ratio of word probabilities predicted from brain for cat and dog

cat	dog
top 10 words in brain distribution (in article): species animal wear light head design time century type size	top 10 words in brain distribution (in article): species animal male female breed human hunt wolf cat live
top 10 words in brain distribution (not in article): horse iron blade steel handle drink lamp metal tool hand	top 10 words in brain distribution (not in article): bird egg feed insect lion water elephant ant habitat forest
times more probable under cat 30 20 10 6 4 2.5 1.25 1 1.25 2.5 4 6 10 20 30 times more probable under dog (words not in the model)
The cat'" ("Felis catus"), also known as the domestic cat'" or "'house cat'" to distinguish it from other felines and felids, is a small predatory carnivorous species of crepuscular mammal that is valued by humans for its companionship and its ability to hunt vermin, snakes, scorpions, and other unwanted household pests. It has been associated with humans for at least 9,500 years. A skilled predator, the cat is known to hunt over 1,000 species for food. It can be trained to obey simple commands. Individual cats have also been known to learn on their own to manipulate simple mechanisms, such as doorknobs. Cats use a variety of vocalizations and types of body language for communication, including meowing, purring, hissing, growling, squeaking, chirping, clicking, and grunting. Cats may be the most popular pet in the world, with over 600 million in homes all over the world. They are also bred and shown as registered pedigree pets. This hobby is known as the "cat fancy." Until recently the cat was commonly believed to have been domesticated in ancient Egypt, where it was a cult animal. However a 2007 study found that the lines of descent of all house cats probably run through as few as five self-domesticating African Wildcats "(Felis silvestris lybica)" circa 8000 BC, in the Near East. Size. Cats typically weigh between 2.5 and 7 kg (5.5–16 pounds); however, some, such as the Maine Coon, can exceed. Some have been known to reach up to due to overfeeding. Conversely, very small cats (less than) have been reported. The largest cat ever was officially reported to have weighed in at about (46 lb 15.25 oz). The smallest cat ever officially recorded weighed around 3 lbs (1.36 kg). Cats average about in height and in head body length (males being larger than females), with tails averaging in length. Skeleton. Cats have 7 cervical vertebrae like almost all mammals, 13 thoracic vertebrae (humans have 12), 7 lumbar vertebrae (humans have 5), 3 sacral vertebrae like most mammals (humans have 5 because of their bipedal posture), and, except for Manx cats, 22 or 23 caudal vertebrae (humans have 3 to 5, fused into an internal coccyx). The extra lumbar and thoracic vertebrae account for the cat's enhanced spinal mobility and flexibility, compared with humans. The caudal vertebrae form the tail, used by the cat as a counterbalance to the body during quick movements. Cats also have free-floating clavicle bones, which allows them to pass their body through any space into which they can fit their heads. Mouth. Cats have highly specialized teeth for the killing of prey and the tearing of meat. The premolar and first molar together compose the carnassial pair on each side of the mouth, which efficiently functions to shear meat like a pair of scissors. While this is present in canids, it is highly developed in felines. The cat's tongue has sharp spines, or papillae, useful for retaining and ripping flesh from a carcass. These papillae are small backward-facing hooks that contain keratin which also assist in their grooming. As facilitated by their oral structure, cats use a variety of vocalizations and types of body language for communication, including meowing, purring, hissing, growling, squeaking, chirping, clicking, and grunting. Ears. Thirty-two individual muscles in each ear allow for a manner of directional hearing: a cat can move each ear independently of the other. Because of this mobility, a cat can move its body in one direction and point its ears in another direction. Most cats have straight ears pointing upward. Unlike dogs, flap-eared breeds are extremely rare (Scottish Folds are one such exceptional mutation.) When angry or frightened, a cat will lay back its ears, to accompany the growling or hissing sounds it makes. Cats also turn their ears back when they are playing, or to listen to a sound coming from behind them. The angle of cats' ears is an important clue to their mood. Legs. Cats, like dogs, are digitigrades. They walk directly on their toes, with the bones of their feet making up the lower part of the visible leg. Cats are capable of walking very precisely, because like all felines they directly register; that is, they place each hind paw (almost) directly in the print of the corresponding forepaw, minimizing noise and visible tracks. This also provides sure footing for their hind paws when they navigate rough terrain. Claws. Like nearly all members of family Felidae, cats have protractable claws. In their normal, relaxed position the claws are sheathed with the skin and fur around the toe pads. This keeps the claws sharp by preventing wear from contact with the ground and allows the silent stalking of prey. The claws on the forefeet are typically sharper than those on the hind feet. Cats can voluntarily extend their claws on one or more paws. They may extend their claws in hunting or self-defense, climbing, "kneading", or for extra traction on soft surfaces (bedspreads, thick rugs, etc.). It is also possible to make a cooperative cat extend its claws by carefully pressing both the top and bottom of the paw. The curved claws may become entangled in carpet or thick fabric, which may cause injury if the cat is unable to free itself. Most cats have five claws on their front paws, and four or five on their rear paws. Because of an ancient mutation, however, domestic and feral cats are prone to polydactylyism, (particularly in the east coast of Canada and north east coast of the United States) and may have six or seven toes. The fifth front claw (the dewclaw) is proximal to the other claws. More proximally, there is a protrusion which appears to be a sixth "finger". This special feature of the front paws, on the inside of the wrists, is the carpal pad, also found on the paws of big cats and dogs. It has no function in normal walking, but is thought to be an anti-skidding device used while jumping. Skin. Cats possess rather loose skin; this allows them to turn and confront a predator or another cat in a fight, even when it has a grip on them. This is also an advantage for veterinary purposes, as it simplifies injections. In fact, the lives of cats with kidney failure can sometimes be extended for years by the regular injection of large volumes of fluid subcutaneously, which serves as an alternative to dialysis. The particularly loose skin at the back of the neck is known as the "scruff", and is the area by which a mother cat grips her kittens to carry them. As a result, cats tend to become quiet and passive when gripped there. This behavior also extends into adulthood, when a male will grab the female by the scruff to immobilize her while he mounts, and to prevent her from running away as the mating process takes place. This technique can be useful when attempting to treat or move an uncooperative cat. However, since an adult cat is heavier than a kitten, a pet cat should never be carried by the scruff, but should instead have its weight supported at the rump and hind legs, and at the chest and front paws. Often (much like a small child) a cat will lie with its head and front paws over a person's shoulder, and its back legs and rump supported under the person's arm. Senses. Cat senses are attuned for hunting. Cats have highly advanced hearing, eyesight, taste, and touch receptors, making the cat extremely sensitive among mammals. Cats' night vision is superior to humans although their vision in daylight is inferior. Cat eyes have a tapetum lucidum and cat eyes that are blue typically lack melanin and hence can show the red-eye effect (see odd-eyed cat). Humans and cats have a similar range of hearing on