380
CHARLES J. KREBS AND JUDITH H. MYERS
growth was found in field animals. Myers and Krebs (1971b) with a
very small sample showed that male M . ochrogaster dispersing from an
increasing population had a low growth potential. No one has monitored,
under standard conditions in the laboratory, growth of microtines taken
from various phases of the population fluctuation.
If specific behavior patterns which were measurable in the laboratory
could be found to be associated with changes in population density,
these might serve as more meaningful genetic markers. We were largely
unsuccessful in our attempts to find a laboratory measurement which
would distinguish dispersing and non-dispersing M . ochrogaster and
M . penmylvanicus (Myers and Krebs, 1971b). Although there is evidence
for genetically determined behavioral types in house mice (Van
Oortmerssen, 1970), no work has begun to determine the genetic
component of behavior in microtines.
There may be ways to obtain individuals of different types if we
let the field situation act as the selection process. As discussed earlier,
dispersal is a particularly important process during the phase of
population increase. A possible explanation for the higher density
of enclosed populations is that the prevention of dispersal has forced
individuals which would normally have left to stay in the population.
Consequently, the quality of individuals of high populations in enclosures should be different from that of natural peak populations.
Our data on dispersal in M . pennsylvanicus (Myers and Krebs, 1971b)
indicated that those individuals with the highest reproductive potential
were dispersing. These were young females which had just become
sexually mature (Fig, 44). Using marker alleles at the transferrin
locus we found that there was a genetic component to dispersal among
females during the phase of population increase.
We suggest that instead of comparing the reproductive potential
of transferrin genotypes, we should look a t the reproduction potential
of dispersers and non-dispersers, of animals from peak populations and
declining populations, of animals from high-density populations in
enclosures (those which have not been selected by dispersal) and from
high-density natural populations. If we are to test the Chitty hypothesis,
we must be concerned with demographically pertinent genotypes rather
than marker loci which may be variously linked with loci under
density selection.
I n summary, data collected on the genetics of cycling vole populations
by the use of electrophoretic variants as genetic markers demonstrate
dramatic genetic changes associated with density changes. In some
cases ( M . ochrogaster and M . penmylvanicus) there are significant
correlations between changes in gene frequency and changes in density,
and these changes show some consistency over two population cycles
CHARLES J. KREBS AND JUDITH H. MYERS
growth was found in field animals. Myers and Krebs (1971b) with a
very small sample showed that male M . ochrogaster dispersing from an
increasing population had a low growth potential. No one has monitored,
under standard conditions in the laboratory, growth of microtines taken
from various phases of the population fluctuation.
If specific behavior patterns which were measurable in the laboratory
could be found to be associated with changes in population density,
these might serve as more meaningful genetic markers. We were largely
unsuccessful in our attempts to find a laboratory measurement which
would distinguish dispersing and non-dispersing M . ochrogaster and
M . penmylvanicus (Myers and Krebs, 1971b). Although there is evidence
for genetically determined behavioral types in house mice (Van
Oortmerssen, 1970), no work has begun to determine the genetic
component of behavior in microtines.
There may be ways to obtain individuals of different types if we
let the field situation act as the selection process. As discussed earlier,
dispersal is a particularly important process during the phase of
population increase. A possible explanation for the higher density
of enclosed populations is that the prevention of dispersal has forced
individuals which would normally have left to stay in the population.
Consequently, the quality of individuals of high populations in enclosures should be different from that of natural peak populations.
Our data on dispersal in M . pennsylvanicus (Myers and Krebs, 1971b)
indicated that those individuals with the highest reproductive potential
were dispersing. These were young females which had just become
sexually mature (Fig, 44). Using marker alleles at the transferrin
locus we found that there was a genetic component to dispersal among
females during the phase of population increase.
We suggest that instead of comparing the reproductive potential
of transferrin genotypes, we should look a t the reproduction potential
of dispersers and non-dispersers, of animals from peak populations and
declining populations, of animals from high-density populations in
enclosures (those which have not been selected by dispersal) and from
high-density natural populations. If we are to test the Chitty hypothesis,
we must be concerned with demographically pertinent genotypes rather
than marker loci which may be variously linked with loci under
density selection.
I n summary, data collected on the genetics of cycling vole populations
by the use of electrophoretic variants as genetic markers demonstrate
dramatic genetic changes associated with density changes. In some
cases ( M . ochrogaster and M . penmylvanicus) there are significant
correlations between changes in gene frequency and changes in density,
and these changes show some consistency over two population cycles