370
CHARLES J. KREBS AND JUDITH H . MYERS
Heavier voles were usually more dominant in aggressive encounters. It
is tempting to interpret these data in terms of population fluctuations.
Two characteristics of peak populations are smaller home ranges
and larger-sized animals. Both these characteristics were found by
Turner (1971) to be related to aggressiveness in male voles. Turner’s
study is a model of the type of careful analysis of aggresssive behavior
that must be done if we are to understand the role of intraspecific
strife in affecting numbers. He is presently continuing observations
to determine whether aggressiveness is related to population density.
Conley (1971) measured aggressive behavior of male and female
M . longicaudus during a peak summer and a decline summer in the
mountains of New Mexico. He found that voles from a peak population
were more aggressive toward each other than were voles from a declining
population. Conley also studied a sympatric population of M . mexicanus
which was at low density during the two years. This population showed
no difference in aggressiveness between years. Aggressive reactions of
male and female voles were the same.
Social antagonism was analyzed by Getz (1972) by the use of multiplecapture live-traps for M . pennsylvanicus. Getz showed that many
multiple captures of two adult males occurred during his five-month
study, and he could see no evidence of great antagonism between males
in his declining population. However, the number of double captures of
adult females was significantly less than expected, which might suggest
some antagonism among females.
The movements of Norwegian lemmings, which are associated with
high densities, may be triggered by the aggressiveness of both male and
female residents (Clough, 1968). Unfortunately, no detailed studies
have been made on changes in agonistic behavior through a population
fluctuation of this legendary lemming.
Frank (1957) has argued that behavioral mechanisms are critical
in determining population changes in M . arvalis. He refers to these
behavioral mechanisms collectively as the “condensation potential”.
There are three major elements in the condensation potential. First,
home ranges can be reduced to small sizes. Both males and females
are territorial, and females drive off their male offspring and allow
the young females to settle in or near their home range. Second,
females can form mother-families and even “great families” at the end
of the breeding season, so that groups of immature animals can overwinter together. These great families break up once breeding begins
in the spring. In some cases related females remain together on a
common territory and bring up their litters in a common nest. Third,
males of M . arvalis show spacing behavior at all times when they are
breeding, and consequently many more females than males occur in
CHARLES J. KREBS AND JUDITH H . MYERS
Heavier voles were usually more dominant in aggressive encounters. It
is tempting to interpret these data in terms of population fluctuations.
Two characteristics of peak populations are smaller home ranges
and larger-sized animals. Both these characteristics were found by
Turner (1971) to be related to aggressiveness in male voles. Turner’s
study is a model of the type of careful analysis of aggresssive behavior
that must be done if we are to understand the role of intraspecific
strife in affecting numbers. He is presently continuing observations
to determine whether aggressiveness is related to population density.
Conley (1971) measured aggressive behavior of male and female
M . longicaudus during a peak summer and a decline summer in the
mountains of New Mexico. He found that voles from a peak population
were more aggressive toward each other than were voles from a declining
population. Conley also studied a sympatric population of M . mexicanus
which was at low density during the two years. This population showed
no difference in aggressiveness between years. Aggressive reactions of
male and female voles were the same.
Social antagonism was analyzed by Getz (1972) by the use of multiplecapture live-traps for M . pennsylvanicus. Getz showed that many
multiple captures of two adult males occurred during his five-month
study, and he could see no evidence of great antagonism between males
in his declining population. However, the number of double captures of
adult females was significantly less than expected, which might suggest
some antagonism among females.
The movements of Norwegian lemmings, which are associated with
high densities, may be triggered by the aggressiveness of both male and
female residents (Clough, 1968). Unfortunately, no detailed studies
have been made on changes in agonistic behavior through a population
fluctuation of this legendary lemming.
Frank (1957) has argued that behavioral mechanisms are critical
in determining population changes in M . arvalis. He refers to these
behavioral mechanisms collectively as the “condensation potential”.
There are three major elements in the condensation potential. First,
home ranges can be reduced to small sizes. Both males and females
are territorial, and females drive off their male offspring and allow
the young females to settle in or near their home range. Second,
females can form mother-families and even “great families” at the end
of the breeding season, so that groups of immature animals can overwinter together. These great families break up once breeding begins
in the spring. In some cases related females remain together on a
common territory and bring up their litters in a common nest. Third,
males of M . arvalis show spacing behavior at all times when they are
breeding, and consequently many more females than males occur in