The adult cheetah weighs from 40 to 65 kg (88 to 140 lb). Its total body length is from 115 to 135 cm (45 to 53 in), while the tail can measure up to 84 cm (33 in) in length. Males tend to be slightly larger than females and have slightly bigger heads, but there is not a great variation in cheetah sizes and it is difficult to tell males and females apart by appearance alone. Compared to a similarly-sized leopard, the cheetah is generally shorter-bodied, but is longer tailed and taller (it averages about 90 cm (35 in) tall) and so it appears more streamlined.
Some cheetahs also have a rare fur pattern mutation: cheetahs with larger, blotchy, merged spots are known as 'king cheetahs'. It was once thought to be a separate subspecies, but it is merely a mutation of the African cheetah. The 'king cheetah' has only been seen in the wild a handful of times, but it has been bred in captivity.
The cheetah's paws have semi-retractable claws[6] (known only in three other cat species - the Fishing Cat, the Flat-headed Cat and the Iriomote Cat) offering the cat extra grip in its high-speed pursuits. The ligament structure of the cheetah's claws is the same as those of other cats; it simply lacks the sheath of skin and fur present in other varieties, and therefore the claws are always visible, with the exception of the dewclaw. The dewclaw itself is much shorter and straighter than other cats.
Adaptations that enable the cheetah to run as fast as it does include large nostrils that allow for increased oxygen intake, and an enlarged heart and lungs that work together to circulate oxygen efficiently. During a typical chase its respiratory rate increases from 60 to 150 breaths per minute.[6] While running, in addition to having good traction due to its semi-retractable claws, the cheetah uses its tail as a rudder-like means of steering to allow it to make sharp turns, necessary to outflank prey who often make such turns to escape.
Unlike "true" big cats, the cheetah can purr as it inhales, but cannot roar. By contrast, the big cats can roar but cannot purr, except while exhaling. However, the cheetah is still considered by some to be the smallest of the big cats. While it is often mistaken for the leopard, the cheetah does have distinguishing features, such as the aforementioned long "tear-streak" lines that run from the corners of its eyes to its mouth. The body frame of the cheetah is also very different from that of the leopard, most notably so in its thinner and longer tail, and unlike the leopard, its spots are not arranged into rosettes.
The cheetah is a vulnerable species. Out of all the big cats, it is the least able to adapt to new environments. It has always proved difficult to breed in captivity, although recently a few zoos have managed to succeed at this. Once widely hunted for its fur, the cheetah now suffers more from the loss of both habitat and prey.
The cheetah was formerly considered to be particularly primitive among the cats and to have evolved approximately 18 million years ago. New research, however, suggests that the last common ancestor of all 40 existing species of felines lived more recently than that - about 11 million years ago. The same research indicates that the cheetah, while highly derived morphologically, is not of particularly ancient lineage, having separated from its closest living relatives (Puma concolor, the cougar, and Puma yaguarondi, the jaguarundi) around five million years ago.[7][8] These felids haven't changed much since they first appeared in the fossil record.