The Herbivore-Based Trophic System 341 



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Jan I Feb |Mar | Apr |May | Jun | Jul | Aug | Sep | Oct | Nov | Dec 



FIGURE 10-4. Mean litter sizes for different age classes 

 of lemmings throughout the year for the period 1952-65. 

 Vertical bars represent ± 1 SE. (After Osborn 1975; based 

 upon data of Pit el ka 1974.) 



from November through April. Far fewer females are pregnant in mid- 

 winter than in summer. Breeding intensity varies more during winter 

 than summer, probably depending upon the snowpack, thermal condi- 

 tions and the availability of food, but there is little direct evidence for 

 this. Lack of winter nests, lack of placental scars in adult females in 

 spring and decreases in the population indicate that during some winters 

 Httle, if any, reproduction occurs (MacLean et al. 1974). 



Litter sizes differed among age classes of lemmings and among sea- 

 sons (Figure 10-4). Older females have larger litters during summer; the 

 mean litter size is eight for adults, seven for subadults and six for juven- 

 iles. During midwinter mean Htter size declines to three. Although sta- 

 tistical analysis (ANOVA, p < 0.05) indicated significant, but minor, dif- 

 ferences in average litter sizes from summer to summer, they did not ap- 

 pear to be related to the phase of the cycle (Osborn 1975). 



Dramatic changes in survival rates from summer to summer do oc- 

 cur, from 70'Vo per 28 days to 10% per 28 days for adults in July, but our 

 knowledge of survival rates throughout a cycle is scanty. Changes in sex 

 ratio indicate that survival of males tends to be lower than survival of fe- 

 males during the summer and higher during the winter, possibly as a re- 

 sult of differential predation. Osborn (1975) developed a computer simu- 

 lation model that allows survival rates and density of sucklings, which 

 are not trappable, to be estimated by a trial and error procedure, given 

 information on age-specific reproductive rates and on population struc- 

 ture. Using the reproductive rates and litter sizes discussed above, to- 

 gether with field observations of age structure and population size during 

 summer, he estimated density of sucklings and survival rates for each 