the low end of the scale, but cats can hear much higher-pitched sounds, up to 64 kHz, which is 1.6 octaves above the range of a human, and even one octave above the range of a dog. A domestic cat's sense of smell is about fourteen times as strong as a human's. Due to a mutation in an early cat ancestor, one of two genes necessary to taste sweetness may have been lost by the cat family. To aid with navigation and sensation, cats have dozens of movable vibrissae (whiskers) over their body, especially their face. Metabolism. Cats conserve energy by sleeping more than most animals, especially as they grow older. The daily duration of sleep varies, usually 12–16 hours, with 13–14 being the average. Some cats can sleep as much as 20 hours in a 24-hour period. The term "cat nap" refers to the cat's ability to fall asleep (lightly) for a brief period and has entered the English lexicon – someone who nods off for a few minutes is said to be "taking a cat nap". Due to their crepuscular nature, cats are often known to enter a period of increased activity and playfulness during the evening and early morning, dubbed the "evening crazies", "night crazies", "elevenses", or "mad half-hour" by some. The temperament of a cat can vary depending on the breed and socialization. Cats with oriental body types tend to be thinner and more active, while cats that have a cobby body type tend to be heavier and less active. The normal body temperature of a cat is between 38 and 39 °C (101 and 102.2 °F). A cat is considered febrile (hyperthermic) if it has a temperature of 39.5 °C (103 °F) or greater, or hypothermic if less than 37.5 °C (100 °F). For comparison, humans have a normal temperature of approximately 36.8 °C (98.6 °F). A domestic cat's normal heart rate ranges from 140 to 220 beats per minute, and is largely dependent on how excited the cat is. For a cat at rest, the average heart rate usually is between 150 and 180 bpm, about twice that of a human (average 80 bpm). Genetics. A 2007 study published in the journal "Science" asserts that all house cats are descended from a group of self-domesticating desert wildcats "Felis silvestris lybica" circa 10,000 years ago, in the Near East. The domesticated cat and its closest wild ancestor are both diploid organisms that possess 38 chromosomes, in which over 200 heritable genetic defects have been identified, many homologous to human inborn errors. Specific metabolic defects have been identified underlying many of these feline diseases. There are several genes responsible for the hair color identified. The combination of them gives different phenotypes. Features like hair length, lack of tail, or presence of a very short tail (bobtail cat) are also determined by single alleles and modified by polygenes. The Cat Genome Project, sponsored by the Laboratory of Genomic Diversity at the U.S. National Cancer Institute Frederick Cancer Research and Development Center in Frederick, Maryland, focuses on the development of the cat as an animal model for human hereditary disease, infectious disease, genome evolution, comparative research initiatives within the family Felidae, and forensic potential. All felines, including the big cats, have a genetic anomaly that may prevent them from tasting sweetness, which is a likely factor for their indifference to or avoidance of fruits, berries, and other sugary foods. Feeding and diet. Cats feed on small prey such as insects, birds, and rodents. Feral cats, or house cats who are free-fed, consume about 8 to 16 small meals in a single day. Despite this, adult cats can adapt to being fed once a day. Cats are classified as obligate carnivores, because their physiology is geared toward efficient processing of meat, and lacks efficient processes for digesting plant matter. The cat cannot produce its own taurine (an essential organic acid), and, as it is contained in flesh, the cat must eat flesh to survive (see Taurine and cats). Similarly as with its teeth, a cat's digestive tract has become specialized over time to suit meat eating, having shortened in length only to those segments of intestine best able to break down proteins and fats from animal flesh. This trait severely limits the cat's ability to properly digest, metabolize, and absorb plant-derived nutrients, as well as certain fatty acids. For example, taurine is scarce in plants but abundant in meats. It is a key amino sulfonic acid for eye health in cats. Taurine deficiency can cause a condition called macular degeneration wherein the cat's retina slowly degenerates, eventually causing irreversible blindness. Despite the cat's meat-oriented physiology, it is still quite common for a cat to supplement its carnivorous diet with small amounts of grass, leaves, shrubs, houseplants, or other plant matter. One theory suggests this behavior helps cats regurgitate if their digestion is upset; another is that it introduces fiber or trace minerals into the diet. In this context, caution is recommended for cat owners because some houseplants are harmful to cats. For example, the leaves of the Easter Lily can cause permanent and life-threatening kidney damage to cats, and Philodendron are also poisonous to cats. The Cat Fanciers' Association has a full list of plants harmful to cats. There are several vegetarian or vegan commercially available cat foods supplemented with chemically synthesized taurine and other added nutrients that attempt to address nutritional shortfalls. Cats can be selective eaters (which may be due in some way to the aforementioned mutation which caused their species to lose sugar-tasting ability). However, cats generally cannot tolerate lack of food for more than 36 hours without risking liver damage. Cats have a fondness for catnip, which is sensed by their olfactory systems. Many enjoy consuming catnip, and most will often roll in it, paw at it, and occasionally chew on it. Cats also can also develop odd eating habits. Some cats like to eat or chew on other things like plastic, paper, string, wool, or even coal. This condition is called pica and can threaten the cat's health depending on the amount and toxicity of the non-food items eaten. The condition's name comes from the Latin word for magpie, a bird which is reputed to eat almost anything. Toxic sensitivity. The liver of a cat is less effective at detoxification than those of other animals, including humans and dogs; therefore exposure to many common substances considered safe for households may be dangerous to them. In general, the cat's environment should be examined for the presence of such toxins and the problem corrected or alleviated as much as possible; in addition, where sudden or prolonged serious illness without obvious cause is observed, the possibility of toxicity must be considered, and the veterinarian informed of any such substances to which the cat may have had access. For instance, the common painkiller paracetamol or acetaminophen, sold under brand names such as Tylenol and Panadol, is extremely toxic to cats; because they naturally lack enzymes needed to digest it, even minute portions of doses safe for humans can be fatal and any suspected ingestion warrants immediate veterinary attention. Even aspirin, which is sometimes used to treat arthritis in cats, is much more toxic to them than to humans and must be administered cautiously. Similarly, application of minoxidil (Rogaine) to the skin of cats, either accidental or by well-meaning owners attempting to counter loss of fur, has sometimes proved fatal. In addition to such obvious dangers as insecticides and weed killers, other common household substances that should be used with caution in areas where cats may be exposed to them include mothballs and other naphthalene products, as well as phenol based products often used for cleaning and disinfecting near cats' feeding areas or litter boxes, such as Pine-Sol, Dettol (Lysol), hexachlorophene, "etc." which, although they are widely used without problem, have been sometimes seen to be fatal. Ethylene glycol, often used as an automotive antifreeze, is particularly appealing to cats, and as little as a teaspoonful can be fatal. Essential oils are toxic to cats and there have been reported cases of serious illnesses caused by tea tree oil, and