450 LAND ANIMALS 



Australian jerboa rats {Conilurus) , the Old World Dipodinae (Jaculus, 

 Alactaga) , the North American Zapodinae (Zapus) , and Heteromyidae 

 (Dipodomys, Perodipus) have adapted themselves to this form of 

 locomotion. But in South America, where rodents are particularly 

 abundant, there are no jumping rodents, and only hare-like running 

 animals occur, such as the viscacha with long hindlegs but also with 

 well-developed forelegs. It is hard to see what advantage this mode of 

 locomotion affords the smaller forms; Spencer 17 thought that their 

 zigzag motion aided them in escaping birds of prey. 



Sight is of great importance in open country, and we find forms 

 such as the giraffe among the mammals, which are especially keen of 

 vision. Other species habitually survey their surroundings; many ro- 

 dents sit up on their haunches to look about, like the hare and the 

 bobak, Cynomys; the kangaroo also has this habit. The flashing white 

 of the buttocks of the prongbuck serves as a signal from great dis- 

 tances and from herd to herd. Despite the use of vision, the sense of 

 smell is even more important to mammals of the grasslands. Sound, 

 though of secondary importance, is audible at greater distances than 

 in the forest and may serve to orient steppe animals, which make use 

 of warning signals like calls and whistles. In general, however, birds 

 and mammals of the plains are quiet, as they are able to keep together 

 by sight ; and noisy groups, like troops of monkeys and flocks of par- 

 rots of the forests are infrequent. 



The flocking habit. — It is particularly striking how often animals 

 congregate in herds in grasslands, much more so than in the forest. 

 The two chief groups of mammals, the digging rodents and the un- 

 gulate animals, are usually found in large companies. The rodents 

 of the same species have their burrows close together, often over wide 

 stretches. Bison, yak, antelopes, and guanacos live in herds. Frequently 

 several species of antelopes mix freely in the same herd, and the zebra 

 and quagga associate with these, just as the kulan {Equus hemionus) 

 of central Asia feeds in company with various wild sheep, Tibetan 

 antelope, and the yak. Ostriches often join such herds in Africa, and 

 guanacos and rheas are found together in Patagonia. Kangaroos, too, 

 are gregarious. Even many carnivores, which are ordinarily solitary, 

 occur in packs in the open country, such as jackals and hyenas; in- 

 deed, even lions hunt together in what are thought to be family groups. 

 Among the birds of the open country, the formation of flocks is com- 

 mon, as among the emus, rheas, and ostriches, the bustards, and 

 partridges. Pteroclurus lives a solitary life in the Kalahari sandy 

 desert, but during the flight to watering places they come together, 

 often forming huge flocks that contain up to sixty thousand individ- 



