Physiology

Physiology
As cats are familiar and easily kept animals, their physiology has been particularly well studied; it generally resembles that of other carnivorous mammals but displays several unusual features probably attributable to cats' descent from desert-dwelling species.[35 ] 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),[54 ] :46 and can tolerate temperatures of up to 56 °C (133 °F) if they have access to water.[63 ]

Cats conserve heat by reducing the flow of blood to their skin and lose heat by evaporation through their mouth. They do not sweat, and pant for heat relief only at very high temperatures[64 ] (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.[65 ] :1 Cats' feces are comparatively dry and their urine is highly concentrated, both of which are adaptations that allow cats to retain as much fluid as possible.[35 ] Their kidneys are so efficient that cats can survive on a diet consisting only of meat, with no additional water,[66 ] and can even rehydrate by drinking seawater.[65 ] :29 [67 ]

Cats are obligate carnivores: their physiology has evolved to efficiently process meat, and they have difficulty digesting plant matter.[35 ] 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.[35 ] 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.[68 ] 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.[35 ] Since cats tend to eat all of their prey, they obtain minerals by digesting animal bones, and a diet composed only of meat may cause calcium deficiency.<sup class="reference" id="cite_ref-MacDonald_35-6">[35 ]

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 that are needed to digest carbohydrates.<sup class="reference" id="cite_ref-Zoran_69-0">[69 ] These traits severely limit the cat's ability to digest and use plant-derived nutrients, as well as certain fatty acids.<sup class="reference" id="cite_ref-Zoran_69-1">[69 ] 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 that cats require,<sup class="reference" id="cite_ref-Gray_2004_70-0">[70 ] and diets containing no animal products pose the risk of causing severe nutritional deficiencies.<sup class="reference" id="cite_ref-Zaghini_71-0">[71 ]

Cats do eat grass occasionally. Proposed explanations include that grass is a source of folic acid or dietary fiber.<sup class="reference" id="cite_ref-EatGrass_72-0">[72 ]