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Rodents and reptiles are generally the most numerous vertebrates in low latitude and re.-ions. They successfully occupy and habitats because of their abilities to adapt when confronted with changes in microclimate and water availability. A variety of physiological and behavioural solutions arc employed by small vertebrates when faced with the challenge of the Sahara desert environment. For example, there is a considerable diversity of responses to chronic dehydration. Reptiles often select their food, avoiding a natural salt overload. When conditions become unfavourable they stop feeding and foraging, and seek favourable microclimates. But reptiles can tolerate changes in water volume and osmotic concentration of body fluids as well reduction of water evaporation through changes in the skin permeability. Various lizards have salt-secreting nasal glands that contribute to electrolyte homeostasis. Nasal secretion is composed mostly of potassium for herbivorous species (Uromastix acanthinurus), and sodium for carnivorous species (Varanus griseus). The mean water flux rates (WFR) are 17 ml and 28.5 ml. k(-1).d(-1) respectively. For herbivores, nasal salt elimination is essential, and initiation of this function depends up on the water content of the plant material consumed. Differences in salt excretory capabilities related essentially to their diets. Insectivorous lizards which lack functioning salt glands (Agama spp.), retain sodium in the extracellular fluid (>300 mOsm/l) during the dry season. However, within a few hours following a sudden thunderstorm, during which insectivorous agamids drink copiously, plasma sodium concentration and cellular dehydration were observed to fall to normal levels.

The relationship between body mass and water evaporation for 46 and habitat species of reptiles was found to be : ml.day(1) = 2.92 kg(0.64), Interestingly a species dwelling in non-arid region, such as Anolis carolinensis anet, can considerably reduce its rate of evaporative water loss (EWL) by low-humidity acclimation. Until now, the plasticity of reptilian physiology has been underestimated.

Desert rodents also exist in deserts through the use of various physiological mechanisms : high urine concentration, dry faeces, low EWL. Their behavioural adaptations (diet selection) can be as effective as their physiological adaptations for water conservation. For example, seeds stored in the burrow at a RH (relative humidity) of 60% contained 30% water whilst the same seeds at the surface contained only 5% water. Some gerbils require seeds with a minimum of approximately 5% water in order to achieve water balance. Thus, they forage at night, when temperatures are lower and humidities higher than during the day. The WFR of the sand rat (Psammomys obesus), which cuts hydrated plants of a very high salt concentration (Chenopodiacca), is very high (400 ml.kg(-1).d(-1)) whereas the Gundi (Ctenodactylus vali), which consumes dry plants has a WFR of 86 ml.kg(-1).d(-1) and a field metabolic rate (FMR) of 520kJ.kg(-1).d(-1). Generally the FMR of desert rodents is >600 kJ.kg(-1).d(-1).

A terrestrial community of small vertebrates in the and Kerkennah islands (Tunisia) was studied during the active season. It was composed essentially of three species : two lacertids and one rodent. For the same body mass, lizards have an FMR of only 3% of that of the rodent. A low rate of energy metabolism as well as a higher capacity to digest dry and low quality plant materials help the Sahara-Arabian jird Meriones (compared to these of Levant vole, Microtus) to cope with the shortage of food characteristic of most desert habitats. The wild Mus musculus, like many desert rodents, is capable of acclimatising to long term restrictions of water availability. Such mice are also capable of adjusting their water balance. They reduce the total water ingestion to 50% of body mass and reduce their water expenditure by decreasing excessible evaporation and urinary water loss. Urine concentration averaged 4500 mOsm.l(-1) (urea: 2500 mMol. l(-1)). Thus, they can produce urine that is as highly concentrated as that of desert rodents : gerbils, jerboas and sand rats.

Various rodents conserve water and energy effectively and obtain adequate water and energy throughout the year without drinking by varying their diet (seeds, vegetation and arthropods) appropriately. Reptiles, because they are ectothermic have generally low water and energy requirements, and thus they have successfully occupied habitats that are generally too environmentally harsh to sustain endotherms such as mammals and birds.