32 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 122 



respects. G. nanus is sensitive to crowding, and captive reproduction 

 is possible only by means of a simultaneous encounter technique. 

 During male-anestrous female encounters, the partners exhibit mutual 

 avoidance or agonistic patterns of behavior. Sandbathing as a means 

 of chemical communication is displayed by this species and, after 

 mating, the male and female return to behavior patterns expressing 

 intolerance. All the preceding attributes also characterize the be- 

 havior of Perognathus parvus, P. inornatus, and P. longimembrls (see 

 p. 22 and Eisenberg, 1963b). Returning again to tables 2 through 

 5, there is a marked similarity between G. nanus and the silky pocket 

 mice with respect to habitat, general ecology, size, and reproduction. 

 This total convergence may well be indicative of a special adaptation 

 syndrome that correlates with the attribute of social intolerance; 

 however, until further comparisons are made on the energy needs 

 and recruitment rates of different desert-adapted species, the selec- 

 tive advantage of the various social systems in all probability will 

 remain obscured. 



Summary 



Selected species from thi-ee rodent families have been studied, in- 

 cluding the following genera: Cricetinae: Peromyscus; Gerbillinae: 

 Tatera, Gerbillus, Dipodillus, Pachyuro7nys, and Meriones; Dipodidae: 

 Jaculus, Allactaga; Heteromjddae : Heteromys, Lioinys, Perognathus, 

 and Dipodomys. Species have been selected in order to give a series 

 of forms that have evolved in a convergent or parallel fashion in 

 adapting to desert environments. 



The discrete behavior patterns exhibit a profound similarity, but 

 species and generic differences can be discerned. Sandbathing has 

 been selected for intensive study and it has been found that functional 

 sandbathing has evolved independently in all xeri tic-adapted forms 

 from the same basic movement patterns. Differences in the frequency 

 of occurrence rather than in the form of the movement have proved to 

 be the most effective criterion for delineating taxon-specific differences. 



Social behavior has been investigated with a standard methodology 

 including staged encounters, pah' tolerance, and group formation tests. 

 Using reproductive success as the criterion for describing a functional 

 social group, it has been found that each species can be characterized 

 into one of three general social types. The different social types 

 appear to represent different degrees of tolerance for the presence of 

 conspecifics. All social types will exhibit similar social pathologies 

 under social stress, but the more social species have higher thresholds 

 of tolerance. It is proposed that species-specific social tolerance rep- 

 resents an adaptation to average densities that are of selective advan- 

 tage to the species under consideration. The adaptive correlates of 

 the differing social systems remain, in part, unknown. 



