Ecology oj Animals 655 



the leaves reduced to mere needles. The animals are adjusted both by 

 their physiology and habits to the rigors of this extreme environment. 



Most of the animals dwelling in the desert regions are quite small 

 and are nocturnal in habit. The mammals include the desert jack 

 rabbit and a large number of rodents such as the kangaroo rats. Rep- 

 tiles are among the characteristic desert forms. Among the inverte- 

 brates are numerous small molluscs, many arachnids, and some insects. 



The chief stresses of this rigorous environment are the lack of water 

 and the evaporative power of the sun. These stresses are met in many 

 ways: structurally the reptiles have heavy scales, many of the rodents 

 excrete dry urine, and utilize the water {produced during metabolism. 

 By burrowing and being active only at night, the animals escape the 

 worst of the heat. 



De-serts are found in many parts of the world ; in the United States 

 portions of the .Southwest can be termed true deserts. 



Tropical Rain Forest. — Through the tropical areas of the world 

 where the rainfall is heaviest are located the luxuriant rain forests with 

 their immensely varied fauna and flora. These forests are found in 

 Central America and in northern South America, in Central Africa, 

 southeast Asia, and the East Indies and neighboring islands. 



These forests differ markedly from the northern ones. Here are 

 found numerous species of trees in contrast to the few species of northern 

 areas. The trees are of varying heights, with the tallest ones forming a 

 continuous canopy over the lower growing forms. Layers can be recog- 

 nized ; thus there is the high canopy, the trees of somewhat lesser height, 

 those of medium height, and finally the sparse shrub layer. Innumerable 

 vines twine from tree to tree, forming intricate lianas. Near the ground, 

 there is but little light and plants must constantly grow upward in their 

 search for light for carrying on their food-making activities. 



As with the plants, there are many different species of animals, many 

 of which are specifically adapted to an arboreal life. The insects like- 

 wise are of tremendous variety. There are numerous species of ants and 

 termites, some of which have developed truly amazing habits. Brilliantly 

 colored birds are lost in the dense foliage. The arboreal animals often 

 have prehensile tails to aid them in their movements through the trees. 

 Undoubtedly no other biome compares with the tropical rain forest in 

 variety and number of animals and plants. .Some of the characteristic 

 animals of the American tropics are the monkeys, ocelots, sloths, Ixja con- 

 strictors, parrots, army ants, and many others. 



