As a familiar and easily-kept animal, the physiology of cats has been particularly well studied and is probably is similar to that of other carnivorous mammals.[24] However, several features of cats' physiology are unusual and are probably due to their descent from desert-dwelling species.[24] For instance, cats are able to tolerate quite high temperatures, with humans starting to feel uncomfortable when their skin temperature passes about 44.5 °C (112 °F), in contrast cats show no discomfort until their skin reaches around 52 °C (126 °F).[75] 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 in both day and night.[76] As well as being tolerant of high temperatures, 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.[24] Indeed, their kidneys are so efficient that cats can survive on a diet consisting only of meat, with no additional water,[77] and can even rehydrate by drinking seawater.[78][79]
In captivity, indoor cats typically live 14 to 20 years, though the oldest known cat, Creme Puff, lived to age 38.[80] Some[who?] say that 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). However, this is not always necessarily true, because a cat kept indoors is more inclined to have a shortened life due to obesity (sometimes leading to heart problems and diabetes) and lack of exercise. Having a cat be neutered, though, is almost universally regarded as a health benefit. Some benefits of neutering are that castrated males cannot develop testicular cancer, spayed females cannot develop uterine or ovarian cancer, and both have a reduced risk of mammary cancer.[81] The lifespan of feral cats is hard to determine accurately, although one study reported a median age of 4.7 years, with a range between 0 to 10 years.[82]
