POPULATION CYCLES IN SMALL MAMMALS
367
The arena may be a neutral arena or a home cage of the animals.
Often only males have been tested, in the possibly mistaken belief that
important aspects of spacing behavior were restricted to males (the
bird analogy), and because reproductive cycles of females complicate
the analysis of their behavior.
The first attempt to see if aggressiveness varied with density was
made by Tamura (1966), working with Microtus californicus. She ran
bouts between 167 adult males brought in from a fluctuating population and tested in home cages in the laboratory. Thirty-three behavioral
variables were recorded from these male-male interactions, and over the
two years of study Tamura could detect no significant patterns of
change in the aggressive components of behavior.
A second attempt to see if male aggressiveness varied with density
was made by Krebs (1970), working with M . ochrogmter and M .
pennsylvanicus. Two classes of behavioral measurements were made.
Exploratory activity was measured in an open field, and aggressiveness
was measured in paired encounters in a neutral arena. These measurements were made on field animals brought into the laboratory for two
days and then returned to the field. Exploratory activity showed some
relationship to population changes, particularly in M . ochrogmter,
but the exploratory behavior scores of individuals were not useful
in predicting either the duration of life or the home range size of the
individual males. We would expect that, if we could measure behavior
accurately and if the measured type of behavior is related to the
demographic machinery of density changes, we could predict individual
attributes such as length of life from the behavioral data. Aggressiveness scores changed significantly in both species such that voles in peak
populations were most aggressive. Aggressive behavior profiles were
obtained for voles from increasing, peak, and declining populations.
Figure 40 illustrates profiles for M . pennsylvanicus. These profiles were
obtained from data collected from 1965 to 1967, and we used them to
predict population changes from 1967 to 1970 (Krebs, 1971). The
attempt to predict population parameters from aggressive behavior
data was only partly successful, and the relations between demography
and aggressive behavior were weak. Krebs (1971) suggested that there
might be three reasons for this: (1) aggressive behavior of female voles
might be more important than male aggressiveness; (2) the set of
behaviors measured might be a poor index of the important spacing
behaviors in social groups in nature; or (3) aggressive behavior may not
be an important factor in causing population changes in voles.
There has been relatively little work on the techniques of measuring
aggressive behavior in wild rodents. We are inclined to think that our
present techniques do measure something which is important in
367
The arena may be a neutral arena or a home cage of the animals.
Often only males have been tested, in the possibly mistaken belief that
important aspects of spacing behavior were restricted to males (the
bird analogy), and because reproductive cycles of females complicate
the analysis of their behavior.
The first attempt to see if aggressiveness varied with density was
made by Tamura (1966), working with Microtus californicus. She ran
bouts between 167 adult males brought in from a fluctuating population and tested in home cages in the laboratory. Thirty-three behavioral
variables were recorded from these male-male interactions, and over the
two years of study Tamura could detect no significant patterns of
change in the aggressive components of behavior.
A second attempt to see if male aggressiveness varied with density
was made by Krebs (1970), working with M . ochrogmter and M .
pennsylvanicus. Two classes of behavioral measurements were made.
Exploratory activity was measured in an open field, and aggressiveness
was measured in paired encounters in a neutral arena. These measurements were made on field animals brought into the laboratory for two
days and then returned to the field. Exploratory activity showed some
relationship to population changes, particularly in M . ochrogmter,
but the exploratory behavior scores of individuals were not useful
in predicting either the duration of life or the home range size of the
individual males. We would expect that, if we could measure behavior
accurately and if the measured type of behavior is related to the
demographic machinery of density changes, we could predict individual
attributes such as length of life from the behavioral data. Aggressiveness scores changed significantly in both species such that voles in peak
populations were most aggressive. Aggressive behavior profiles were
obtained for voles from increasing, peak, and declining populations.
Figure 40 illustrates profiles for M . pennsylvanicus. These profiles were
obtained from data collected from 1965 to 1967, and we used them to
predict population changes from 1967 to 1970 (Krebs, 1971). The
attempt to predict population parameters from aggressive behavior
data was only partly successful, and the relations between demography
and aggressive behavior were weak. Krebs (1971) suggested that there
might be three reasons for this: (1) aggressive behavior of female voles
might be more important than male aggressiveness; (2) the set of
behaviors measured might be a poor index of the important spacing
behaviors in social groups in nature; or (3) aggressive behavior may not
be an important factor in causing population changes in voles.
There has been relatively little work on the techniques of measuring
aggressive behavior in wild rodents. We are inclined to think that our
present techniques do measure something which is important in