POPULATION CYCLES IN SMALL MAMMALS
299
rates of individuals tend to be low in declining populations (Krebs et al.,
1969). Consequently, if weight at sexual maturity is equal in increasing
and declining populations, age at sexual maturity must be greater in
declining populations. If we had a reliable indicator of age in voles, we
could investigate this deduction directly.
We conclude that the age of sexual maturity is an important variable
in the reproductive strategy of microtine rodents. Age a t sexual
maturity is increased in peak populations and perhaps also in declining
populations. More work is required to quantify these trends in species
which can be aged.
5 . Sex ratio
If the sex ratio is sufficiently disturbed from the typical 50% males
two things may happen. First, if there are too few males, females might
go unmated and consequently the pregnancy rate would decrease.
We have not been able to find any evidence from voles or lemmings
that females ever experience such limitation. Second, a shift in the
population sex ratio might regulate its density. Increasing populations
might have a higher percentage of females to increase the reproductive
output, while peak populations might equalize the sex ratio or even
favor males (Williams, 1966, p. 148). Unfortunately, natural selection
does not seem to operate in such a way to maximize population fitness.
Many studies on microtines have commented on sex ratios, but few
have analyzed the variables which affect the observed sex ratios.
Males are typically less abundant than females, but the sex ratio does
not correlate with population density in Microtus pennsylvanicus or
M . ochrogaster (Myers and Krebs, 1971a).
Abnormal sex ratios (20-30y0 males) in the wood lemming, Myopus
schisticolor, occur in field populations because some females produce
only female offspring and other females produce both sexes (Kalela and
Oksala, 1966). There is as yet little information on how this abnormal
sex ratio varies in relation to population density. Kalela and Oksala
(1966) describe a declining population of the wood lemming in which
the sex ratio increased from about 20-30y0 males in the peak year to
about 47% males in the year of decline. Whether this change was caused
by movement of animals or differential mortality is not known.
Except for the interesting case of the wood lemming, there is no
indication that variations in sex ratios are associated with population
fluctuations in voles and lemmings.
6. Summary
Reproductive changes are part of the machinery which drives the
population cycle. Not all components of reproduction are involved,
299
rates of individuals tend to be low in declining populations (Krebs et al.,
1969). Consequently, if weight at sexual maturity is equal in increasing
and declining populations, age at sexual maturity must be greater in
declining populations. If we had a reliable indicator of age in voles, we
could investigate this deduction directly.
We conclude that the age of sexual maturity is an important variable
in the reproductive strategy of microtine rodents. Age a t sexual
maturity is increased in peak populations and perhaps also in declining
populations. More work is required to quantify these trends in species
which can be aged.
5 . Sex ratio
If the sex ratio is sufficiently disturbed from the typical 50% males
two things may happen. First, if there are too few males, females might
go unmated and consequently the pregnancy rate would decrease.
We have not been able to find any evidence from voles or lemmings
that females ever experience such limitation. Second, a shift in the
population sex ratio might regulate its density. Increasing populations
might have a higher percentage of females to increase the reproductive
output, while peak populations might equalize the sex ratio or even
favor males (Williams, 1966, p. 148). Unfortunately, natural selection
does not seem to operate in such a way to maximize population fitness.
Many studies on microtines have commented on sex ratios, but few
have analyzed the variables which affect the observed sex ratios.
Males are typically less abundant than females, but the sex ratio does
not correlate with population density in Microtus pennsylvanicus or
M . ochrogaster (Myers and Krebs, 1971a).
Abnormal sex ratios (20-30y0 males) in the wood lemming, Myopus
schisticolor, occur in field populations because some females produce
only female offspring and other females produce both sexes (Kalela and
Oksala, 1966). There is as yet little information on how this abnormal
sex ratio varies in relation to population density. Kalela and Oksala
(1966) describe a declining population of the wood lemming in which
the sex ratio increased from about 20-30y0 males in the peak year to
about 47% males in the year of decline. Whether this change was caused
by movement of animals or differential mortality is not known.
Except for the interesting case of the wood lemming, there is no
indication that variations in sex ratios are associated with population
fluctuations in voles and lemmings.
6. Summary
Reproductive changes are part of the machinery which drives the
population cycle. Not all components of reproduction are involved,