Adaptations of Desert Animals
Two characteristics of the desert i.e., high temperature and scarcity of rainfall determine the occurrence, distribution and adaptations of desert animals.
In addition to these the highly characteristic “spaced distribution” of the desert vegetation also affects the desert fauna.
The characteristic animals of the desert are insects, small rodents, and reptiles. Birds and mammals are comparatively rare or absent. Poikilothermic animals, which match their internal temperature to the external, are adapted to live easily in the desert.
Insects and reptiles of the desert have impervious integument and excrete nitrogen in the form of uric acid. In desert insects, the evaporation from respiratory surface is reduced to the minimum the internally invaginated spiracle system. Rodents of the desert can live on dry seeds, succulent cacti and other plants that store water and do not require drinking water. They stay in burrows during the day to avoid evaporation and water loss and conserve water by excreting highly concentrated urine and by not using water for temperature regulation. Hibernation is necessary for many ectothermic animals of the desert.
Desert snakes and lizards hibernate 0.5 m or more in sand, under rocks or in burrows of other animals. Some ants and crickets burrow deeply into the ground. Burrows of kangaroo rat penetrate 50-65 cm below the surface. Protection of eye, ear and nostril against the sand is an important adaptation. In the burrowing snake, Typhlops, the eyes are covered by minute shields. In camel, the eyes are well protected by long eye lashes and are kept high above die ground by long neck. The ear opening of desert animals are also well protected by hair or scales.
The presence of poison glands is also an adaptive feature of desert animals. Protection against natural enemies is achieved through protective colouration or spiny covering. Their colour generally matches with their surroundings. The so-called homed lizard, Phrynosoma, of the western American deserts and the spiny devil, Moloch horridus, of Australia are classical examples of the desert animals having a spiny covering on their body.
Physiological adaptations of desert animals are no less interesting. Some of them, for example the desert lizard, Sauromalus obesus, have the mechanism for selective cooling of blood to the brain. In some African gazelles and ungulates the brain is supplied with cool blood. Some desert reptiles, when exposed to heat, increase their respiration frequency and breathe with an open mouth, a situation reminiscent of panting.
Other lizards use saliva regurgitated from the mouth over the throat region to achieve cooling. A camel (Camelus dromedarius) in the desert uses several methods to reduce heat gain. Firstly, the heat is stored by an increase in body temperature.
In camel deprived of water the body temperature in the morning may be about 34°C, which rises to 41”C in the late afternoon. This 7°C increase in body temperature corresponds to about 29000 kcal of heat, which equals a saving of 5 litres of water. The increase in body temperature decreases the heat flow from the environment, the fur reducing the heat gain from the environment.
A camel can tolerate a greater degree of water depletion of the body, and when water is available, it may drink more than one third of its body weight (Schmidt- Nielsen, 1964). Further adaptations to desert life are splayed hooves, which are ideal for walking on sand and the hump which stores fat. Thus, adaptations of desert animals are actually the adjustments to protect themselves against high temperatures, to live without water, and to conserve water as far as possible.
Adaptations of Desert Plants:
Extreme desert is without any vegetation and rainfall. However, some deserts receive less than 5 cm of rain per year. Deserts include arid regions, which contain considerable vegetation, commonly called xerophytes, in the form of desert bushes and shrubs, succulents such as cacti which store water, and other small plants which avoid drought by growing only when there is adequate moisture. In some deserts, plants survive as seeds for several years until a little rainfall provides conditions suitable for growth and flowering.
The ephemeral plants of the desert can complete their entire life cycle in a few weeks. Geophytes of the desert avoid periods of drought by surviving as underground bulbs or corns. Succulents such as cacti (Opuntia) and Euphorbia have adaptations enabling them to survive above ground for the whole year.
They possess thick cuticles, a very low surface area to volume ratio and sunken stomata, which open during night so as to minimise water loss in transpiration. In succulents, the stem is flattened and the function of leaf is performed by stem. In Opuntia, leaves are reduced to spines.
The stem epidermal surface is covered with waxy material, which prevents water loss, by cuticular transpiration. In some desert plants like Aloe and Agave the leaves are thick leathery or succulent. Absence of broad leaves and abundance of spines further protect desert plants from being eaten by animal consumers. The roots in perennial xerophytes are very deep. The stem is usually covered with corky layers, which reduce water loss. Many of these plants produce gums and resins as in Acacia; their presence also helps in formation of layers, which prevent water loss.