Test 3
Tuesday, January 19, 2016

Domestic cats are similar in size to the other members of the genus Felis, typically weighing between 4 and 5 kg (9 and 10 lb).^[26] Some breeds, however, such as the Maine Coon, can occasionally exceed 11 kg (25 lb). Conversely, very small cats, less than 2 kg (4 lb), have been reported.^[46] The world record for the largest cat is 21 kg (50 lb).^[47] The smallest adult cat ever officially recorded weighed around 1 kg (2 lb).^[47] Feral cats tend to be lighter as they have more limited access to food than house cats. In the Boston area, the average feral adult male will weigh 4 kg (9 lb) and average feral female 3 kg (7 lb).^[48] Cats average about 23–25 cm (9–10 in) in height and 46 cm (18 in) in head/body length (males being larger than females), with tails averaging 30 cm (12 in) in length.^[49]

Cats have seven cervical vertebrae, as do almost all mammals; 13 thoracic vertebrae (humans have 12); seven lumbar vertebrae (humans have five); three sacral vertebrae like most mammals (humans have five); and a variable number of caudal vertebrae in the tail (humans retain three to five caudal vertebrae, fused into an internal coccyx).^[50]:11 The extra lumbar and thoracic vertebrae account for the cat's spinal mobility and flexibility. Attached to the spine are 13 ribs, the shoulder, and the pelvis.^[50] :16 Unlike human arms, cat forelimbs are attached to the shoulder by free-floating clavicle bones which allow them to pass their body through any space into which they can fit their head.^[51]Cat skull

The cat skull is unusual among mammals in having very large eye sockets and a powerful and specialized jaw.^[52]:35 Within the jaw, cats have teeth adapted for killing prey and tearing meat. When it overpowers its prey, a cat delivers a lethal neck bite with its two long canine teeth, inserting them between two of the prey's vertebrae and severing its spinal cord, causing irreversible paralysis and death.^[53] Compared to other felines, domestic cats have narrowly spaced canine teeth, which is an adaptation to their preferred prey of small rodents, which have small vertebrae.^[53] The premolar and first molar together compose thecarnassial pair on each side of the mouth, which efficiently shears meat into small pieces, like a pair of scissors. These are vital in feeding, since cats' small molars cannot chew food effectively.^[52]:37

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.^[54] 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 fore paw, minimizing noise and visible tracks. This also provides sure footing for their hind paws when they navigate rough terrain. Unlike most mammals, when cats walk, they use a "pacing" gait; that is, they move the two legs on one side of the body before the legs on the other side. This trait is shared with camels andgiraffes. As a walk speeds up into a trot, a cat's gait changes to be a "diagonal" gait, similar to that of most other mammals (and many other land animals, such as lizards): the diagonally opposite hind and fore legs move simultaneously.^[55]

Like almost all members of the Felidae, cats have protractable and retractable claws.^[56] In their normal, relaxed position, the claws are sheathed with the skin and fur around the paw's 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 fore feet are typically sharper than those on the hind feet.^[57] 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. Most cats have five claws on their front paws, and four on their rear paws.^[58] The fifth front claw (the dewclaw) is proximal to the other claws. More proximally 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 antiskidding device used while jumping. Some breeds of cats are prone to polydactyly (extra toes and claws).^[58] These are particularly common along the northeast coast of North America.^[59]

Cats are obligate carnivores: their physiology has evolved to efficiently process meat, and they have difficulty digesting plant matter.^[21] In contrast to omnivores such as rats, which only require about 4% protein in their diet, about 20% of a cat's diet must be protein.^[21] Cats are unusually dependent on a constant supply of the amino acid arginine, and a diet lacking arginine causes marked weight loss and can be rapidly fatal.^[67] Another unusual feature is that the cat cannot produce taurine, with taurine deficiency causing macular degeneration, wherein the cat's retina slowly degenerates, causing irreversible blindness.^[21]Cats conserve heat by reducing the flow of blood to their skin and lose heat by evaporation through their mouths. Cats have minimal ability to sweat, with glands located primarily in their paw pads,^[62]and pant for heat relief only at very high temperatures^[63] (but may also pant when stressed). A cat's body temperature does not vary throughout the day; this is part of cats' general lack of circadian rhythms and may reflect their tendency to be active both during the day and at night.^[64]:1 Cats' feces are comparatively dry and their urine is highly concentrated, both of which are adaptations to allow cats to retain as much water as possible.^[21] Their kidneys are so efficient, they can survive on a diet consisting only of meat, with no additional water,^[65] and can even rehydrate by drinking seawater.^[64]:29[66]Cats are familiar and easily kept animals, and their physiology has been particularly well studied; it generally resembles those of other carnivorous mammals, but displays several unusual features probably attributable to cats' descent from desert-dwelling species.^[21] For instance, cats are able to tolerate quite high temperatures: Humans generally start to feel uncomfortable when their skin temperature passes about 38 °C (100 °F), but cats show no discomfort until their skin reaches around 52 °C (126 °F),^[52]:46 and can tolerate temperatures of up to 56 °C (133 °F) if they have access to water.^[61]

