FIG. 25-1 Grassland ani- 

 mals. Clockwise, bison, 

 pronghorn, coyote, badger 

 (courtesy U.S. Forest ^j. 



animals that occupy similar habitats, although often 

 quite unrelated taxonomically, is of particular interest 

 and may best be shown in table form (Table 25-1). 



ANIMAL ADJUSTMENTS 



It is in the grasslands throughout the world 

 that the large herbivorous ungulates reach their 

 largest populations. Their adaptations for feeding on 

 grasses and the high productivity of grasses, which in 

 fact is stimulated by moderate grazing, gives an effi- 

 cient food coaction of high energy utilization. A for- 

 est cannot support such large populations of grazing 

 animals since the herb layer is less luxuriant, com- 

 posed more of the broad-leaved herbs, and shrub and 

 tree foliage cannot tolerate continued browsing. The 

 evolutionary development of these large ungulate 

 populations had to await the evolution of these ex- 

 tensive grasslands in mid-Tertiary time (Stirton 

 1947). Previous to the formation of the grasslands 

 in North America, we may suppose that the ancestral 

 forerunners of bison and pronghorn were largely lim- 

 ited to serai grassy stages in the Arcto-tertiary and 

 perhaps Madro-tertiary floras. 



It is common for these large ungulates to feed 

 in large herds. The primitive population of bison in-. 

 North America is estimated at 50-60 million animals, 

 perhaps an average of 6 per sq km (15 per sq mi). 

 Few occurred west of the Rockies even in colonial 

 times, but to the east and north the species penetrated 

 far into forested areas. Herds containing 20,000 indi- 

 viduals were common and an occasional herd reached 



a pupulaliuii as liigli as 4 million animals ( Seton 1909) . 

 When attacked by wolves or other predators the bulls 

 formed a circle facing outward with the cows and 

 calves inside. The animals shed their fur in the sum- 

 mer and were greatly annoyed by flies, mosquitoes, 

 and the penetrating seeds of the needle grass. They 

 relieved their miseries by rolling in wallows, covering 

 themselves with mud. Some calves were killed by 

 wolves, coyotes, bears, and mountain lions. Adults 

 died in consequence of bogging down in sloughs or 

 swamps, breaking through thin ice when crossing 

 rivers in winter, and of old age. Disease apparently 

 was never common. The animals regularly grazed 

 until mid-morning, traveled sometimes 16 km (10 

 miles) to a water hole, rested and chewed their cud 

 during mid-day, and grazed again in the evening. A 

 herd would graze an area intensively for several days 

 or weeks, then move to some other area. In these 

 movements and migrations, the animals commonly 

 traveled in single file. There is question, however, 

 whether north-south migrations were very regular 

 and extensive (Roe 1951). 



The pronghorn antelope occurs in the drier grass- 

 lands, including California, and their primitive popu- 

 lation is estimated at 4 per sq km (10 per sq mi). 

 They traveled in herds of 100-200, sometimes 2000. 

 When they were scattered in feeding, the approach 

 of a predator was quickly signalled from one to the 

 other by raising the hair in the white rump patch so 

 that it flashed like a tin pan reflecting the sun. 



Safety for these animals depended largely upon 

 fleetness of foot, and some of the fastest-running 

 animals in the world occur in this biome (Craig 



328 Geographic distribution of communities 



