POPULATION CYCLES IN SMALL MAMMALS
297
individual is exposed. Therefore, slow growth during the summer
causes underestimation of the age using the Lidicker and MacLean
(1969) technique, and so this technique has the same biases as the use
of body weight as an estimator of age. A new method for aging wild
rodents based on the fractions of soluble and insoluble proteins in the
eye lens is described by Otero and Dapson (1972). As the individual
ages a larger portion of the lens protein becomes insoluble in water.
This method is supposed to be less influenced by environmental factors
than other aging techniques. If one has detailed knowledge of a population’s breeding seasons and mortality rates, one can use weight as a
reasonable index of age, particularly for young animals.
Natural history observations have established that age at sexual
maturity is variable in microtines and that changes in the rate of
sexual maturation of young voles and lemmings are a major driving
force behind population cycles. Young Norwegian lemmings about
20 days old (25 g) were found pregnant in the summer of increase by
Koshkina and Khalansky (1962), while almost none of the summerborn young lemmings became mature in the following year of peak
density. Mullen (1965) records delayed maturation of male brown
lemmings in a peak summer. Kalela (1957) in a detailed investigation
of Clethrionomys rufocanus in Finland found that the maturation rate
of the early summer young was strongly affected by population density:
Proportion of early summer young mature
Males
Females
1954 (increase)
almost all
almost all
1955 (peak)
almost none
majority
1956 (decline)
majority
majority
Koshkina (1965) showed that maturation of Clethrionomys rutilus was
inversely related to population density (Fig. 16). Note that there were
always fewer males maturing then females. The same observation was
made by Zejda (1967) for C. glareolus.
Few studies on weight at sexual maturity have utilized the quantitative techniques of Leslie et al. (1945) to estimate the median body
weight at sexual maturity. Figure 17 shows changes in size at sexual
maturity in the brown lemming over a cycle in numbers. Note that
males are more strongly depressed in maturation than females. I n the
decline year of 1961 no young males matured, although young females
did mature at about four weeks of age. Keller and Krebs (1970) show
that the median weight at sexual maturity was higher in peak populations of Microtus pennsylvanicus and M . ochrogaster but equal in
increasing and declining populations. This work illustrates some of the
problems of using weight as an index of age. We know that growth
297
individual is exposed. Therefore, slow growth during the summer
causes underestimation of the age using the Lidicker and MacLean
(1969) technique, and so this technique has the same biases as the use
of body weight as an estimator of age. A new method for aging wild
rodents based on the fractions of soluble and insoluble proteins in the
eye lens is described by Otero and Dapson (1972). As the individual
ages a larger portion of the lens protein becomes insoluble in water.
This method is supposed to be less influenced by environmental factors
than other aging techniques. If one has detailed knowledge of a population’s breeding seasons and mortality rates, one can use weight as a
reasonable index of age, particularly for young animals.
Natural history observations have established that age at sexual
maturity is variable in microtines and that changes in the rate of
sexual maturation of young voles and lemmings are a major driving
force behind population cycles. Young Norwegian lemmings about
20 days old (25 g) were found pregnant in the summer of increase by
Koshkina and Khalansky (1962), while almost none of the summerborn young lemmings became mature in the following year of peak
density. Mullen (1965) records delayed maturation of male brown
lemmings in a peak summer. Kalela (1957) in a detailed investigation
of Clethrionomys rufocanus in Finland found that the maturation rate
of the early summer young was strongly affected by population density:
Proportion of early summer young mature
Males
Females
1954 (increase)
almost all
almost all
1955 (peak)
almost none
majority
1956 (decline)
majority
majority
Koshkina (1965) showed that maturation of Clethrionomys rutilus was
inversely related to population density (Fig. 16). Note that there were
always fewer males maturing then females. The same observation was
made by Zejda (1967) for C. glareolus.
Few studies on weight at sexual maturity have utilized the quantitative techniques of Leslie et al. (1945) to estimate the median body
weight at sexual maturity. Figure 17 shows changes in size at sexual
maturity in the brown lemming over a cycle in numbers. Note that
males are more strongly depressed in maturation than females. I n the
decline year of 1961 no young males matured, although young females
did mature at about four weeks of age. Keller and Krebs (1970) show
that the median weight at sexual maturity was higher in peak populations of Microtus pennsylvanicus and M . ochrogaster but equal in
increasing and declining populations. This work illustrates some of the
problems of using weight as an index of age. We know that growth