Abstract:
A dispersal polymorphism may exist in emigrants from cyclic populations
of Microtus '~nnsylvanicus biasing trap-revealed movements
of unenclosed animals in favour of sedentary or colonizing individuals.
The dispersal tendency of emigrants from an enclosed population was
investigated by releasing animals via tubes into one of two adjacent
enclosures, one vacant and one inhabited. Individuals from the enclosed
population were monitored for age, sex, weight and electrophoretically
detectable serum transferrin genotype in an intensive live-trapping
program.
In 1973 the minimum number alive in the introduced enclosed
study population reached approximately l67/ha when breeding stopped
in October. In 1974 intensive breeding increased the population
density to 333/ha by mid-July when a long decline in numbers and
breeding intensity began without an intervening plateau. An
adjacent unenclosed area had a much lower density and longer breeding
season in 1974. The growth rate of young males in the enclosed
population tended to be lowest during the decline period in 1974.
Survival of the enclosed population was high throughout but was
lowest during the decline phase in both sexes, especially males.
Low transferrin heterozygote survival during the decline coincided
with a significant heterozygote deficiency in females whereas in males
genotype frequencies did not depart from Hardy-Weinberg equilibrium
values throughout th.e study. Twenty-nine suitable ani.mals were
released during the decline in five periods from July to November 1974.
The proportions of males and transferrin heterozygotes in the released
graun were generally greater than in the source population~ In the
test enclosures 21% of the released animals continued their movement
through the vacant area while 41% (no significant difference) moved
through the inhabited enclosure. In the vacant test area, females
had a greater tendency than males to continue dispersal whereas no
difference was noted in the inhabited area. Low frequency of captures
in the tubes, predator disturbances and cold weather forced the
termination of the study.
The role of dispersal as a population regulating mechanism was
further substantiated. The genetic differences between emigrant and
resident animals lend support to Howard's hypothesis that a genetic
polymorphism influences the tendency to disperse. Support is also
given to Myers' and Krebs' contention that among dispersers an
additional density dependent polymorphism influences the distance
dispersed.
