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C H a L E S J. m E B S AND JUDITH H. MYERS
might be more subject to loss by predation, bad weather, or stress
diseases. Note that only a small mortality change is necessary to
cause a population decline (cf. Fig. 24). A 10-15% drop in the probability of surviving per month would be sufficient to account for most
of the losses in declining populations. We are thus looking for a steady
mortality factor of small magnitude rather than a catastrophic mortality
factor of large magnitude.
We have shown previously that there seems to be little dispersal
during the decline phase, and most of the losses seem to be deaths in
situ. This result, if it is a general one, is difficult to reconcile with the
results of behavioral studies. The muskrat and red grouse models
would suggest that social intolerance should produce considerable
dispersal, and these dispersing animals (or those pushed into marginal
habitats) should suffer high mortality. But with voles we must, at
present, postulate that social intolerances in the decline phase somehow
lead to mortality but without much dispersal movement.
We cannot specify now what the behavioral attributes of dispersing
voles and lemmings should be. Most of the studies cited above suggest
that animals in peak populations are the most aggressive. We might
guess that more aggressive populations would produce more dispersal,
but this is not the case since dispersal rate is highest during the phase
of increase. Myers and Krebs (1971b) compared the aggressive behavior
profiles of male Microtus pennsylvanicus which had dispersed with
those of resident males, but found no clear and consistent differences
in the types of variables scored by Krebs (1970). Overall the dispersing
males tended to show characteristics of aggressiveness, and in one case
the “increase” behavior type (Krebs, 1970) was over-represented in
dispersing M . ochrogaster. We feel there must be some behavioral
reason why some individuals disperse and others remain as residents.
Perhaps the important traits which we would like to measure are so
transitory they disappear after dispersal has occurred. More work is
clearly required on these behavioral problems.
Behavioral interactions between individual rodents have been shown
to have strong effects on reproduction and growth in confined populations (review by Christian, 197 lb). The physiological and endocrinological mechanisms involved in the suppression of reproduction and
growth have been particularly well studied in rodents by Christian and
his co-workers. The main gap at present is the specific application of
these findings to the details of changes found in field populations.
For example, we need to know whether there is a certain behavioral
milieu which permits winter breeding in some years and another
milieu which stops the breeding season early.
If behavioral changes are important in population cycles, we must
C H a L E S J. m E B S AND JUDITH H. MYERS
might be more subject to loss by predation, bad weather, or stress
diseases. Note that only a small mortality change is necessary to
cause a population decline (cf. Fig. 24). A 10-15% drop in the probability of surviving per month would be sufficient to account for most
of the losses in declining populations. We are thus looking for a steady
mortality factor of small magnitude rather than a catastrophic mortality
factor of large magnitude.
We have shown previously that there seems to be little dispersal
during the decline phase, and most of the losses seem to be deaths in
situ. This result, if it is a general one, is difficult to reconcile with the
results of behavioral studies. The muskrat and red grouse models
would suggest that social intolerance should produce considerable
dispersal, and these dispersing animals (or those pushed into marginal
habitats) should suffer high mortality. But with voles we must, at
present, postulate that social intolerances in the decline phase somehow
lead to mortality but without much dispersal movement.
We cannot specify now what the behavioral attributes of dispersing
voles and lemmings should be. Most of the studies cited above suggest
that animals in peak populations are the most aggressive. We might
guess that more aggressive populations would produce more dispersal,
but this is not the case since dispersal rate is highest during the phase
of increase. Myers and Krebs (1971b) compared the aggressive behavior
profiles of male Microtus pennsylvanicus which had dispersed with
those of resident males, but found no clear and consistent differences
in the types of variables scored by Krebs (1970). Overall the dispersing
males tended to show characteristics of aggressiveness, and in one case
the “increase” behavior type (Krebs, 1970) was over-represented in
dispersing M . ochrogaster. We feel there must be some behavioral
reason why some individuals disperse and others remain as residents.
Perhaps the important traits which we would like to measure are so
transitory they disappear after dispersal has occurred. More work is
clearly required on these behavioral problems.
Behavioral interactions between individual rodents have been shown
to have strong effects on reproduction and growth in confined populations (review by Christian, 197 lb). The physiological and endocrinological mechanisms involved in the suppression of reproduction and
growth have been particularly well studied in rodents by Christian and
his co-workers. The main gap at present is the specific application of
these findings to the details of changes found in field populations.
For example, we need to know whether there is a certain behavioral
milieu which permits winter breeding in some years and another
milieu which stops the breeding season early.
If behavioral changes are important in population cycles, we must