Cute Cat

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 only at very high temperatures. Unusually, 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. Cats' feces are usually dry and their urine is also highly concentrated, both of which are adaptations that allow cats to retain as much fluid as possible. Their kidneys are so efficient that cats can survive on a diet consisting only of meat, with no additional water, and can even rehydrate by drinking seawater.
Cats are obligate carnivores: their physiology has evolved to efficiently process meat, and they have difficulty digesting plant matter. 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. 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. Another unusual feature is that the cat also cannot produce the amino acid taurine, with taurine deficiency causing macular degeneration, where the cat's retina slowly degenerates, causing irreversible blindness. 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.
A cat's digestive tract is also 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. These traits severely limit the cat's ability to digest and use plant-derived nutrients, as well as certain fatty acids. 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, and diets containing no animal products pose the risk of causing severe nutritional deficiencies.