18 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 122 



Sandbathing is a trait shared by all desert-adapted rodents. It 

 would appear that increased sebaceous secretion is a necessary con- 

 dition to reduce evaporative water loss through the epidermis. In 

 addition, many desert rodents have a dense pelage with a concomitant 

 increase in sebaceous glands as an adaptation to extremes of cold 

 during the desert night (Sokolov, 1962). With the increase in 

 sebaceous glands and secretion, one finds a corresponding necessity to 

 dress tlie pelage. Since all species of rodents studied appear to dry 

 their fur when it is moistened by means of either extending and flexing 

 the body while lying on their side or ventrum or rolling over, the con- 

 clusion is unavoidable that ritualized sandbathing has evolved from 

 the same set of basic movements in all rodent families. It is inter- 

 esting to note that selection has favored a relatively stereotyped 

 pattern that varies in a species-specific manner (see table 7). The 

 higher taxonomic categories show less uniformity; liowever, the sand- 

 bathing Heteromyidae are remarkably uniform with their tendency 

 to integrate side rubs and ventral rubs. This characteristic tends to 

 set oft' the Heteromyidae from the Gerbilhnae and Dipodidae. 



Since sandbathing has the dual function of dressing the pelage and 

 leaving a chemical trace of presumptive communicatory value, the 

 evolutionary origins of sandbathing are inextricably tied to marking. 

 Marking by means of the perineal drag probably had its origin in a 

 common cleaning movement that consisted of ^viping the anal-genital 

 area on the substrate after urination or defecation. The stretch in- 

 volving extension and flexion of the body also frequently accompanies 

 ehmination after the animal has awakened from prolonged sleep. 

 Thus, selection could favor a combined ventral rub with perineal drag 

 as a marking movement if the sebaceous secretions of the ventral 

 epidermis had some inherent communicatory function that affected 

 reproductive success or survival of the genotype. Such a ritualized 

 marking pattern appears to have arisen as an independent element in 

 Meriones, whereas in the Heteromyidae the ventral rubbing with its 

 marking function has been combined Avith side rubbing in a functional 

 sandbathing sequence (see table 9). 



Patterns of Social Behavior 



In a recent review of rodent social behavior (Eisenberg, 1966), I 

 attempted to outline the origins and evolution of the various social 

 systems to be found ^dthin the Rodentia. Social systems may be 

 classified into two categories: solitary and communal. The com- 

 munal systems have several subtypes including monogamous, polyga- 

 mous, and family band groupings. For convenience, I will restrict 

 the discussion in this paper to three categories: solitary, pair tolerance, 

 and communal. The latter category corresponds to the family band 



