424 LAND ANIMALS 



of the African tropics and Tragulus of the Asiatic tropics are true 

 forest dwellers. 



The arboreal habit. — The animals of the tropical rain-forest are 

 characterized by great development of the arboreal habit. This is 

 particularly noticeable among the vertebrates. In the densely forested 

 New Guinea, and northern Queensland, kangaroos of the genus Den- 

 drolagus have developed this habit despite the fact that their structure 

 is not easily adjusted to tree life. The arboreal habit imposes certain 

 size limitations which are particularly striking among mammals. 

 Climbing not only involves great muscular effort for large animals, 

 but the branches and vines offer inadequate support, so that swinging 

 from tree to tree, so easy for a monkey, may become impossible with 

 increasing size, and the gorilla, largest of the apes, is mainly a ground 

 dweller. Of the three species of American anteaters, only the smallest 

 is completely arboreal, while the largest is entirely limited to the 

 ground. 



Climbing mechanisms are varied among arboreal animals, but nu- 

 merous convergent adaptations can be discovered. Sharp claws are 

 often present among climbing birds, mammals, lizards, or insects. In 

 the neotropical sloths, the claws have developed into powerful hooks. 

 Many tree-dwelling vertebrates have developed opposable toes. The 

 tree frogs of the ranid genus Chiromantis, of West Africa, have two 

 opposing digits of the hand while both inner finger and inner toe 

 are opposable in the neotropical Phyllomedusa; chameleons, and a few 

 other lizards, have opposable digits on fore and hind feet. Many birds 

 have two toes permanently turned backwards; others can turn a second 

 toe so at will. This allows a stronger hold upon branches than when 

 but one toe is so turned; parrots furnish a familiar example. Among 

 the mammals the thumb and great toe are opposed to the rest of the 

 digits, in the marsupials often the second digit as well. The Didelphiaae 

 among the marsupials have only the hind feet for grasping; the Pha- 

 langeridae have the hands modified also, which of course is con- 

 spicuously true of the monkeys. 



The presence of sucking disks on the feet or digits is a widespread 

 adjustment for climbing. It is well developed and widespread among 

 the climbing amphibians, not only among the Hylidae but also among 

 the Bufonidae and Ranidae. The adhesive organs on the feet of the 

 geckonid lizards are especially well developed. Thiroptera from Brazil 

 and Cheiromeles from the Sunda Islands, among bats, have such 

 sucking disks on their digits; Tarsius has them on the balls of its feet, 

 as have also such diverse forms as the tree porcupine, Erethizon, the 

 tree coney, Dendrohyrax, and the monkey, Inuus speciosus. Tree-dwell- 



