size, teeth (which give clues to diet), local 

 climate conditions, and vegetation, as well 

 as comparisons with some kinds of mod- 

 em mammals, can help us reconstruct the 

 social systems of fossil horses. During the 

 Eocene, the small, forest-dwelling brows- 

 ers, such as Hyracotherium, were prob- 

 ably solitary or lived in small bands within 

 small home ranges, as do modem forest- 

 dwelling tapirs and Chinese water deer. In 

 contrast, more open landscapes and mixed 

 forest and grassland habitats of the 

 Miocene enabled early horses to broaden 

 their range of behaviors. Both the kind of 

 territoriality observed today in Grevy's 

 zebra and the more nomadic life in bands, 

 or harems, seen in Burchell's zebra, have 

 probably existed in horses since the 

 Miocene. 



Fossils from individual quarries some- 

 times represent particular populations of 

 extinct horses. They also provide insight 

 into the longevity and reproductive biol- 

 ogy of various species. Little Hyra- 

 cotherium probably had a potential life 

 span of three or four years. Females would 

 have given birth to at least one foal a year, 

 although, based on local climate recon- 

 stmctions, breeding cycles in the Eocene 

 were not synchronized or concentrated at 

 any particular season. During the Mio- 

 cene, however, the average life span of 

 horses increased to about nine to fourteen 

 years, depending upon the species. Within 

 local populations, many of these horses 

 gave birth during the season when food 

 was most abundant. 



Starting about eight million years ago, 

 horse diversity dropped drastically, retum- 

 ing to pre-Miocene levels of only three to 

 five species at any given fossil locality in 

 the Northern Hemisphere. Studies of an- 



In a panorama depicting life ten to 

 fifteen million years ago on the 

 Great Plains, three-toed open- 

 country grazers dominate the 

 foreground; one, detecting the 

 furtive cat in the ground cover, 

 neighs a warning to herd 

 members. Other three-toed grazers 

 and one-toed Pliohippus, near the 

 elm tree, gather in small bands, or 



harems. At the far right, a 



contingent of three-toed browsers 



barely emerge from their forest 



home. Crocodiles, rhinos, and 



shovel-tusked elephants share the 



valley stream. 



Painting by Marianne Collins 



cient climate indicate increased global 

 aridity. This, in tum, seems to have led to 

 less productive land ecosystems. The hey- 

 day of horses ended. In addition, competi- 

 tion with cud-chewing, hoofed herbivores 

 such as deer and bison may also have af- 

 fected horse diversity. By two million 

 years ago, only the single horse genus 



Equus, consisting of a few species, re- 

 mained in the Northem Hemisphere. 



About three million years ago, during 

 the Pliocene, Equus emigrated from North 

 America across the Bering land bridge 

 into the Old World and, after the formation 

 of another dry-land connection to the 

 south, crossed the isthmus of Panama 



64 Natural History 4/94 



