POPULATION CYCLES IN SMALL MAMMALS
319
Clethrionomys rutilus and C. gapperi and found no change in mean
body weight. Fuller presents weight data for two years only, the peak
and decline summers for C. gapperi and two apparent peak summers
for C. rutilus. Further data are needed for increasing and low populations,
but it is puzzling that he found no differences in the declining population of C. gapperi. Elliott (1969) claimed that C. gapperi populations do
not fluctuate cyclically in most of their distributional range. If high
populations of C. gapperi represent irregular irruptions rather than
regulm cycles, we might use the high body weight criterion to distinguish
these two classes of population fluctuations. Regardless of our classification scheme, however, it would seem important to find other microtine
populations which do not behave as predicted.
Another approach to the study of growth over a rodent cycle is
to compute “indices of condition”. LeCren (1961) proposed a relative
condition factor obtained as the ratio
observed weight
weight predicted from body length
Condition factors of this type have been widely used in fish population
studies. We have tried to use LeCren’s index of condition to investigate
fluctuations of Microtus pennsylvanicus and M . ochrogaster in southern
Indiana (unpublished data). Snap-trap samples were obtained over six
years, and body length and weight were taken during standard
autopsies (Keller and Krebs, 1970). We pooled all the data to calculate
the body weight (Y)-body length (X) regression for each species, and
then referred individual voles to this common regression to get the
predicted weight. Figure 29 shows our results for M . ochrogaster,
which reached peak densities in 1966 and 1969 in our study areas.
It is apparent that there are large changes in “condition’) of voles from
year to year. We could detect no clear trends related to density,
however. Condition was “poor” in increasing populations in 1965 and
average’’ to “good” in declining populations in 1967. There was no
clear seasonal trend, and this may reflect the relatively mild winters
of southern Indiana. We concluded from our analysis that there were
real differences from year to year in relative condition but these differences were not related to cyclic density changes.
There is an array of more sophisticated methods for determining
relative condition of rodents. Krebs (1964a) used an arbitrary fat
index to judge the amount of stored fat on lemmings (Lemmw and
Dicrostonyz). There was no relation between the amount of fat which
lemmings had stored and population density. Batzli and Pitelka (1971)
extracted the fat from Microtw californicus carcasses and also were
not able to associate levels of stored fat with population changes.
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319
Clethrionomys rutilus and C. gapperi and found no change in mean
body weight. Fuller presents weight data for two years only, the peak
and decline summers for C. gapperi and two apparent peak summers
for C. rutilus. Further data are needed for increasing and low populations,
but it is puzzling that he found no differences in the declining population of C. gapperi. Elliott (1969) claimed that C. gapperi populations do
not fluctuate cyclically in most of their distributional range. If high
populations of C. gapperi represent irregular irruptions rather than
regulm cycles, we might use the high body weight criterion to distinguish
these two classes of population fluctuations. Regardless of our classification scheme, however, it would seem important to find other microtine
populations which do not behave as predicted.
Another approach to the study of growth over a rodent cycle is
to compute “indices of condition”. LeCren (1961) proposed a relative
condition factor obtained as the ratio
observed weight
weight predicted from body length
Condition factors of this type have been widely used in fish population
studies. We have tried to use LeCren’s index of condition to investigate
fluctuations of Microtus pennsylvanicus and M . ochrogaster in southern
Indiana (unpublished data). Snap-trap samples were obtained over six
years, and body length and weight were taken during standard
autopsies (Keller and Krebs, 1970). We pooled all the data to calculate
the body weight (Y)-body length (X) regression for each species, and
then referred individual voles to this common regression to get the
predicted weight. Figure 29 shows our results for M . ochrogaster,
which reached peak densities in 1966 and 1969 in our study areas.
It is apparent that there are large changes in “condition’) of voles from
year to year. We could detect no clear trends related to density,
however. Condition was “poor” in increasing populations in 1965 and
average’’ to “good” in declining populations in 1967. There was no
clear seasonal trend, and this may reflect the relatively mild winters
of southern Indiana. We concluded from our analysis that there were
real differences from year to year in relative condition but these differences were not related to cyclic density changes.
There is an array of more sophisticated methods for determining
relative condition of rodents. Krebs (1964a) used an arbitrary fat
index to judge the amount of stored fat on lemmings (Lemmw and
Dicrostonyz). There was no relation between the amount of fat which
lemmings had stored and population density. Batzli and Pitelka (1971)
extracted the fat from Microtw californicus carcasses and also were
not able to associate levels of stored fat with population changes.
<<