tea tree oil-based flea treatments and shampoos. Many human foods are somewhat toxic to cats; theobromine in chocolate can cause theobromine poisoning, for instance, although few cats will eat chocolate. Toxicity in cats ingesting relatively large amounts of onions or garlic has also been reported. Even such seemingly safe items as cat food packaged in pull tab tin cans have been statistically linked to hyperthyroidism; although the connection is far from proven, suspicion has fallen on the use of bisphenol A-based plastics, another phenol based product as discussed above, to seal such cans. Many houseplants are at least somewhat toxic to many species, cats included and the consumption of such plants by cats is to be avoided. Sociability. For cats, life in close proximity with humans (and other animals kept by humans) amounts to a "symbiotic social adaptation" which has developed over thousands of years. It has been suggested that, ethologically, the human keeper of a cat functions as a sort of surrogate for the cat's mother, and that adult domestic cats live their lives in a kind of extended kittenhood, a form of behavioral neoteny. Cats may express affection towards their human companions, especially if they imprint on them at a very young age and are treated with consistent affection. Regardless of the average sociability of any given cat or of cats in general, there are still any number of cats who meet or exceed the negative feline stereotype insofar as being poorly socialized. Older cats have also been reported to sometimes develop aggressiveness towards kittens, which may include biting and scratching; this type of behavior is known as Feline Asocial Aggression. Cohabitation. One may see natural house cat behavior by observing feral domestic cats, which are social enough to form colonies. Each cat in a colony holds a distinct territory, with sexually active males having the largest territories, and neutered cats having the smallest. Between these territories are neutral areas where cats watch and greet one another without territorial conflicts. Outside these neutral areas, territory holders usually aggressively chase away stranger cats, at first by staring, hissing, and growling, and if that does not work, by short but noisy and violent attacks. Despite cohabitation in colonies, cats do not have a social survival strategy, or a pack mentality. This mainly means that an individual cat takes care of all basic needs on its own (e.g., finding food, and defending itself), and thus cats are always lone hunters; they do not hunt in groups as dogs or lions do. Cats frequently tonguebathe themselves (see hygiene). The chemistry of their saliva, expended during their frequent grooming, appears to be a natural deodorant. Thus, a cat's cleanliness would aid in decreasing the chance a prey animal could notice the cat's presence. By contrast, dog odor is an advantage in hunting, for a dog is a pack hunter; part of the pack stations itself upwind, and its odor drives prey towards the rest of the pack stationed downwind. This requires a cooperative effort, which in turn requires communication skills. No such communication skills are required of a lone hunter. Fighting. When engaged in feline-to-feline combat for self-defense, territory, reproduction, or dominance, fighting cats make themselves appear more impressive and threatening by raising their fur and arching their backs, thus increasing their apparent size. Cats also behave this way while playing. Attacks usually comprise powerful slaps to the face and body with the forepaws as well as bites, but serious damage is rare; usually the loser runs away with little more than a few scratches to the face, and perhaps the ears. Cats will also throw themselves to the ground in a defensive posture to rake with their powerful hind legs. Normally, serious negative effects will be limited to possible infections of the scratches and bites, though these have been known to sometimes kill cats if untreated. In addition, such fighting is believed to be the primary route of transmission of feline immunodeficiency virus (FIV). Sexually active males will usually be in many fights during their lives, and often have decidedly battered faces with obvious scars and cuts to the ears and nose. Not only males will fight; females will also fight over territory or to defend their kittens. Play. Domestic cats, especially young kittens, are known for their love of play. This behavior mimics hunting and is important in helping kittens learn to stalk, capture, and kill prey. Many cats cannot resist a dangling piece of string, or a piece of rope drawn randomly and enticingly across the floor. This well known love of string is often depicted in cartoons and photographs, which show kittens or cats playing with balls of yarn. It is probably related to hunting instincts, including the common practice of kittens hunting their mother's and each other's tails. If string is ingested, however, it can become caught in the cat’s stomach or intestines, causing illness, or in extreme cases, death. Due to possible complications caused by ingesting a string, string play is sometimes replaced with a laser pointer's dot, which some cats will chase. While caution is called for, there are no documented cases of feline eye damage from a laser pointer, and the combination of precision needed and low energy involved make it a remote risk. A common compromise is to use the laser pointer to draw the cat to a prepositioned toy so the cat gets a reward at the end of the chase. A regular flashlight with a well-focused light spot has been commonly used in such play for decades, preceding the availability of consumer laser pointers. Cats will also engage in play fighting, with each other and with human partners. Humans "wrestling" with a supine cat, however, should be wary: if the cat is overstimulated or startled it may decide that the play has turned serious and cease to pull its punches; this can lead to serious scratches and occasionally even bites. Hunting. Much like their big cat relatives, domestic and feral cats are very effective predators. Domestic felines ambush or pounce upon and immobilize vertebrate prey using tactics similar to those of leopards and tigers. Having overpowered such prey, a cat delivers a lethal neck bite with its long canine teeth that either severs the prey's spinal cord with irreversible paralysis to prey, causes fatal bleeding by puncturing the carotid artery or the jugular vein, or asphyxiates the prey by crushing its trachea. One poorly understood element of cat hunting behavior is the presentation of prey to human owners. Ethologist Paul Leyhausen proposed that cats adopt humans into their social group, and share excess kill with others in the group according to the local pecking order, in which humans are placed at or near the top. However, anthropologist and animal scientist Desmond Morris in his 1986 book "Catwatching" suggests that when cats bring home mice or birds they have caught, they are teaching their human to hunt, or helping their human as if feeding (his words) "an elderly, inept kitten". Another possibility is that presenting the kill might be a relic of a kitten's behavior of demonstrating for its mother's approval that it has developed the necessary skill for hunting. Indoor cats will often retain their hunting instinct and deliver small household items to their owners, such as watches, pens, pencils, and other objects they can carry in their mouths. Reproduction. Cats are seasonally polyestrous, which means they may have many periods of heat over the course of a year. A heat period lasts about 4 to 7 days if the female is bred; if she is not, the heat period lasts longer. Multiple males will be attracted to a female in heat. The males will fight over her, and the victor wins the right to mate. At first, the female will reject the male, but eventually the female will allow the male to mate. The female will give a loud yowl as the male pulls out of her. After mating, the female will give herself a thorough wash. If a male attempts to breed with her at this point, the female will attack him. Once the female is done grooming, the cycle will repeat. The male cat's penis has spines which point