A cat's gastrointestinal tract is adapted to meat eating, being much shorter than that of omnivores and having low levels of several of the digestive enzymes needed to digest carbohydrates.^[68] These traits severely limit the cat's ability to digest and use plant-derived nutrients, as well as certain fatty acids.^[68] Despite the cat's meat-oriented physiology, several vegetarian or vegan cat foods have been marketed that are supplemented with chemically synthesized taurine and other nutrients, in attempts to produce a complete diet. However, some of these products still fail to provide all the nutrients cats require,^[69] and diets containing no animal products pose the risk of causing severe nutritional deficiencies.^[70]

Cats do eat grass occasionally. A proposed explanation is that cats use grass as a source of folic acid. Another proposed explanation is that it is used to supply dietary fiber.^[71]

increasing the eye's sensitivity to dim light.^[72] Another adaptation to dim light is the large pupils of cats' eyes. Unlike some big cats, such as tigers, domestic cats haveslit pupils.^[73] These slit pupils can focus bright light without chromatic aberration, and are needed since the domestic cat's pupils are much larger, relative to their eyes, than the pupils of the big cats.^[73] At low light levels a cat's pupils will expand to cover most of the exposed surface of its eyes.^[74] However, domestic cats have rather poor color vision and (like most nonprimate mammals) have only two types of cones, optimized for sensitivity to blue and yellowish green; they have limited ability to distinguish between red and green.^[75] A 1993 paper reported a response to middle wavelengths from a system other than the rodswhich might be due to a third type of cone. However, this appears to be an adaptation to low light levels rather than representing true trichromatic vision.^[76]

Cats have excellent hearing and can detect an extremely broad range of frequencies. They can hear higher-pitched sounds than either dogs or humans, detecting frequencies from 55 Hz to 79,000 Hz, a range of 10.5 octaves, while humans and dogs both have ranges of about 9 octaves.^[77][78] Cats can hear ultrasound, which is important in hunting^[79] because many species of rodents make ultrasonic calls.^[80] However, they do not communicate using ultrasound like rodents do. Cats' hearing is also sensitive and among the best of any mammal,^[77] being most acute in the range of 500 Hz to 32 kHz.^[81] This sensitivity is further enhanced by the cat's large movable outer ears (their pinnae), which both amplify sounds and help detect the direction of a noise.^[79]

Cats have relatively few taste buds compared to humans. Domestic and wild cats share a gene mutation that keeps their sweet taste buds from binding to sugary molecules, leaving them with no ability to tastesweetness.^[88] Their taste buds instead respond to amino acids, bitter tastes, and acids.^[89]Cats have an acute sense of smell, due in part to their well-developed olfactory bulb and a large surface of olfactory mucosa, about 5.8 cm² (0.90 in²) in area, which is about twice that of humans.^[82] Cats are sensitive to pheromones such as 3-mercapto-3-methylbutan-1-ol,^[83] which they use to communicate through urine spraying and marking with scent glands.^[84] Many cats also respond strongly to plants that contain nepetalactone, especially catnip, as they can detect that substance at less than one part per billion.^[85] About 70—80% of cats are affected by nepetalactone.^[86] This response is also produced by other plants, such as silver vine (Actinidia polygama) and the herb valerian; it may be caused by the smell of these plants mimicking a pheromone and stimulating cats' social or sexual behaviors.^[87]

To aid with navigation and sensation, cats have dozens of movable whiskers (vibrissae) over their body, especially their faces. These provide information on the width of gaps and on the location of objects in the dark, both by touching objects directly and by sensing air currents; they also trigger protective blink reflexes to protect the eyes from damage.^[52]:47

Most breeds of cat have a noted fondness for settling in high places, or perching. In the wild, a higher place may serve as a concealed site from which to hunt; domestic cats may strike prey by pouncing from a perch such as a tree branch, as does a leopard.^[90] Another possible explanation is that height gives the cat a better observation point, allowing it to survey its territory. 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.^[91] This is known as the cat righting reflex. An individual cat 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 is around 90 cm (3 ft). Cats without a tail (e.g. Manx cats) 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 little used for this feat.^[92]