The extinction of the camel at about the same time as human died out left a niche that was distinctly unattractive to any other animal. For a large animal to exist in desert conditions a quite remarkable physiology is required. The camel, for instance, was able to lose about 30 per cent of its body weight through dehydration without ill-effects, and it stored all the subcutaneous fat of its body in one lump, leaving the rest of the body free to radiate heat. It could tolerate fluctuations in its body temperature to some extent and had thick nostril covers and eyelids that effectively kept dust and sand out of its nose and eyes.
After some 50 million years of evolution these features have all developed again in one animal - the desert leaper, Aquator adepsicautus. The leaper is descended from the rodents, possibly one of the jerboas or sand rats, and has grown large - adult males may reach more than 3 metres from nose to tail. The tail is the most unusual feature of this animal; it is here that all its subcutaneous fat is stored. The fat is not a water store, but a store of food that enables the leaper to go for long periods without eating when food is unavailable. When the fat store is full the animal's body is well balanced and it can leap quickly along on its hind limbs. In this condition it can undertake journeys of 100 kilometres or more between waterholes and oases. It has broad, horny pads on the toes of its hind feet which prevent it from sinking into the sand and give it a good grip on naked rock.
The rocky areas of the desert are the habitat preferred by the grobbit, Ungulamys cerviforme. This rodent is about 60 centimetres long, excluding the tail, and has hooves developed on its third and fourth digits enabling it to run about the craggy landscape of the rocky desert. The second and fifth digits of its front feet have small claws that almost touch the hooves when the foot is bent, allowing the grobbit to grasp and pull down branches and feed on them. The grobbit lives in packs all over the rocky desert zone of the African and Asian sub-continents.
In the deserts, large predators are not common and very few meat-eating mammals of any description are found. The khilla, Carnosuncus pilopodus, however, descended from the insectivores, is one of the few. Standing about 60 cm high at the shoulder, it is largely nocturnal and spends most of the day hidden in a network of burrows excavated in soft sand. At night it hunts small mammals and obtains most of the water it needs from the moisture contained in their flesh.
Most desert animals are sandy yellow in colour to blend in with the surroundings and have white undersurfaces that counteract the effects of shade and give them a two-dimensional appearance. That this colour scheme is the result of evolutionary pressure is a belief supported by the darker appearance of animals found on black-grey lava areas and the almost white forms of the same animals found in salt-pan regions.
Animals that are not camouflaged are predominantly black. Predatory birds, reptiles and the most poisonous and unpalatable arthropods fall into this category. The colour resemblance may be due to a form of mimicry in which for some reason black is an advantageous colour for certain predators, and all others adopt the same colour to derive some similar benefit.