then 2 winters and a full and partial hunting season before 

 she reaches breeding age. The statistical probability of the 

 father living to the first breeding season for the daughter is 

 28%, based on average mortality rates observed during this 

 study. Survival probabilities for the father decline 

 considerably from that for subsequent breeding seasons. 

 Probability of inbreeding is relatively small for this 

 population, but the fact that at least some potential for 

 inbreeding remains may be beneficial (Shields 1983). 



The occurrence of matrilineal groups probably had several 

 survival advantages that outweighed the disadvantage of 

 increased resource competition within an area. As population 

 numbers increase, additional deer put increasing pressure on 

 the resources of an area. When mature females allow their 

 female offspring to rejoin them after early fawn-rearing, 

 thereby forming matrilineal groups, they ensure that the 

 resources within their home range, although somewhat 

 diminished for them, are beneficial to and passed on to their 

 genetic heirs rather than to unrelated deer. Kin-selection 

 principles (Maynard-Smith 1964 and Hamilton 1964) can explain 

 much of the rationale for the formation of matrilineal groups. 



Matrilineal groups are also especially beneficial to the 

 offspring of deer that only move to "winter range" during the 

 most severe winters. Continued association with the matriarch 

 ensures that they will eventually learn the location of 

 "winter ranges". The fact that yearling males are primarily 

 associated with female groups also increases the chances that 

 they will learn the locations of "wintering areas" even though 

 they are usually genetically unrelated to the female groups. 

 This may also be beneficial to the females because they are 

 increasing the survival odds of a potential mate of new 

 genetic stock. 



Our data enabled us to address the issue of factors 

 determining group size as discussed for white-tailed deer by 

 Hirth (1977). The general consensus has been that group size 

 varies with habitat type and that relationship evolved as a 

 means of minimizing losses to predation and possibly 

 optimizing feeding efficiency. Our data indicated that other 

 factors also are involved. Further, the general perception 

 that mule deer are more gregarious than white-tailed deer 

 (DeVos et al . 1967) may not result from inherent social 

 differences, but rather is related to differences in habitat 

 use and fawn survival . 



The habitat on our study area was a mixture of moderately 

 dense cover interspersed with relatively open forest cover and 

 very open shrub-grasslands. Deer had a variety of cover 

 options, but dense cover was limited. Our data indicated that 

 group size for adults was dependent on fawn survival. Females 



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