POPULATION CYCLES IN SMALL MAMMALS
359
population was maintained at approximately 40 individuals while the
other population increased, with provision of additional food, to a
peak density of 158 individuals. After maintaining high densities for
several months the uncropped population declined. Houlihan ( 1963)
compared eosinophil levels between these two populations. In the
months before and after the decline of the uncropped population,
eosinophil counts were higher than those of individuals from the
moderate density control population. This indicates increased adrenal
activity if eosinophil is a valid measure. However, in the month of the
decline of the high density population, the eosinophil counts were
significantly higher than in the other two months. Therefore, it was not
possible to interpret these results in a simple manner, and Houlihan
concluded that eosinophil levels should not be used as an indication of
adrenal stress.
Mullen (1965) comes to the same negative conclusion as Houlihan in
regard to the use of eosinophil levels as a measure of adrenal activity.
Rather than using the number of eosinophils as his index, Mullen
determined the percent of total leucocytes represented by eosinophils.
Therefore, although there is a trend for the eosinophil level to be low
in individuals taken from the 1960 peak population, this was found to
be the result of a greater number of leucocytes all together, which
lowers the percent of eosinophils, although the absolute number of
eosinophils was actually higher in 1960 than in other years. Crowding
lemmings in the laboratory did not influence eosinophil levels.
Determination of non-protein nitrogen in the blood is another physiological index in microtines. Houlihan (1963) used this measure on his
artificial high and low density Microtus populations. The trends in this
measure were the same in both populations. Houlihan did find several
indications of physiological derangement in his declining population.
These were: (1) a 30% decrease in thyroid activity, (2) an increase in
the time required for blood clotting and a change in the quality of the
blood clots, and (3) an increased susceptibility to the blood sampling
procedures resulting in more deaths, particularly among males. At the
time of the decline there was severe fighting and wounding. Although
these enclosed populations were maintained outdoors, they were
artificial in that additional food had to be added for the high density
(equivalent to 47701acre) to be reached. The degree of similarity of this
microtine decline to a natural decline still remains a question.
Other physiological assays performed by Mullen (1965) on brown
lemmings were leucocyte, erythrocyte and reticulocyte counts, and
hematocrit values. Two factors complicate the interpretation of these
data. Pirst, experiments in which these characteristics were determined
for lemmings kept under crowded conditions and kept singly in the
359
population was maintained at approximately 40 individuals while the
other population increased, with provision of additional food, to a
peak density of 158 individuals. After maintaining high densities for
several months the uncropped population declined. Houlihan ( 1963)
compared eosinophil levels between these two populations. In the
months before and after the decline of the uncropped population,
eosinophil counts were higher than those of individuals from the
moderate density control population. This indicates increased adrenal
activity if eosinophil is a valid measure. However, in the month of the
decline of the high density population, the eosinophil counts were
significantly higher than in the other two months. Therefore, it was not
possible to interpret these results in a simple manner, and Houlihan
concluded that eosinophil levels should not be used as an indication of
adrenal stress.
Mullen (1965) comes to the same negative conclusion as Houlihan in
regard to the use of eosinophil levels as a measure of adrenal activity.
Rather than using the number of eosinophils as his index, Mullen
determined the percent of total leucocytes represented by eosinophils.
Therefore, although there is a trend for the eosinophil level to be low
in individuals taken from the 1960 peak population, this was found to
be the result of a greater number of leucocytes all together, which
lowers the percent of eosinophils, although the absolute number of
eosinophils was actually higher in 1960 than in other years. Crowding
lemmings in the laboratory did not influence eosinophil levels.
Determination of non-protein nitrogen in the blood is another physiological index in microtines. Houlihan (1963) used this measure on his
artificial high and low density Microtus populations. The trends in this
measure were the same in both populations. Houlihan did find several
indications of physiological derangement in his declining population.
These were: (1) a 30% decrease in thyroid activity, (2) an increase in
the time required for blood clotting and a change in the quality of the
blood clots, and (3) an increased susceptibility to the blood sampling
procedures resulting in more deaths, particularly among males. At the
time of the decline there was severe fighting and wounding. Although
these enclosed populations were maintained outdoors, they were
artificial in that additional food had to be added for the high density
(equivalent to 47701acre) to be reached. The degree of similarity of this
microtine decline to a natural decline still remains a question.
Other physiological assays performed by Mullen (1965) on brown
lemmings were leucocyte, erythrocyte and reticulocyte counts, and
hematocrit values. Two factors complicate the interpretation of these
data. Pirst, experiments in which these characteristics were determined
for lemmings kept under crowded conditions and kept singly in the