RODENTS OF LIBYA 115 



Bir Fergian, Tripolitania Province, are intermediate in color and 

 cranial characters between gerbils from the Fezzan and those from 

 Cyrenaica. 



The Sand Sea of Calanscio to the east and the Sand Sea of Rebianna 

 to the south apparently do not act as absolute barriers to the move- 

 ments of these gerbils. Likewise, the volcanic ramparts of the Gebel el 

 Harug el Asued of eastern Tripolitania and western Cyrenaica do not 

 entirely limit the east-west dispersal of this species in Libya. Three 

 specimens from the Wadi er Rueis, 340 kilometers west-northwest of 

 Tazerbo are indistinguishable from those from the type locality in 

 color and cranial characters. A pathway for gene exchange between 

 populations of gerbils from the Fezzan and Cyrenaica is provided by 

 the numerous small oases that circumscribe the Gebel el Harug el 

 Asued. When specimens become available from these areas, they 

 probably will show characters allying them to populations of gerbils 

 in both the Fezzan and Cyrenaica. 



Cranially, gerbils from the coastal areas near Agedabia, Marble 

 Arch, and El Agheila are indistinguishable from those from the type 

 locality in the Saharan interior. In color, however, these animals 

 from the coast are much paler dorsally with greater suffusion of 

 whitish hairs. This trend toward darker dorsal color increases pro- 

 gressively toward the south and reaches its extreme in gerbils from 

 Tazerbo and Cufra Oases. A single specimen, 325105, from 150 kilo- 

 meters south of Gialo is intermediate in dorsal color between gerbils 

 from Tazerbo and Gialo Oases. Similarly, specimens from Gasr es 

 Sahabi, which is located midway between Gialo Oasis and the coast 

 are lighter than those from Gialo but darker than those from the 

 coastal deserts. Thus a north-south, clinal gradient in dorsal color 

 is demonstrable within populations of gerbils from Cyrenaica. This 

 gradient in dorsal color reaches its greatest intensity in gerbils in- 

 habiting the southern oases. 



Ecological observations. These gerbils range throughout the 

 sandy environs of the oases, where they are frequently associated 

 with debris near the bases of palm trees. They also occur in sandy 

 margins of wadis and frequently inhabit the margins of the sand 

 seas or "Ramleh" where unstable, mountainous dunes are wide- 

 spread. The larger sand seas, however, such as Rebianna and Calanscio, 

 contain vast areas of constantly shifting dunes which probably are 

 not inhabited by these mice. Apparently sand seas of small extent 

 are not barriers to dispersal. The widely scattered distribution of 

 this subspecies and the collecting sites at 150 kilometers south of 

 Gialo and from the Wadi er Rueis confirm this. 



In the coastal areas of Cyrenaica this species is supplanted by 

 Gerbillus eatoni, which prefers the more densely vegetated margins 

 of the coastal plain. 



