icily, and inetaniorphose into adults. Animals previ- 

 ously aggregated around water holes scatter widely 

 over the surrounding country. However, as the rains 

 stop and the ponds again dry up, populations contract 

 sharply, and many species go back into aestivation, 

 sometimes for years, until the next wet period occurs. 

 Xot only invertebrates but also birds and mammals 

 largely confine their reproductive activities to the 

 rainy season (Bu.xton 1923). 



In the northern cooler portions of the desert, 

 hibernation over winter is necessary for many cold- 

 blooded forms. Lizards and snakes may hibernate 

 one-half meter or more in sand, under rocks, or in 

 burrows of other animals (Cowles 1941). Rattle- 

 snakes hibernate in natural cavities on hillsides or 

 elsewhere. 



Although much of the desert soil is a hard, 

 gravelly pavement, loose soil and sand dunes are 

 not uncommon. Many reptiles have special structural 

 and behavior adaptations to cope with sandy habitats 

 so that a good herpetofauna occurs in such places 

 (Mosauer 1935). The texture and hardness of the 

 soil, its depth, slope, and color influence the niche 

 segregation of small mammals (Hardy 1945). The 

 light gray, yellow, and brown tone of desert soils is 

 reflected in the pale coloration of many desert birds 

 and mammals. Desert animals are generally less 

 heavily pigmented and are smaller in size than close 

 relatives in humid regions (Gloger's rule). It is not 

 certain how much of this is a response to the arid 

 climate (Buxton 1923, Sumner 1925) or high light 

 intensities (Meinertzhagen 1950), and how much is 

 a response to the color of the soil. Deer mice and 

 pocket mice occurring in the White Sands National 

 Monument of New Mexico are very light in color, 

 but a few kilometers away, in the Tularoosa lava beds, 

 their color is very dark (Dice and Blossom 1937). 

 This blending coloration doubtless furnishes protec- 

 tion from the attacks of predators on moonlit nights. 

 Even in humid climates, the dark coloration of ani- 

 mals may be an adaptation to the darker colored 

 vegetation and ground litter (Bowers 1960). 



The fish of the desert present features of special 

 interest. The small ponds and pools are widely iso- 

 lated from each other and are without outlets to the 

 sea. The salt concentration in some of them is high 

 as a result of centuries of continuous evaporation of 

 water. Some spring-fed pools contain only a few 

 dozen or a few hundred individuals, but because of 

 their isolation these individuals have evolved into 

 distinct varieties or species found nowhere else. 

 These various fish populations, however, show a 

 relation to each other. In many cases, particularly in 

 the Great Basin, the ponds are deep holes that per- 

 sisted after the drying up of large shallow Pleistocene 

 lakes, such as Lake Lahontan and Lake Bonneville. 

 The fish in these ponds are descendants, therefore. 



of po])ulations formerly widespread throughout the 

 Pleistocene lakes (Hubbs 1940a, Hubbs and Miller 

 P)48). 



HLIM.AN KKLATIONS 



In semi-deserts, there is production of 

 cattle, shee[), goats, horses, and camels, but the carry- 

 ing capacity of the land is low. The stock needs to 

 have access to springs, ponds, or rivers which are of 

 course widely scattered. Nomadic primitive people 

 roam the semi-deserts of Arabia, Africa, and Aus- 

 tralia in search of pasture for their herds. 



Since the scanty rainfall does not wash away the 

 salts, the soil is fertile where irrigation is possible. 

 Vegetable and other crops and fruit can be raised 

 advantageously. Where the ground water table comes 

 close to the surface locally, oases of vegetation occur, 

 even in otherwise extreme desert, and may support 

 small settlements. On the whole, however, man does 

 not find the desert an amenable habitat. 



SUMMARY 



Deserts, like grasslands, occur in all conti- 

 nents. They develop in areas with very low annual 

 precipitation and generally high temperatures. The 

 vegetation consists of scrubby, sclerophyllous, small- 

 leaved, often widely-spaced shrubs, bushes, or cacti, 

 which are commonly covered with prickles or thorns. 

 Succession is not conspicuous and the prevailing 

 vegetation varies considerably with local soil and 

 moisture conditions. Desert scrub and basin sage- 

 brush biociations are distinguished in North America. 

 These communities commenced to emerge as distinct 

 biotic entities in mid-Pliocene, as organisms became 

 adapted to the increasing aridity of the climate. 



The most prominent animals of the desert are 

 small herbivorous rodents and reptiles. Animals tend 

 to avoid extremely high temperatures by becoming 

 nocturnal, spending much of their time in shady 

 places, or burrowing into the ground. Many desert 

 animals are able to go a long time without drinking 

 water, getting what they need from succulent foods 

 and the oxidation of fats and carbohydrates. There 

 is reabsorption of much water from urinary wastes, 

 or uric acid is excreted instead of urea. After periods 

 of rain, there is temporarily a rich expansion of plant 

 and animal reproductive activities. Many reptiles 

 have structural and behavioral adaptations to cope 

 with sandy habitats. There is a general tendency for 

 desert animals to be lighter in color and smaller in 

 size than close relatives in humid regions. Since 

 ponds and lakes are widely isolated, speciation among 

 fishes has developed extensively. 



Desert biome 339 



