POPULATION CYCLES I N SMALL MAMMALS
385
building. To cope with predators was perhaps a more serious challenge,
particularly with ground predators such as weasels, and we might
imagine that even higher reproductive potentials would be necessary
to keep from going extinct. Those rodent species which could overcome
these limitations now found themselves up against another problem.
Excessive reproductive capacity carries with it the seeds of habitat
destruction and starvation, particularly in areas where predators are
less common. From this challenge rodent species must have evolved
some form of self-regulation. This could have been achieved by
individual selection or by group selection. There are obvious advantages
to a group of rodents from not destroying the habitat, and we can
easily see that group selection would favor the evolution of selfregulation. The more dificult question is whether individual selection
would lead in the same direction.
Spacing behavior can evolve readily on the basis of individual
selection. A vole which begins to drive away his neighbours by physical
aggression would be at a selective advantage when crowding became
serious. Brown (1964) has discussed the evolution of territoriality in
birds, and has pointed out that aggressiveness is primarily a behavioral
response to competition for resources which are in short supply. Since
food for voles and lemmings cannot be defended readily, we would
suggest that breeding space is probably the resource in short supply
as density rises.
Once spacing behavior begins to evolve, the next problem is to set
the upper limits to such behavior. If some aggressiveness is good in
high-density populations, is not more aggressiveness even better?
There are some obvious limits to excessive aggressiveness. Very
aggressive rodents might find it impossible to mate, or they might
destroy their own offspring. There are at least some vague upper limits
to aggressiveness. But why does the population not stabilize at some
intermediate level of aggressiveness? I n order to fluctuate periodically,
a microtine population must continually be overshooting the"optimum"
level of aggressiveness.
The solution to this dilemma may lie in the fluctuating seasonal
environments in which microtines live. Two extreme end-points can
be selected toward. At the low density end of the scale there is no
premium on aggressive behavior, and the selective premium falls on
genotypes capable of breeding late into the fall and winter, maturing
early, and growing rapidly. At the high density end of the scale, there
is a great premium on aggressive behavior, and reproductive behavior
is shifted toward deferring maturity until a later date when density has
fallen. SchaEer and Tamarin (1973) have analyzed how reproductive
effort should be expended in fluctuating populations in order to
385
building. To cope with predators was perhaps a more serious challenge,
particularly with ground predators such as weasels, and we might
imagine that even higher reproductive potentials would be necessary
to keep from going extinct. Those rodent species which could overcome
these limitations now found themselves up against another problem.
Excessive reproductive capacity carries with it the seeds of habitat
destruction and starvation, particularly in areas where predators are
less common. From this challenge rodent species must have evolved
some form of self-regulation. This could have been achieved by
individual selection or by group selection. There are obvious advantages
to a group of rodents from not destroying the habitat, and we can
easily see that group selection would favor the evolution of selfregulation. The more dificult question is whether individual selection
would lead in the same direction.
Spacing behavior can evolve readily on the basis of individual
selection. A vole which begins to drive away his neighbours by physical
aggression would be at a selective advantage when crowding became
serious. Brown (1964) has discussed the evolution of territoriality in
birds, and has pointed out that aggressiveness is primarily a behavioral
response to competition for resources which are in short supply. Since
food for voles and lemmings cannot be defended readily, we would
suggest that breeding space is probably the resource in short supply
as density rises.
Once spacing behavior begins to evolve, the next problem is to set
the upper limits to such behavior. If some aggressiveness is good in
high-density populations, is not more aggressiveness even better?
There are some obvious limits to excessive aggressiveness. Very
aggressive rodents might find it impossible to mate, or they might
destroy their own offspring. There are at least some vague upper limits
to aggressiveness. But why does the population not stabilize at some
intermediate level of aggressiveness? I n order to fluctuate periodically,
a microtine population must continually be overshooting the"optimum"
level of aggressiveness.
The solution to this dilemma may lie in the fluctuating seasonal
environments in which microtines live. Two extreme end-points can
be selected toward. At the low density end of the scale there is no
premium on aggressive behavior, and the selective premium falls on
genotypes capable of breeding late into the fall and winter, maturing
early, and growing rapidly. At the high density end of the scale, there
is a great premium on aggressive behavior, and reproductive behavior
is shifted toward deferring maturity until a later date when density has
fallen. SchaEer and Tamarin (1973) have analyzed how reproductive
effort should be expended in fluctuating populations in order to