362
CHARLES J. KREBS AND JUDITH H. MYERS
support the social stress theory for population control but that they do
suggest that disease or metabolic disorder may be associated with the
population decline. The question remains as to whether the susceptibility to disease of individuals in high-density populations is greater.
Further investigation of the comparative survival of individuals with
high and low leucocyte counts and reticulocyte counts should be done.
After the decline, the levels of these two blood parameters returned to
normal (Mullen, 1965). Did individuals with high leucocyte and
reticulocyte levels have higher mortality or did the physiology of
surviving individuals change? Studies of this sort could be done by
taking blood samples from individuals at intervals in the field.
Hematological tests of adrenal activity have been unsuccessful and
the search for hypoglycemia associated with declining microtine
populations seems to have been futile. However, the use of blood cell
counts as physiological indicators deserves further consideration.
The extent of ectoparasitism in populations of Microtus californicus
was found to vary with population density (Batzli and Pitelka, 1971).
More animals were infested by fleas, lice and mites in the autumn
following the peak population density than in the previous autumn of
population increase. Ectoparasitism may be a useful characteristic for
assay of the general condition of voles. Because there is most likely a
relationship between the condition of the animals and the ectoparasite
load this approach to the investigation of stress and population density
deserves further consideration.
While dead animals are rarely found following a decline in microtine
populations, Fitzgerald (1972) reports finding dead M . montanus in
nests the spring after a winter decline. Dead and dying M . califomicus
were observed on Grizzly Island, California during the decline in
population during the late summer of 1971 (Myers, personal observation), and Rauch (1950) observed dying lemmings during the 1949
decline in Alaska. Voles from declining populations which are brought
into the laboratory seem usually to survive well (Newson and Chitty,
1962; Krebs, 1966; Andrews and Strohbehn, 1971). Because we know
almost nothing of the characteristics of dying microtines or if, in fact,
the animals are dying in situ during the decline, we cannot judge the
relation of the stress theory to mortality in natural populations of
cycling rodents.
There is one set of observations which remains difficult to explain
with the “stress hypothesis”: that enclosed populations are able to
reach much higher densities than are observed in natural populations.
Microtines can both live and reproduce a t densities much higher than
occur naturally. Proponents of the stress hypothesis argue that social
stress is not a simple function of population density, and consequently
CHARLES J. KREBS AND JUDITH H. MYERS
support the social stress theory for population control but that they do
suggest that disease or metabolic disorder may be associated with the
population decline. The question remains as to whether the susceptibility to disease of individuals in high-density populations is greater.
Further investigation of the comparative survival of individuals with
high and low leucocyte counts and reticulocyte counts should be done.
After the decline, the levels of these two blood parameters returned to
normal (Mullen, 1965). Did individuals with high leucocyte and
reticulocyte levels have higher mortality or did the physiology of
surviving individuals change? Studies of this sort could be done by
taking blood samples from individuals at intervals in the field.
Hematological tests of adrenal activity have been unsuccessful and
the search for hypoglycemia associated with declining microtine
populations seems to have been futile. However, the use of blood cell
counts as physiological indicators deserves further consideration.
The extent of ectoparasitism in populations of Microtus californicus
was found to vary with population density (Batzli and Pitelka, 1971).
More animals were infested by fleas, lice and mites in the autumn
following the peak population density than in the previous autumn of
population increase. Ectoparasitism may be a useful characteristic for
assay of the general condition of voles. Because there is most likely a
relationship between the condition of the animals and the ectoparasite
load this approach to the investigation of stress and population density
deserves further consideration.
While dead animals are rarely found following a decline in microtine
populations, Fitzgerald (1972) reports finding dead M . montanus in
nests the spring after a winter decline. Dead and dying M . califomicus
were observed on Grizzly Island, California during the decline in
population during the late summer of 1971 (Myers, personal observation), and Rauch (1950) observed dying lemmings during the 1949
decline in Alaska. Voles from declining populations which are brought
into the laboratory seem usually to survive well (Newson and Chitty,
1962; Krebs, 1966; Andrews and Strohbehn, 1971). Because we know
almost nothing of the characteristics of dying microtines or if, in fact,
the animals are dying in situ during the decline, we cannot judge the
relation of the stress theory to mortality in natural populations of
cycling rodents.
There is one set of observations which remains difficult to explain
with the “stress hypothesis”: that enclosed populations are able to
reach much higher densities than are observed in natural populations.
Microtines can both live and reproduce a t densities much higher than
occur naturally. Proponents of the stress hypothesis argue that social
stress is not a simple function of population density, and consequently