backwards. Upon withdrawal of the penis, the spines rake the walls of the female's vagina, which may cause ovulation. Because this does not always occur, females are rarely impregnated by the first male with which they mate. Furthermore, cats are superfecund; that is, a female may mate with more than one male when she is in heat, meaning different kittens in a litter may have different fathers. The gestation period for cats is approximately 63–65 days. The size of a litter averages three to five kittens, with the first litter usually smaller than subsequent litters. Kittens are weaned at between six and seven weeks, and cats normally reach sexual maturity at 5–10 months (females) and to 5–7 months (males). Cats are ready to go to new homes at about 12 weeks old (the recommended minimum age by Fédération Internationale Féline), or when they are ready to leave their mother. Cats can be surgically sterilized (spayed or castrated) as early as 6–8 weeks to limit unwanted reproduction. This surgery also prevents undesirable sex-related behavior, such as territory marking (spraying urine) in males and yowling (calling) in females. If a cat is neutered after such behavior has been learned, however, then the behavior may persist. Hygiene. Cats are known for their fastidious cleanliness. They groom themselves by licking their fur, employing their hooked papillae and saliva. As mentioned, their saliva is a powerful cleaning agent and deodorant. Many cats also enjoy grooming humans or other cats. Sometimes the act of grooming another cat is initiated as an assertion of superior position in the pecking order of a group (dominance grooming). Some cats occasionally regurgitate hairballs of fur that have collected in their stomachs as a result of their grooming. Longhaired cats are more prone to this than shorthaired cats. Hairballs can be prevented with certain cat foods and remedies that ease elimination of the hair and regular grooming of the coat with a comb or stiff brush. Scratching. Cats are naturally driven to periodically hook their front claws into suitable surfaces and pull backwards, in order to clean the claws and remove the worn outer sheath, as well as exercise and stretch their muscles. This scratching behavior seems enjoyable to the cat, and even declawed cats will go through elaborate scratching routines with every evidence of great satisfaction, despite the total lack of results. Some researchers believe this is due to scent glands located in their pads, and that scratching is effectively a part of marking territory. Fondness for heights. Most breeds of cat have a noted fondness for settling in high places, or perching. Animal behaviorists have posited a number of explanations, the most common being that height gives the cat a better observation point, allowing it to survey its territory and become aware of activities of people and other pets in the area. In the wild, a higher place may serve as a concealed site from which to hunt; domestic cats are known to strike prey by pouncing from such a perch as a tree branch, as does a leopard. Height, therefore, can also give cats a sense of security. During a fall from a high place, a cat can reflexively twist its body and right itself using its acute sense of balance and flexibility. This is known as the cat's "righting reflex". It always rights itself in the same way, provided it has the time to do so, during a fall. The height required for this to occur in most cats (safely) is around 90 cm (3 feet). Cats without a tail also have this ability, since a cat mostly moves its hind legs and relies on conservation of angular momentum to set up for landing, and the tail is in fact little used for this feat. However, cats' fondness for high spaces can dangerously test the righting reflex. The American Society for the Prevention of Cruelty to Animals warns owners to safeguard the more dangerous perches in their homes, to avoid "high-rise syndrome", where an overconfident cat falls from an extreme height. Habitat. The African Wildcat ("Felis silvestris lybica"), ancestor of the domestic cat, is believed to have evolved in a desert climate, as evident in the behavior common to both the domestic and wild forms. Wildcats ("Felis sylvestris") are native to all continents other than Australia and Antarctica, although feral cats have become apex predators in the Australian Outback where they are menaces to wildlife. Their feces are usually dry, and cats prefer to bury them in sandy places. Urine is highly concentrated, which allows the cat to retain as much fluid as possible. They are able to remain motionless for long periods, especially when observing prey and preparing to pounce. In North Africa there are still small wildcats that are probably related closely to the ancestors of today's domesticated cat breeds. Being closely related to desert animals, cats enjoy heat and solar exposure, often sleeping in a sunny area during the heat of the day, as part of a general preference for warm temperatures. Where humans typically start to feel uncomfortable when their skin temperature gets higher than about 44.5 °C (112 °F), by contrast cats do not start to show signs of discomfort until their skin reaches about 52 °C (126 °F). Overall, cats can easily withstand the heat and cold of a temperate climate, so long as the cold is not for extended periods. Although certain breeds such as the Norwegian Forest Cat and Maine Coon have developed heavier coats of fur than other cats, they have little resistance against moist cold (e.g., fog, rain, and snow) and struggle to maintain their 39 °C (102 °F) body temperature when wet. In direct relation to that fact, most cats dislike immersion in water. One major exception is the Turkish Van breed which has an unusual fondness for water. Abyssinians and Bengals are also reported to be more tolerant of water than most cats. Impact of hunting. The domestic cat hunts and eats over a thousand species, many of them invertebrates, especially insects — many big cats will eat fewer than a hundred different species. Although theoretically big cats can kill most of these species as well, they often do not due to the relatively low nutritional content that smaller animals provide for the effort. An exception is the leopard, which commonly hunts rabbits and many other smaller animals. Even well-fed domestic cats may hunt and kill birds, mice, rats, scorpions, cockroaches, grasshoppers, and other small animals in their environment. As a consequence of their exceptional hunting ability, cats can be quite destructive to ecosystems in which they are not native, where local species have not had time to adapt to feline introduction. In some cases, cats have contributed to or caused extinctions — for example, see the case of the Stephens Island Wren. Due to their hunting behavior, in many countries feral cats are considered pests. Domestic cats are occasionally also required to have contained cat runs or to be kept inside entirely, as they can be hazardous to locally endangered bird species. For instance, various municipalities in Australia have enacted such legislation. In some localities, owners fit their cat with a bell in order to warn prey of its approach (although some cats may figure out how and when the bell works, thereby learning more careful movements to avoid the ringing). Domestication. In 2004, a grave was excavated in Cyprus that contained the skeletons, laid close to one another, of both a human and a cat. The grave is estimated to be 9,500 years old, pushing back the earliest known feline-human association significantly. Like some other domesticated animals, cats live in a mutualistic arrangement with humans. It is believed that the benefit of removing rats and mice from humans' food stores outweighed the trouble of extending the protection of a human settlement to a formerly wild animal, almost certainly for humans who had adopted a farming economy. Unlike the dog, which also hunts and kills rodents, the cat does not eat grains, fruits, or vegetables. In modern rural areas, farms often have dozens of semi-feral cats. Hunting in the barns and the fields, they kill and eat rodents that would otherwise spoil large parts of the grain crop. Many pet cats successfully hunt and