340 



G. O. Batzli et al. 



Year of Cycle 



FIGURE 10-3. The percentage of female lem- 

 mings pregnant in each age class during the 

 course of a cycle. Sample sizes range from 10 to 

 746 and include all females collected by Pitelka 

 (1973) during 1952-65. Data were collected for 

 only one winter (1962-63). The shaded bars sepa- 

 rate summer and winter and indicate times when 

 mean air temperatures are near 0°C. (After 

 Osborn 1975.) 



Demographic changes accompany these population fluctuations. 

 Suppression of breeding, indicated by a low incidence of pregnancies, oc- 

 curs in May just before snowmelt and in September during freeze-up 

 (Figure 10-3). When lemmings do breed at these times, nests lie exposed 

 on the surface because burrows are filled with ice or water or the snow 

 cover is not well developed. Summer breeding appears to decline regu- 

 larly in late August, although less so in pre-high years, but the resump- 

 tion of breeding in early summer varies, depending upon temperature 

 and the timing of snowmelt (Mullen 1968). During these breeding pauses 

 the population structure shifts toward the older age classes, and density 

 declines. Once the summer season begins, the population reproduces 

 maximally — nearly every female is pregnant by mid-July — and the popu- 

 lation structure shifts toward the younger age classes. If survival is high, 

 the population increases rapidly. In general older females become preg- 

 nant more frequently than younger ones. The breeding intensity of adults 

 varies little from summer to summer, but juveniles and subadults breed 

 much less during the summer of a high population (Figure 10-3). 



Little is known about the winter breeding season except that it lasts 



