POPULATION CYCLES I N SMALL MAMMALS
379
To test these predictions, field populations of M. ochrogaster were
established in two-acre enclosures in Indiana with three founder
populations of 20 individuals all of one genotype (Gaines et al., 1971).
Three such introductions were made in one year so that by the end of
the experiment we had data for three populations of each of the three
transferrin genotypes. The surprising result was that we could find no
statistical differences between populations established with different
genotypes. Populations of each of the genotypes were capable of rapid
population increase. In all three populations started with heterozygotes
there was an increase in the frequency of the TfE allele by the end of the
experiment, which covered four months and included two generations.
Because of this we suggest that when the TfF/TfF genotypes are free
from competition with the other genotypes they are able to do well,
but in mixed populations they are at a disadvantage.
The failure to verify predictions about the effect of selection of
M. ochrogaster transferrin genotypes might be explained by the fact
that the enclosures, which prevented dispersal, disrupted the normal
demographic machinery. We suggest that perturbations of field
populations may be more meaningful (Gaines et al., 1971; Krebs, 1971).
This could be accomplished by switching homozygotes of two types
between populations and observing the demographic response.
Genetic markers can be used as the basis for classification of microtines into increase, peak or decline categories, but there will be considerable noise in the classification system. On the basis of this work with
transferrin in M. ochrogaster and M . pennsylvanicus, Tamarin, Gaines
and Krebs were able to distinguish increase and decline types based on
the comparative success of the genotypes during these population
phases. We would like to be able more directly to distinguish increase,
peak and decline individuals based on those attributes which give them
the advantage during these population phases. A pertinent question is:
Is there a rapid growth genotype which is favoured during the increase
phase or is there a genotype which is capable of withstanding crowded
conditions of peak populations but is less resistant to mortality factors?
The problem is to devise a system in which the heritability of the factors
can be scored.
Heritability studies of ecologically important attributes would seem
to offer a promising area for future work. For example, heritability of
growth could be determined for microtine rodents. Few studies have
attempted to determine the growth potential of animals in cycling
populations. Newson and Chitty (1962) found animals of high and low
growth potentials in declining populations. Krebs (1966) showed that
some voles from a declining population of M. californicus would grow
to large sizes if they were removed to the laboratory, although little
379
To test these predictions, field populations of M. ochrogaster were
established in two-acre enclosures in Indiana with three founder
populations of 20 individuals all of one genotype (Gaines et al., 1971).
Three such introductions were made in one year so that by the end of
the experiment we had data for three populations of each of the three
transferrin genotypes. The surprising result was that we could find no
statistical differences between populations established with different
genotypes. Populations of each of the genotypes were capable of rapid
population increase. In all three populations started with heterozygotes
there was an increase in the frequency of the TfE allele by the end of the
experiment, which covered four months and included two generations.
Because of this we suggest that when the TfF/TfF genotypes are free
from competition with the other genotypes they are able to do well,
but in mixed populations they are at a disadvantage.
The failure to verify predictions about the effect of selection of
M. ochrogaster transferrin genotypes might be explained by the fact
that the enclosures, which prevented dispersal, disrupted the normal
demographic machinery. We suggest that perturbations of field
populations may be more meaningful (Gaines et al., 1971; Krebs, 1971).
This could be accomplished by switching homozygotes of two types
between populations and observing the demographic response.
Genetic markers can be used as the basis for classification of microtines into increase, peak or decline categories, but there will be considerable noise in the classification system. On the basis of this work with
transferrin in M. ochrogaster and M . pennsylvanicus, Tamarin, Gaines
and Krebs were able to distinguish increase and decline types based on
the comparative success of the genotypes during these population
phases. We would like to be able more directly to distinguish increase,
peak and decline individuals based on those attributes which give them
the advantage during these population phases. A pertinent question is:
Is there a rapid growth genotype which is favoured during the increase
phase or is there a genotype which is capable of withstanding crowded
conditions of peak populations but is less resistant to mortality factors?
The problem is to devise a system in which the heritability of the factors
can be scored.
Heritability studies of ecologically important attributes would seem
to offer a promising area for future work. For example, heritability of
growth could be determined for microtine rodents. Few studies have
attempted to determine the growth potential of animals in cycling
populations. Newson and Chitty (1962) found animals of high and low
growth potentials in declining populations. Krebs (1966) showed that
some voles from a declining population of M. californicus would grow
to large sizes if they were removed to the laboratory, although little