kill rabbits, rodents, birds, lizards, frogs, fish, and large insects by instinct, but might not eat their prey. In modern urban areas, some people find feral and free-roaming pet cats annoying and intrusive. Unaltered cats can engage in persistent nighttime calling (termed caterwauling) and defecation or "marking" of private property. Indoor confinement of pets and TNR programs for feral cats can help; some people also use cat deterrents to discourage cats from entering their property. In captivity, indoor cats typically live 14 to 20 years, though the oldest known cat lived to age 36. Domesticated cats tend to live longer if they are not permitted to go outdoors (reducing the risk of injury from fights or accidents and exposure to diseases) and if they are neutered. Some such benefits are: castrated male cats cannot develop testicular cancer, spayed female cats cannot develop ovarian cancer, and both have a reduced risk of mammary cancer. Interaction with humans. Human attitudes toward cats vary widely. Some people keep cats for casual companionship as pets. Cats are also bred and shown as registered pedigree pets, in a hobby known as the cat fancy. Because of their small size, domesticated house cats pose almost no major danger to adult humans — the main hazard is the possibility of infection (e.g., cat scratch disease, or, rarely, rabies) from a cat bite or scratch. Cats can also potentially inflict severe scratches or puncture an eye, though this is quite rare (although dogs have been known to be blinded by cats in fights, where the cat specifically and accurately targeted the eyes of the larger animal). Allergens. Allergic reactions to cat dander and or cat saliva inspire one of the most common reasons people cite for disliking cats. Some humans who are allergic to	The dog'" ("Canis lupus familiaris",) is a domesticated subspecies of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The domestic dog has been one of the most widely kept working and companion animals in human history. The domestication of the gray wolf took place in a handful of events roughly 15,000 years ago in central Asia. The dog quickly became ubiquitous across culture in all parts of the world, and was extremely valuable to early human settlements. For instance, it is believed that the successful emigration across the Bering Strait might not have been possible without sled dogs. As a result of the domestication process, the dog developed a sophisticated intelligence that includes unparalleled social cognition and a simple theory of mind that is important to their interaction with humans. These social skills have helped the dog to perform in myriad roles, such as hunting, herding, protection, and, more recently, assisting handicapped individuals. Currently, there are estimated to be 400 million dogs in the world. Over the 15,000 year span that the dog had been domesticated, it diverged into only a handful of landraces, groups of similar animals whose morphology and behavior have been shaped by environmental factors and functional roles. Humans did not take an active, intentional role in this process until the last few hundred years. As the modern understanding of genetics developed, humans began to intentionally breed dogs for a wide range of specific traits. Through this process, the dog has developed into hundreds of varied breeds, and shows more behavioral and morphological variation than any other land mammal. For example, height measured to the withers ranges from a few inches in the Chihuahua to a few feet in the Irish Wolfhound; color varies from white through grays (usually called "blue'") to black, and browns from light (tan) to dark ("red" or "chocolate") in a wide variation of patterns; coats can be short or long, coarse-haired to wool-like, straight, curly, or smooth. It is common for most breeds to shed this coat, but non-shedding breeds are also popular. Etymology and related terminology. "Dog" is the common use term that refers to members of the subspecies "Canis lupus familiaris". The term is sometimes used to refer to a wider range of species: it can be used to refer to any mammal belonging to the family Canidae, which includes wolves, foxes, jackals, and coyotes; it can be used to refer to the subfamily of Caninae, or the genus Canis, also often called the "true dogs". Some members of the family have "dog" in their common names, such as the raccoon dog and the African wild dog. A few animals have "dog" in their common names but are not canids, such as the prairie dog and the dog fish. The English word "dog" can be traced back to the Old English "docga", a "powerful breed of canine". The term may derive from Proto-Germanic "*dukkōn", represented in Old English "finger-docce" ("finger-muscle"). Due to the linguistically archaic structure of the word, the term "dog" may ultimately derive from the earliest layer of Proto-Indo-European vocabulary, reflecting the role of the dog as the earliest domesticated animal. The English word "hound", which refers to a specific breed group in English, means "dog" in general in other Germanic languages; it is cognate to German "hund", Dutch "hond", common Scandinavian "hund", and Icelandic "hundur". "Hound" itself is derived from the Proto-Indo-European "*kwon-", which is also the direct root of the Greek κυων (kuōn) and the indirect root of the Latin "canis" through the variant form "*kani-". In breeding circles, a male canine is referred to as a "dog", while a female is called a "bitch". A group of offspring is a "litter". The father of a litter is called the "sire", and the mother is called the "dam". Offspring are generally called "pups" or "puppies" until they are about a year old. The process of birth is "whelping". Taxonomy and evolution. The domestic dog was originally classified as "Canis familiaris" and "Canis familiarus domesticus" by Linnaeus in 1758, and is currently classified as "Canis lupus familiaris", a subspecies of the gray wolf "Canis lupus", by the Smithsonian Institution and the American Society of Mammalogists. Overwhelming evidence from behavior, vocalizations, morphology, and molecular biology led to the contemporary scientific understanding that a single species, the gray wolf, is the common ancestor for all breeds of domestic dogs, however the timeframe and mechanisms by which dogs diverged are controversial. The current consensus among biologists and archaeologists is that no one can be sure when dogs were domesticated. There is conclusive evidence that dogs genetically diverged from their wolf ancestors at least 15,000 years ago but most believe domestication to have occurred much earlier. The evidence cited for an earlier divergence comes from archaeological findings and mitochondrial DNA studies, both of which are inconclusive. The archaeological evidence demonstrates that the domestication of dogs occurred prior to 15,000 years ago. 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. The earliest dog fossils, two large skulls from Russia and a mandible from Germany, date from roughly 14,000 years ago. Their likely ancestor is the large Eurasian wolf ("Canis lupus lupus"). Remains of smaller dogs from Natufian cave deposits in the Middle East have been dated to around 10,000 to 12,000 years ago. There is a great deal of archealogical 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. DNA studies have provided a wider range of possible divergence dates, from 15,000 to 40,000 years ago, to as much as 100,000 to 140,000 years ago. This evidence depends on a number of assumptions that others claim are violated. Genetic studies are based in comparisons of genetic diversity between species, and depend on a calibration date, such as the wolf-coyote divergence date, which is estimated to be roughly 1 million years ago. If this divergence date is closer to 750,000 or 2 million years ago, then genetic analyses 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, which could bias DNA analyses to support an earlier divergence. 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, assumes 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. Genetic analyses indicate all dogs are likely descended from a handful of domestication events with a small number of founding females, although there is evidence that domesticated dogs interbred with local populations of wild wolves on several occasions. 