38 U.S. NATIONAL MUSEUM BULLETIN 275 



G. campestris, however, seems to be the fissures and recesses of rocky 

 outcroppings. Because of this broad range of ecological tolerance, G. 

 campestris is the most widespread rodent in Libya. In this respect it 

 parallels rather closely the ecological and genetic plasticity of the 

 deer mouse, Peromyscvs maniculatus (Wagner) , of North America. 



When G. amoenus and G. campestris inhabit the same oasis, the 

 latter is almost invariably found to occupy the interior, more lush 

 portions of the oasis, and G. amoenus is usually confined to the drier, 

 peripheral zones. Thus, habitat exclusion is clearly operative. 



Habitat exclusion is further demonstrated by these two species 

 along the coastal escarpment where G. campestris is confined to rocky 

 outcroppings or rock-strewn uplands, and G. amoenus invariably 

 occupies the intervening pockets of sand and gravel. 



Two sibling species of jerboas, Jaculus deserti and Jaculus jaculus, 

 are sympatric over large parts of the Libyan interior but show subtle 

 habitat preferences. Jaculus deserti occurs in the outlying hamadas 

 and sparsely vegetated margins of wadis, whereas J. jaculus is con- 

 fined more to margins of sand seas and dune areas associated with 

 oases. Only rarely were both species found to occur in the same local 

 area, and then the two species were never collected from the same 

 portion of the trapline. 



Sympatry of species of Libyan rodents is not linlited to these few 

 examples. In many areas of Libya, several species of genetically and 

 morphologically distinct rodents occur in the same geographic area 

 and demonstrate marked habitat exclusion. 



In some oases, Gerbillus gerbillus, Gerbillus pyramidum, Gerbillus 

 campestris, Gerbillus amoenus, Meriones caudatus, Jaculus jaculus, 

 and Mus musculus are all present. In these oases, each species or 

 species pair tends to inhabit a particular niche with consequent ex- 

 clusion of the other species. The dense growth of sedges and other 

 mesophytic plants of the interior of these oases are occupied exclu- 

 sively by Gerbillus campestris and Mus musculus. In open sandy areas 

 and around bases of palm trees, Gerbillus gerbillus and Gerbillus 

 pyramidum are more abundant. Meriones caudatus is confined to 

 sandy hummocks supporting shrubby young palms, and Jaculus 

 jaculus and Gerbillus amoenus are confined largely to peripheral areas 

 of oases in the zone of tamarix and outlying dunes. 



Many additional factors probably prevent genetic exchange between 

 closely related sympatric species of Libyan rodents. Some of these 

 additional isolating mechanisms include behavioral barriers to mating, 

 differences in the time of the breeding season, and mechanical incom- 

 patabilities, such as differences in the size and shape of the external 

 genitalia. 



