RODENTS OF LIBYA 35 



Pachuromys duprasi; Acomys cahirinus; Hystrix cristata; and Psam- 

 momys vexillaris. 



Most of the above species are more highly specialized than the 

 polytypic species discussed earlier; consequently, they are less variable 

 genetically and show a more limited range of phenotypic expression. 

 In such genetically impoverished species, natural selection has fewer 

 opportunities to alter genotypic and phenotypic composition of the 

 population, and morphological characteristics of these species tend 

 to remain relatively uniform, sometimes over large areas. Usually 

 the geographic ranges of these genetically static species tend to be 

 small because of the narrow ecological tolerance of the individuals 

 comprising the population. This tendency for more specialized forms 

 to have smaller geographic ranges is exemplified by Spalax ehrenbergi 

 Gerbillus kaiseri, Gerbillus eatoni, and Gerbillus henleyi, all of which 

 have ranges confined to northern Cyrenaica or areas near the Libyan 

 coast. On the other hand, species such as Gerbillus gerbillus, Gerbillus 

 campestris, and Jaculus jaculus, which have broad genetic constitu- 

 tions, occur virtually throughout Libya. 



Geographic Variation in Response to Selective Factors of the Habitat 



Morphological variation in rodent populations in Libya usually does 

 not conform to climatic or ecogeographical rules, such as Bergmann's 

 Ride and Allen's Rule. In some species, trends in morphological 

 gradients are directly opposite those predicted by the rules. These 

 ecogeographical rules actually have a broad application and are nor- 

 mally apparent only in faunas spanning entire continents. Conforma- 

 tion, therefore, cannot really be expected on such a small scale. 

 In Libya, those species that do conform probably do so by coincidence. 



Geographic variation in response to selective factors of the habitat 

 is most apparent in the cryptic coloration of rodents living on differ- 

 ent types of substrate. Coloration of many subspecies and popula- 

 tions of Libyan rodents closely corresponds to the color of the sub- 

 strate on and in which they live. 



The subspecies of Gerbillus campestris and Meriones caudatus 

 demonstrate this substrate adaptation very well. On the Cyrenaican 

 Plateau, representatives of Gerbillus campestris brunnescens are all 

 of a dark chestnut dorsal color, reflecting the dark color of the sub- 

 strate. Farther south and west in the desert areas of Libya, gerbils 

 representing Gerbillus campestris dodsoni are markedly paler in dorsal 

 color in response to the pale color of the sand and desert soils. The 

 dorsal color of G. c. dodsoni is quite varied and several distinct color 

 patterns are sometimes present within the same local population. 

 This polymorphism in dorsal color is discussed in greater detail below. 