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. 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, Saluki, Afghan Hound, Tibetan Terrier, Lhasa Apso, Chow Chow, Pekingese, Shar-Pei, Shi Tzu, Akita, Shiba Inu, Alaskan Malamute, Siberian Husky, and Samoyed. 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 recreated more recently. There is a great deal of controversy surrounding the evolutionary framework for the domestication of dogs. 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 these canine species to the presence of the close relatives of modern man. If dogs were domesticated, as believed, roughly 15,000 years ago, 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. Biology. Domestic dogs have been selectively bred for millennia for various behaviors, sensory capabilities, and physical attributes. 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. 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 dog was a Yorkshire Terrier, who stood only 6.3 cm (2.5 in) at the shoulder, 9.5 cm (3.75 in) in length along the head-and-body, and weighed only 113 grams (4 ounces). The largest known dog was an English Mastiff which weighed 155.6 kg (343 lbs) and was 250 cm (8.2 feet) from the snout to the tail. The tallest dog is a Great Dane that stands 106.7 cm (42.2 in) at the shoulder. Sight. The dog's visual system is engineered to serve the purposes of a hunter. 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), their visual discrimination for moving objects is very high; dogs have been shown to be able to discriminate between humans (i.e., identifying their owner) from distances up to a mile. 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. The tapetum is a reflective surface behind the retina that reflects light back to give the photoreceptors a second chance to catch the photons. Like most mammals, dogs are dichromats and have color vision equivalent to red-green color blindness in humans. The eyes of different breeds of dogs have different shapes, dimensions, and retina configurations. 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. 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. Hearing. The frequency range of dog hearing is approximately 40 Hz to 60,000 Hz, which means that dogs can detect sounds outside both ends of the human auditory spectrum. Additionally, dogs have ear mobility which allows them to rapidly pinpoint the exact location of a sound. 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. Smell. While the human brain is dominated by a large visual cortex, the dog brain is largely dominated by an olfactory cortex. 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. The bloodhound exceeds this standard with nearly 300 million receptors. Dogs can discriminate odors at concentrations nearly 100 million times lower than humans can. Coat. The coats of domestic dogs are either "double", made up of a coarse topcoat and a soft undercoat, 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. This is a pattern that predators can learn to watch for. A countershaded animal will have dark coloring on its upper surfaces and light coloring below, which reduces its general visibility. Thus many breeds will have an occasional "blaze", stripe, or "star" of white fur on their chest or underside. 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). In some breeds, puppies can be born with a short tail or no tail at all. This occurs more frequently in those breeds that are frequently docked and thus have no breed standard regarding the tail. Types and breeds. While all dogs are genetically very similar, natural selection and selective breeding have reinforced certain characteristics in certain populations of dogs, giving rise to dog types and dog breeds. Dog types are broad categories based on function, genetics, or characteristics. Dog breeds are groups of animals that possess a set of inherited characteristics that distinguishes them from other animals within the same species. Modern dog breeds are non-scientific classifications of dogs kept by modern kennel clubs. Purebred dogs of one breed are genetically distinguishable from purebred dogs of other breeds, but the means by which kennel clubs classify dogs is unsystematic. Systematic analyses of the dog genome has revealed only four major types of dogs that can be said to be statistically distinct. These include the "old world dogs" (e.g., Malamute and Shar-Pei), "Mastiff"-type (e.g., Labrador Retriever), "herding"-type (e.g., St. Bernard), and "all others" (also called "modern"- or "hunting"-type). Health. Dogs are susceptible to various diseases, ailments, and poisons, some of which can affect humans. To defend against many common diseases, dogs are often vaccinated. Some breeds of dogs are prone to certain genetic ailments such as elbow or hip dysplasia, blindness, deafness, pulmonic stenosis, cleft palate, and trick knees. Two serious medical conditions particularly affecting dogs are pyometra, affecting unspayed females of all types and ages, and bloat, which affects the larger breeds or deep-chested dogs. Both of these are acute conditions, and can kill rapidly. Dogs are also susceptible to parasites such as fleas, ticks, and mites, as well as hookworms, tapeworms, roundworms, and heartworms. Dogs are also vulnerable to some of the same health conditions as humans, including diabetes, dental and heart disease, epilepsy, cancer, hypothyroidism, and arthritis. Mortality. The typical lifespan of dogs varies widely among breeds, but for most the median longevity, the age at which half the dogs in a population have died and half are still alive, ranges from 10 to 13 years. Individual dogs may live well beyond the median of their breed. The breed with the shortest lifespan (among breeds for which there is a questionnaire survey with a reasonable sample size) is the Dogue de Bordeaux, with a median longevity of about 5.2 years, but several breeds, including Miniature Bull Terriers, Bulldogs, Nova Scotia Duck-Tolling Retrievers, Bloodhounds, Irish Wolfhounds, Greater Swiss Mountain Dogs, Great Danes, and Mastiffs, are nearly as short-lived, with median longevities of 6 to 7 years. The longest-lived breeds, including Toy Poodles, Border Terriers, Miniature Dachshunds, Miniature Poodles, and Tibetan Spaniels, have median longevities of 14 to 15 years. The median longevity of mixed breed dogs, taken as an average of all sizes, is one or more years longer than that of purebred dogs when all breeds are averaged. The dog widely reported to be the longest-lived is "Bluey," who died in 1939 and was claimed to be 29.5 years old at the time of his death; however, the Bluey record is anecdotal and unverified. The longest verified records are of dogs living for 24 years. Predation. Although wild dogs, like wolves, are apex predators, they can be killed in territory disputes with wild animals. Furthermore, in areas where both dogs and other large predators live, dogs can be a major food source for big cats or canines. Reports from Croatia indicate that dogs are killed more frequently than sheep. Wolves in Russia apparently limit feral dog populations. In Wisconsin, more compensation has been paid for dog losses than livestock. Some wolf pairs have been reported to prey on dogs by having one wolf lure the dog out into heavy brush where the second animal waits in ambush. In some instances, wolves have displayed an uncharacteristic fearlessness of humans and buildings when attacking dogs, to the extent that they have to be beaten off or killed. Coyotes and big cats have also been known to attack dogs. Leopards in particular are known to have a predilection for dogs, and have been recorded to kill and consume them regardless of the dog's size or ferocity. Tigers in Manchuria, Indochina, Indonesia, and Malaysia, are reputed to kill dogs with the same vigor as leopards. Striped Hyenas are major predators of village dogs in Turkmenistan, India, and the Caucasus. Diet. Despite its descent from wolves, the domestic dog is an omnivore, though it is classified in the order Carnivora. Unlike an obligate carnivore, such as a member