POPULATION CYCLES IN SMALL MAMMALS
303
FIG. 20. Survivorship curves for Mkrotua californicua from time of first live-trap
capture for animals from two expanding populations and two low or declining
populations. Adult males only. (After Krebs, 1966.)
difference between the sexes was caused by lower survival rates in the
males :
Survival rates per 14 days
Males
Females
November 1962-March 1963
0.84
0.90
MarchJune 1963
0.74
0.91
Males always survive less well than females during the breeding season,
but beyond this normal difference, some mortality factor affected the
males but not the females in this population from March to June.
Further details of the pattern of survival changes in cyclic vole
populations were obtained on Microtuspennsylvanicus and M. ochrogaster
by Krebs et al. (1969). Figure 22 illustrates one series of survival
estimates for M. penmylvanicus. Survival rates differed little in
303
FIG. 20. Survivorship curves for Mkrotua californicua from time of first live-trap
capture for animals from two expanding populations and two low or declining
populations. Adult males only. (After Krebs, 1966.)
difference between the sexes was caused by lower survival rates in the
males :
Survival rates per 14 days
Males
Females
November 1962-March 1963
0.84
0.90
MarchJune 1963
0.74
0.91
Males always survive less well than females during the breeding season,
but beyond this normal difference, some mortality factor affected the
males but not the females in this population from March to June.
Further details of the pattern of survival changes in cyclic vole
populations were obtained on Microtuspennsylvanicus and M. ochrogaster
by Krebs et al. (1969). Figure 22 illustrates one series of survival
estimates for M. penmylvanicus. Survival rates differed little in