102 U.S. NATIONAL MUSEUM BULLETIN 275 



p. 45), G. gerbillus can be distinguished by its shorter hind feet, longer 

 rostrum, longer ears, longer molariform toothrow, and markedly 

 larger size of body and cranium. 



Remarks. Previous workers in Libyan mammals recognized little 

 variation in G. gerbillus, and all of Libya was included within the range 

 of the nominate subspecies. Much larger series are now available 

 from Libya, and this species is now known to be represented by five 

 subspecies and not one as previously supposed. 



In many areas of Libya, isolated populations of these gerbils occur 

 throughout seemingly barren wastelands. These areas support only a 

 meager vegetative cover, and frequently plant life is entirely wanting. 

 These widely scattered populations are characterized by common 

 morphological characters which indicate at least some degree of 

 genetic relationship. Apparently, seasonal or occasional rains in these 

 desert areas enable plants, primarily grasses, to undergo their brief 

 life cycle and produce abundant quantities of seed which allow small 

 populations of these gerbils to subsist until the next period of mois- 

 ture. By successive seasonal or yearly storms, populations of repro- 

 ductively effective size are able to progressively extend their range. 

 These periods of rainfall may occur within the same year or be spaced 

 over a period of several years. In this manner, small populations of 

 gerbils would be able to "bridge" vast areas in the desert which 

 otherwise would prove insurmountable. 



These shifting populations illustrate another significant aspect of 

 distribution. Failure of seasonal or periodic rains can cause the ex- 

 termination of "bridging" populations just as their occurrence can 

 perpetuate such populations. Failure of rains thus sets up isolating 

 barriers. Occurrence or lack of rain insures a fluctuating pattern of 

 genetic "input" and "output" over the range of the species and also 

 makes it probable that the pattern observed now will be altered in 

 subsequent sampling. 



Ecological observations. Members of this species are the most 

 ubiquitous of the subgenus Gerbillus in Libya. They are not known 

 to occur in habitats lacking sand and are widely distributed through- 

 out the oases, serirs, and hamadas of the interior. They are commonest 

 along the sandy margins of the oases but occur also in the coastal 

 areas wherever sandy areas are present. They are unknown from the 

 coastal areas of Tripolitania and are not found in Cyrenaica north of 

 the Cyrenaican Plateau. In these areas, they are supplanted by 

 Gerbillus eatoni and members of the Gerbillus pyramidum group. 



Throughout their range in Libya they are sympatric with many 

 other species of rodents but most frequently occur with Gerbillus 

 pyramidum and Jaculus jaculus. 