of the cat family with its shorter small intestine, a dog is neither dependent on meat-specific protein nor a very high level of protein in order to fulfill its basic dietary requirements. Dogs are able to healthily digest a variety of foods, including vegetables and grains, and can consume a large proportion of these in their diet. In the wild, canines often eat available plants and fruits. Reproduction. In domestic dogs, sexual maturity begins to happen around age six to twelve months for both males and females, although this can be delayed until up to two years old for some large breeds. This is the time at which female dogs will have their first estrous cycle. They will experience subsequent estrous cycles biannually, during which the body prepares for pregnancy. At the peak of the cycle, females will come into estrus, being mentally and physically receptive to copulation. Because the ova survive and are capable of being fertilized for a week after ovulation, it is possible for a female to mate with more than one male. Adolescence for most domestic dogs is around 12 to 15 months, beyond which they are for the most part more adult than puppy. Domestication has selectively bred for higher libido and earlier and more frequent breeding cycles in dogs than in their wild ancestors, and dogs remain reproductively active until old age. Dogs bear their litters roughly 56 to 72 days after fertilization, with an average of 63 days, although the length of gestation can vary. An average litter consists of about six puppies, though this number may vary widely based on the breed of dog. Toy dogs generally produce from one to four puppies in each litter, while much larger breeds may average as many as twelve. Some dog breeds have acquired traits through selective breeding that interfere with reproduction. Male French Bulldogs, for instance, are incapable of mounting the female. For many dogs of this breed, the female must be artificially inseminated in order to reproduce. Spaying and neutering. Neutering refers to the sterilization of animals, usually by removal of the male's testicles or the female's ovaries and uterus, in order to eliminate the ability to procreate and reduce sex drive. Because of the overpopulation of dogs in some countries, animal control agencies, such as the American Society for the Prevention of Cruelty to Animals (ASPCA), advise that dogs not intended for further breeding should be neutered, so that they do not have undesired puppies that may have to be destroyed later. According to the Humane Society of the United States, 3–4 million dogs and cats are put down each year in the United States and many more are confined to cages in shelters because there are many more animals than there are homes. Spaying or castrating dogs helps keep overpopulation down. Local humane societies, SPCAs, and other animal protection organizations urge people to neuter their pets and to adopt animals from shelters instead of purchasing them. Several notable public figures have spoken out against animal overpopulation, including Bob Barker. On his game show, "The Price Is Right", Barker stressed the problem at the end of every episode, saying: "Help control the pet population. Have your pets spayed or neutered." The current host, Drew Carey, makes a similar plea at the conclusion of each episode. Neutering reduces problems caused by hypersexuality, especially in male dogs. Spayed female dogs are less likely to develop some forms of cancer, affecting mammary glands, ovaries, and other reproductive organs. However, neutering increases the risk of urinary incontinence in female dogs, and prostate cancer in males, as well as osteosarcoma, hemangiosarcoma, cruciate ligament rupture, obesity, and diabetes mellitus in either gender. The hormonal changes involved with sterilization can change the animal's personality and metabolism. Recent studies proved that spayed and neutered dogs in general are more aggressive towards people and other dogs, and more fearful and sensitive to touch than dogs than had not been sterilized. Spaying or neutering very young animals ("early-age spay"), can result in increased health concerns later on in life for both sexes. Incontinence in female dogs is made worse by spaying too early. In both males and females, alteration causes changes in hormones during development. This inhibits the natural signals needed for proper body development, leading to larger animals with greater risk for hip dysplasia, osteoporosis, and other joint disorders. Other studies have shown, however, that early-age neutering of male dogs is associated with no major risks compared to neutering at the more traditional age of six months. Behavior and intelligence. Although dogs have been the subject of a great deal of Behaviorist psychology (e.g.,Pavlov's Dog), they do not enter the world with a psychological "blank slate". Rather, dog behavior is affected by genetic factors as well as environmental factors. Domestic dogs exhibit a number of behaviors and predispositions that were inherited from wolves. The grey wolf is a social animal that has evolved a sophisticated means of communication and social structure. The domestic dog has inherited some of these predispositions, but many of the salient characteristics in dog behavior have been largely shaped by selective breeding by humans. Thus, some of these characteristics, such as the dog's highly developed social cognition, are found only in primitive forms in grey wolves. Intelligence. "Intelligence" is an umbrella term that encompasses the faculties involved in a wide range of mental tasks, such as learning, problem-solving, and communication. The domestic dog has a predisposition to exhibit a social intelligence that is uncommon in the animal world. Dogs are capable of learning in a number of ways, such as through simple reinforcement learning (e.g.,classical or operant conditioning) and by observation. Dogs go through a series of stages of cognitive development. They are not born with an understanding of object permanence; the understanding that objects which are not being actively perceived still remain in existence. The understanding of object permanence occurs during as the infant is learning the coordination of secondary circular reactions, as described by Jean Piaget. That is, as the infants learn to interact intentionally with objects around it. For dogs, this occurs at roughly 8 weeks of age. Handlers of working dogs such as herding dogs and sled dogs know that pups learn behaviors quickly by following examples set by experienced dogs. Many of these behaviors are allelomimetic in that they depend on a genetic predisposition to learn and imitate behaviors of other dogs. This form of intelligence is not peculiar to those tasks dogs have been bred to perform, but can be generalized to myriad abstract problems. Adler & Alder demonstrated this by giving Dachshund puppies the task of learning to pull a cart by tugging on an attached piece of ribbon in order to get a reward in the cart. Puppies that watched an experienced dog successfully retrieve the reward in this way learned the task fifteen times faster than those who were left to solve the problem on their own. In addition to learning by example from other dogs, dogs have also been shown to learn by mimicking human behaviors. In another study, puppies were presented with a box, and shown that when a handler pressed a lever, a ball would roll out of the box. The handler then allowed the puppy to play with the ball, making it an intrinsic reward. The pups were then allowed to interact with the box. Roughly three-quarters of the puppies subsequently touched the lever, and over half successfully released the ball, compared to only 6 percent in a control group that did not watch the human manipulate the lever. Furthermore, the ability for dogs to learn by example has been shown to be as effective as operant conditioning. McKinley and Young have demonstrated that dogs show equivalent accuracy and learning times when taught to identify an object by operant conditioning as they do when taught by human example. Their study found that handing an object between experimenters who then use its name in a sentence successfully taught an observing dog