnessworldrecords.com/world-records/smallest-dog-living-(height) (last accessed 23 November 2016).
  48. ^ Nieber, C., S. Denzau, E. P. Malkemper, J. C. Gross, H. Burda, M. Winklhofer, et al., 2016. Cryptochrome 1 in retinal cone photoreceptors suggests a novel functional role in mammals. Scientific Reports, 6: 21848.
  49. ^ Hart, V., P. Novakova, E. P. Malkemper, S. Begail, V. Hanzal, M. Jezek, et al., 2013. Dogs are sensitive to small variations of the Earth’s magnetic field. Frontiers in Zoology, 10: 80.
  50. ^ Coren, S., 2004. //How dogs think//. First Free Press. Simon & Schuster.
  51. ^ Miklosi, A., 2009. //Dog behaviour, evolution, and cognition//. Oxford University Press.
  52. ^ “Frequency range of dog hearing,” by Elert, G. & T. Condon. The Physics Factbook, 2003. Retrieved from http://hypertextbook.com/facts/2003/TimCondon.shtml (last accessed 6 November 2016).
  53. ^ “Dog sense of hearing”. Seeifido.com. Retrieved from http://www.k9puppydogs.com/html/dog_sense_of_hearing.htm (last accessed 6 November 2016).
  54. ^ Coren, S. & S. Hodgson, 2007. //Understanding your dog for dummies//. Hoboken, N. J. Chichester: Wiley John Wiley, distributor.
  55. ^ Correa, J. E., 2016. The dog’s sense of smell. Alabama A&M and Auburn Universities. Retrieved from http://www.aces.edu/pubs/docs/U/UNP-0066/UNP-0066.pdf (last accessed 6 November 2016).
  56. ^ Coren, S., 2010. //How to speak dog: mastering the art of dog-human communication//. Simon & Schuster, New York.
  57. ^ “Dog”. Wikipedia, 2 November 2016. Retrieved from http://en.wikipedia.org/wiki/Dog (last accessed 6 November 2016).
  58. ^ O’Neill, D. G., D. B. Church, P. D. McGreevy, P. C. Thomson & D. C. Brodbelt, 2013. Longevity and mortality of owned dogs in England. The Veterinary Journal, 198(3): 638-643.
  59. ^ “Dog longevity,” created by K. M. Cassidy. Breed Data Summary, 2007. Retrieved from http://users.pullman.com/lostriver/breeddata.htm (last accessed 6 November 2016).
  60. ^ “Sexual maturity – spay and neuter”. Pets911. Retrieved from https://web.archive.org/web/20090610035254/http://www.pets911.com/hosted/buffalo/puppy/article.php?num=11045 (last accessed 9 November 2016).
  61. ^ “Canis lupus familiaris”, by T. Dewey & S. Bhagat. Animal Diversity Web, 2002. Retrieved from http://animaldiversity.org/site/accounts/information/Canis_lupus_familiaris.html (last accessed 9 November 2016).
  62. ^ Heidenberger, E. & J. Unshelm, 1990. Changes in the behavior of dogs after castration. Tierarztl Prax., 18(1): 69-75.
  63. ^ Morrison, W. B., 1998. //Cancer in dogs and cats// //(1st ed.)//. Williams and Wilkins.
  64. ^ “Sterilisation Programme”. Society for the Prevention of Cruelty to Animals, Singapore. Retrieved from http://www.spca.org.sg/services_sterilisation.asp (last accessed 9 November 2016).
  65. ^ Wheeler, Q. D. & R. Meier, (Eds.), (2000). Species concepts and phylogenetic theory: A debate. New York, NY: Columbia University Press.
  66. ^ Wayne, R. K., B. Van Valkenburgh, P. W. Kat, T. K. Fuller, W. E. Johnson & S. J. O’Brien, 1989. Genetic and morphological divergence among sympatric canids. J. Hered., 80: 447-454.
  67. ^ Dragoo, J. W. & R. L. Honeycutt, 1997. Systematics of mustelid-like carnivores. J. Mamm., 78:426-443.
  68. ^ Wayne, R. K., E. Geffen, D. J. Girman, K. P. Koepfli, L. M. Lau & C. R. Marshall, 1997. Molecular systematics of the Canidae. Syst. Biol., 46(4): 622-653.
  69. ^ Gray, A. P., 1954. //Mammalian hybrids, a check-list with bibliography//. Bucks, U.K.L Farnham Royal.
  70. ^ Vila, C., P. Savolainen, J. E. Maldonado, I. R. Amorim, J. E. Rice, R. L. Honeycutt, et al., 1997. Multiple and ancient origins of the domestic dog. Science, 276: 1687-1689.