358
CHARLES J. RREBS AND JUDITH H. MYERS
season during periods of peak lemming densities, or to high mortality
in the decline.
It is possible that the adrenal steroid levels could be experimentally
increased in a wild population by the injection of hormones into
microtines. However, we understand so little of how these hormones
work that analysis of the results of such an experiment could be too
complicated to reveal any insight. The alternative might be to use
sedatives to decrease social stress, but again manipulations of body
chemistry can have undesirable side-effects.
In addition to all this work on the adrenal gland, various other
physiological indices of condition have been studied by some workers.
Hematological characteristics might be used as indicators of the
physiological condition of animals. Newson and Chitty (1962) searched
for an association between reticulocytosis and anemia and population
decline in Microtus agrestis. While they found seasonal variation in
hemoglobin levels of voles, the animals were not anemic during the
population decline and they failed to find a physiological abnormality
which might explain the decline.
An extensive investigation of the blood of brown lemmings was undertaken by Mullen (1965) in a search for physiological changes which might
be associated with density. Blood glucose levels were measured to test
for hypoglycemia. Hypoglycemia (low blood sugar) has had a part in
€he history of small mammal cycles since the report by Green and
Larson (1938) that snowshoe hares in Minnesota suffered a decline
because of “shock disease” characterized by low blood sugar, degeneration
of the liver, and failure to store glycogen. Chitty (1959) refuted this
work and recorded his own failure to fhd “shock disease” in Microtus
agrestis. Also Chitty (1959) found that low glycogen reserves occurring
in experimental laboratory populations of M . agrestis did not increase
mortality. Houlihan (1963) found that blood sugar of an enclosed
population of M . californicus decreased when the food supply was low
but during a period of sharp decline (37% lost in two weeks) the blood
sugar level of individuals in this population was the same as in a
control population of moderate density which did not decline.
Similarly, while Mullen (1965) found variation in the blood sugar
level of brown lemmings, he was not able to associate this variation
with population density or density changes.
It has been suggested that there is a negative relationship between
the number of circulating eosinophils and adrenocortical activity in
rodents (Speirs and Meyer, 1949). If this is so, determination of
eosinophil levels can be used as a means of assaying adrenal activity.
Houlihan (1963) studied eosinophil levels in two enclosed (120 ft2
outdoor pens) populations of Microtus californicus. The density of one
1
CHARLES J. RREBS AND JUDITH H. MYERS
season during periods of peak lemming densities, or to high mortality
in the decline.
It is possible that the adrenal steroid levels could be experimentally
increased in a wild population by the injection of hormones into
microtines. However, we understand so little of how these hormones
work that analysis of the results of such an experiment could be too
complicated to reveal any insight. The alternative might be to use
sedatives to decrease social stress, but again manipulations of body
chemistry can have undesirable side-effects.
In addition to all this work on the adrenal gland, various other
physiological indices of condition have been studied by some workers.
Hematological characteristics might be used as indicators of the
physiological condition of animals. Newson and Chitty (1962) searched
for an association between reticulocytosis and anemia and population
decline in Microtus agrestis. While they found seasonal variation in
hemoglobin levels of voles, the animals were not anemic during the
population decline and they failed to find a physiological abnormality
which might explain the decline.
An extensive investigation of the blood of brown lemmings was undertaken by Mullen (1965) in a search for physiological changes which might
be associated with density. Blood glucose levels were measured to test
for hypoglycemia. Hypoglycemia (low blood sugar) has had a part in
€he history of small mammal cycles since the report by Green and
Larson (1938) that snowshoe hares in Minnesota suffered a decline
because of “shock disease” characterized by low blood sugar, degeneration
of the liver, and failure to store glycogen. Chitty (1959) refuted this
work and recorded his own failure to fhd “shock disease” in Microtus
agrestis. Also Chitty (1959) found that low glycogen reserves occurring
in experimental laboratory populations of M . agrestis did not increase
mortality. Houlihan (1963) found that blood sugar of an enclosed
population of M . californicus decreased when the food supply was low
but during a period of sharp decline (37% lost in two weeks) the blood
sugar level of individuals in this population was the same as in a
control population of moderate density which did not decline.
Similarly, while Mullen (1965) found variation in the blood sugar
level of brown lemmings, he was not able to associate this variation
with population density or density changes.
It has been suggested that there is a negative relationship between
the number of circulating eosinophils and adrenocortical activity in
rodents (Speirs and Meyer, 1949). If this is so, determination of
eosinophil levels can be used as a means of assaying adrenal activity.
Houlihan (1963) studied eosinophil levels in two enclosed (120 ft2
outdoor pens) populations of Microtus californicus. The density of one
1