Past social experience may explain the variation in rates 

 of emigration among yearling females. Petrusewicz (1963) 

 found that experimentally induced disruptions to social 

 structure could result in changes in density of stable 

 laboratory populations of mice. Population growth could be 

 induced by both removal and addition of several new mice to 

 the population. This suggested that different social 

 structures may tolerate different levels of density. Terman 

 (1962, 1963) for deer mice, Calhoun (1963) for Norway rats, 

 and Hunter and Davies (1963) for Scottish blackface sheep have 

 shown that early social experience is important in determining 

 subseguent spacing behavior. 



Both periods of high emigration during our study were 

 preceded by at least 2 years of poor fawn recruitment, when 

 few yearling females were added to matrilineal social groups. 

 Under those circumstances, mature females enjoy relative 

 solitude and little social interaction at the time of 

 parturition. A sudden increase in the number of yearlings 

 recruited (spring 1980, 1981, and 1987) could elicit very 

 aggressive behavior by mature females around their parturition 

 territory. On the other hand, after several years of 

 increasing recruitment and the addition of yearling females to 

 social groups during autumn and winter (1981-83), mature 

 females may be more used to the presence of other deer such 

 that yearlings are tolerated nearby as long as they don't 

 approach the new fawn. 



Thus, past social experience, as influenced by 

 recruitment of yearling females, could explain both pre- 

 saturation and saturation dispersal. During this study, both 

 periods of dispersal occurred during good forage and weather 

 conditions that followed several years of poor fawn 

 recruitment. These conditions served to enhance the 

 opportunity for success by dispersers (colonizers). 



Regardless of the reason for its occurrence, emigration 

 of yearling females reduced the growth rate of the female 

 population during years that it was documented. During 

 1979-80, net emigration of yearling females reduced the 

 instantaneous growth rate of the female population from 0.353 

 to 0.258; during 1980-81 the reduction was from 0.262 to 

 0.212; and during 1986-87 the reduction was from 0.220 to 

 0.085. Even though many of those yearlings were incorporated 

 into populations occupying adjacent marginal habitats, their 

 loss reflected "mortality" to the mule deer population on our 

 study area. 



Overall, emigration did not appear to occur in a manner 

 or to the extent that it would singly control mule deer 

 numbers. Rather, it was one of many factors that at times, 

 reduced populations in core areas below those which would have 



310 



