400 



G. O. Batzli et al. 



TABLE 10-10 Life History Characteristics of Lemmus and 

 Rangifer 



•Assume no deaths and that female lemming produces one litter per month (3.5 99 per litter), 

 begins reproducing at two months and can produce nine months of the year. A female caribou 

 produces one litter per year (0.5 99 litter"'). 



growth rate, the total weight of the Utter at weaning is 1.1 times that of 

 the mother. A weanling caribou weighs only 0.4 as much as its mother. 

 The relative investment of a female lemming in each litter must therefore 

 be much greater than that of a female caribou. The same would be true 

 during a life span because the number of litters produced is similar. Sur- 

 viving to maximum age and reproducing at a maximum rate, a lemming 

 could produce 14 litters and a caribou 18. During a normal hfe span both 

 species might be expected to produce one or two Utters. A longer life ap- 

 pears to compensate somewhat for slower development and lower repro- 

 ductive rates in caribou so that the total production of litters is similar. It 

 is the number of individuals per litter, resulting in the large biomass of 

 the litter relative to the mother, that produces a greater investment in off- 

 spring per female lemming. These results are consistent with those of 

 Millar (1977), who concluded that litter size is the most important factor 

 affecting the reproductive efforts of mammals in general. 



Just as the ratio of body size remains similar for lemmings and cari- 

 bou at any stage in their life cycle, two important measures of survival. 



I 



