28 



PROCEEDINGS OF THE NATIONAL MUSEUM 



latent in all species and only threshold differences correlate with a 

 given social type (see p. 29). 



In both Lidicker's study (1965) and my own, the species of Pero- 

 myscus show a tendency to increase their reproductive activity in the 

 spring or autumn, following a cessation of reproduction after having 

 achieved a density limit in the previous season. In one case a group 

 of P. eremicus partially reared two Utters, whereas two other eremicus 

 groups showed increased male-male aggression in the spring but the 

 females did not cycle. P. crinitus exhibited similar patterns with 

 spring reproduction in one case and male-male antagonism in two 

 cases (see table 22). 



One may ask if a normally sohtary species can be subjected to early 

 experience that would be conducive to the production of more socially 

 tolerant adult behavior. This may be the case \vith certain labile 

 species that are adapted to exist at varying population densities. I 

 have found, however, that strongly solitary species will not adapt to 

 enforced proximity even as juveniles. It is true that juvenile groups 

 can be maintained together for long periods of time, but normal re- 

 production does not take place. In figure 9 one can observe the re- 

 version to agonistic behavior within eight days after a group of 



100%i 



M 



■ 



4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 



DAYS AFTER SEPARATION 



Figure 9. — Changing proportions of responses during Perognalhus parvus littermate en- 

 counters after separation at 70 days of age (S= sexual behaviors; C= contact-promoting 

 behaviors; Antagonistic behaviors). 



Perognathus parvus littermates were separated. Not only is normal 

 reproduction affected, but prolonged social experience often appears 

 to have Uttle permanent conditioning effect with this species and 

 others. Littermate groups of typically sohtary rodents that did not 

 exhibit normal reproduction included Perognathus parvus, Dipodomys 

 nitratoides , Gerbillus nanus, and Liomys pictus. 



