POPULATION CYCLES IN SMALL MAMMALS
363
the “stress” level of a natural population at 100 per acre could be equal
to the stress level of an artificial population at a caged density of
10 000 per acre. Whether or not one accepts this argument, the point
we wish to make is that more attention should be paid to the behavioral
interactions which must cause social stress. Excessive preoccupation
with physiological measurements may have sidetracked us from more
relevant behavioral aspects of social stress.
In summary, the stress hypothesis suggests that microtine populations
peak and decline because of physiological deterioration of adrenal
functions. Although this theory is popular even in the communications
media, we can find no evidence from natural populations to support it.
Few studies have discovered any apparent relationships between
adrenal functions and population changes, and none of the characteristic features of reproduction, mortality, dispersal, or growth which we
discussed previously have been associated with physiological measurements of adrenal changes.
E. B E H A V I O R
The behavior hypothesis suggests that interactions between individual animals are critical in causing population fluctuations in
small rodents. The behavior hypothesis is an intrinsic hypothesis
since it states that a necessary factor preventing unlimited increase is
a change in the behavior of individuals in the population. We will
consider this theory separately from the stress hypothesis and the
genetic hypothesis because no mechanisms of behavioral change or
inheritance are specified. We inquire, aa a first approximation, only
whether behavioral interactions change during a population cycle.
The primary behavior which might be involved in population events
is spacing behavior. There has never been any doubt that voles and
lemmings show hostility toward one another, and space themselves over
the habitat; people differ greatly in how to interpret such a fact.
Watson and Moss (1970) have carefully reviewed how population
limitation can be achieved by means of aggression in vertebrates, and
they cite three conditions which are necessary to show that aggressive
behavior limits the density of breeding animals:
1. there must be a substantial “surplus” population which does not
2. these surplus animals must be capable of breeding if the more
3. the breeding animals must not be completely depleting some
breed;
dominant animals are removed;
resource, such as food or nesting sites.
363
the “stress” level of a natural population at 100 per acre could be equal
to the stress level of an artificial population at a caged density of
10 000 per acre. Whether or not one accepts this argument, the point
we wish to make is that more attention should be paid to the behavioral
interactions which must cause social stress. Excessive preoccupation
with physiological measurements may have sidetracked us from more
relevant behavioral aspects of social stress.
In summary, the stress hypothesis suggests that microtine populations
peak and decline because of physiological deterioration of adrenal
functions. Although this theory is popular even in the communications
media, we can find no evidence from natural populations to support it.
Few studies have discovered any apparent relationships between
adrenal functions and population changes, and none of the characteristic features of reproduction, mortality, dispersal, or growth which we
discussed previously have been associated with physiological measurements of adrenal changes.
E. B E H A V I O R
The behavior hypothesis suggests that interactions between individual animals are critical in causing population fluctuations in
small rodents. The behavior hypothesis is an intrinsic hypothesis
since it states that a necessary factor preventing unlimited increase is
a change in the behavior of individuals in the population. We will
consider this theory separately from the stress hypothesis and the
genetic hypothesis because no mechanisms of behavioral change or
inheritance are specified. We inquire, aa a first approximation, only
whether behavioral interactions change during a population cycle.
The primary behavior which might be involved in population events
is spacing behavior. There has never been any doubt that voles and
lemmings show hostility toward one another, and space themselves over
the habitat; people differ greatly in how to interpret such a fact.
Watson and Moss (1970) have carefully reviewed how population
limitation can be achieved by means of aggression in vertebrates, and
they cite three conditions which are necessary to show that aggressive
behavior limits the density of breeding animals:
1. there must be a substantial “surplus” population which does not
2. these surplus animals must be capable of breeding if the more
3. the breeding animals must not be completely depleting some
breed;
dominant animals are removed;
resource, such as food or nesting sites.