44 U.S. NATIONAL MUSEUM BULLETIN 2 75 



Zoogeographical Considerations 



Effects of the Pleistocene on the Dispersal of Saharan and Libyan Rodents 



No account of Libyan (or Saharan) mammals should minimize the 

 effects of the Pleistocene upon them and its obvious role in having 

 influenced their present and past distribution. Henry Fairfield Osborn 

 (1915, p. 225) summarized the importance of this period in North 

 Africa by stating "Thus in no region of the world have more profound 

 changes occurred during and since Pleistocene times than in Africa 

 north of the Sahara Desert." In fact, it is probably true that most 

 of the present distributional patterns of Libyan rodents are directly 

 related to Pleistocene climates. 



The Pleistocene was characterized by widespread glaciation in 

 the northern parts of the world. As a response to broad climatic 

 fluctuations during this period, four successive glaciations (in the 

 northern hemisphere) alternated with milder interglacial periods 

 following the withdrawal of the ice sheets. Estimates vary regarding 

 the duration of the Pleistocene, but it probably endured for perhaps 

 1,000,000 years. The time of the withdrawal of the last continental 

 glaciation is also not firmly fixed, and estimates range from 10,000 

 to 25,000 years. 



Widespread glaciation in northern and southern latitudes was not 

 the only manifestation of prevailing cold during the Pleistocene. 

 Temperatures in the North Temperate Zone are thought to have been 

 significantly lower than those of the present. This lowering of the 

 temperature was probably synchronous over most of the Northern 

 Hemisphere (Flint, 1947). During the intervening interglacial periods, 

 climates were apparently warmer and sometimes drier than those of 

 the pluvial periods. 



As a result of lower prevailing temperatures, the amount of rainfall 

 increased significantly over much of the world, and there was an 

 accordant shifting of the temperate rain zones into the low latitude, 

 subtropical deserts. 



These changes in the Pleistocene climate produced manifold changes 

 in the physical configuration in many regions of the Northern Hemi- 

 sphere. Lakes in the arid portions of all continents increased in size 

 and became reduced in rhythm with the advance and recession of 

 the successive glacial stages. The Dead Sea, the great saline lakes 

 of Asia (the Caspian, Aral, and Lake Balkash), and the large lakes of 

 East Africa have lacustrine deposits high above their present shore- 

 lines, indicating marked expansion during the Ice Age, followed by 

 shrinking during dry interglacial periods. 



As glacial'ice accumulated or receded on the continents, there was 

 a synchronous lowering or raising of sea level throughout the world. 