each object's name, allowing the dog to subsequently retrieve the item. Dogs also demonstrate sophisticated social cognition by associating behavioral cues with abstract meanings. One such class of social cognition involves the understanding that others are conscious agents. This is called a theory of mind, and is an area in which dogs excel. Research has shown that dogs are capable of interpreting subtle social cues, and appear to recognize when a human or dog's attention is focused on them. To test this, researchers devised a task in which a reward was hidden underneath one of two buckets. The experimenter then attempted to communicate with the dog to indicate the location of the reward by using a wide range of signals: tapping the bucket, pointing to the bucket, nodding to the bucket, or simply looking at the bucket. The results showed that domestic dogs were better than chimpanzees, wolves, and human infants at this task, and even young puppies with limited exposure to humans performed well. Dr. Stanley Coren, an expert on dog psychology, states that these results demonstrated the social cognition of dogs can exceed that of even our closest genetic relatives, and that this capacity is a recent genetic acquisition which distinguishes the dog from its ancestor, the wolf. Studies have also investigated whether dogs engaged in partnered play change their behavior depending on the attention-state of their partner. Those studies showed that play signals were only sent when the dog was holding the attention of its partner. If the partner was distracted, the dog instead engaged in attention-getting behavior before sending a play signal. Dr. Coren has also argued that dogs demonstrate a sophisticated theory of mind by engaging in deception, which he supports with a number of anecdotes, including one example where a dog hid a stolen treat by sitting on it until the rightful owner of the treat left the room. Although this could have been accidental, Coren suggests that the thief understood that the treat's owner would be unable to find the treat if it were out of view. Together, the empirical data and anecdotal evidence points to dogs possessing at least a limited form of theory of mind. Interactions with humans. Domestic dogs inherited a complex social hierarchy and behaviors from their ancestor, the wolf. Dogs are pack animals with a complex set of behaviors related to determining each dog's position in the social hierarchy, and they exhibit various postures and other means of nonverbal communication that reveal their states of mind. These sophisticated forms of social cognition and communication may account for their trainability, playfulness, and ability to fit into human households and social situations, and these attributes have earned dogs a unique relationship with humans despite being potentially dangerous apex predators. Although experts largely disagree over the details of dog domestication, it is agreed that human interaction played a significant role in shaping the subspecies. Shortly after domestication, dogs became ubiquitous in human populations, and spread throughout the world. Emigrants from Siberia likely crossed the Bering Strait with dogs in their company, and some experts suggest that use of sled dogs may have been critical to the success of the waves that entered North America roughly 12,000 years ago. Dogs were an important part of life for the Athabascan population in North America, and were their only domesticated animal. Dogs also carried much of the load in the migration of the Apache and Navajo tribes 1,400 years ago. Use of dogs as pack animals in these cultures often persisted after the introduction of the horse to North America. Work. Dogs have lived and worked with humans in so many roles that they have earned the unique nickname, "man's best friend", a phrase which is used in other languages as well. They have been bred for herding livestock, hunting (e.g.,pointers, hounds), keeping living spaces clear of rats, guarding, helping fishermen with nets, and pulling loads, in addition to their roles as companions. More recently, dogs have taken on a number of roles under the general classification of service dogs, which provide assistance to individuals with physical or mental disabilities. Service dogs include guide dogs, utility dogs, assistance dogs, hearing dogs, and psychological therapy dogs. Some dogs owned by epileptics have even 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. Sports and shows. Owners of dogs often enter them in competitions, whether show (breed conformation shows) or sports, including dog racing and dog sledding. Conformation shows, also referred to as breed shows, are a type of dog show in which a judge familiar with a specific dog breed evaluates individual purebred dogs for how well the dogs conform to their established breed type as described in their breed standard. As the breed standard only deals with the externally observable qualities of the dog (such as appearance, movement, and temperament), separately tested-for qualities are not part of the judging in conformation shows (such as tests for ability, health, or any other specific tests for characteristics that cannot be directly observed). Physical characteristics. Compared to equally sized wolves, dogs tend to have 20% smaller skulls, 10% smaller brains, as well as proportionately smaller teeth than other canid species. Dogs require fewer calories to function than wolves. Their diet of human refuse in antiquity made the large brains and jaw muscles needed for hunting unnecessary. It is thought by certain experts that the dog's limp ears are a result of atrophy of the jaw muscles. The skin of domestic dogs tends to be thicker than that of wolves, with some Inuit tribes favoring the former for use as clothing due to its greater resistance to wear and tear in harsh weather. The paws of a dog are half the size of those of a wolf, and their tails tend to curl upwards, another trait not found in wolves. Behavior. Dogs tend to be poorer than wolves at observational learning, being more responsive to instrumental conditioning. Feral dogs show little of the complex social structure or dominance hierarchy present in wolf packs. For dogs, other members of their kind are of no help in locating food items, and are more like competitors. Feral dogs are primarily scavengers, with studies showing that unlike their wild cousins, they are poor ungulate hunters, having little impact on wildlife populations where they are sympatric. However, feral dogs have been reported to be effective hunters of reptiles in the Galapagos islands, and free ranging pet dogs are more prone to predatory behavior toward wild animals. Despite common belief, domestic dogs can be monogamous. Breeding in feral packs can be, but does not have to be restricted to a dominant alpha pair (despite common belief, such things also occur in wolf packs). Male dogs are unusual among canids by the fact that they mostly seem to play no role in raising their puppies, and do not kill the young of other females to increase their own reproductive success. Some sources say that dogs differ from wolves and most other large canid species by the fact that they do not regurgitate food for their young, nor the young of other dogs in the same territory. However, this difference was not observed in all domestic dogs. Regurgitating of food by the females for the young as well as care for the young by the males has been observed in domestic dogs, dingos as well as in other feral or semi-feral dogs. Regurgitating of food by the females and direct choosing of only one mate has been observed even in those semi-feral dogs of direct domestic dog ancestry. Also regurgitating of food by males has been observed in free-ranging domestic dogs. Trainability. Dogs display much greater tractability than tame wolves, and are generally much more responsive to coercive techniques involving fear, aversive stimuli, and force than wolves, which are most responsive toward positive conditioning and rewards. Unlike tame wolves, dogs tend to respond more to voice than hand signals. Although they are less difficult to control than wolves, they can be comparatively more difficult to teach than a motivated wolf.