POPULATION CYCLES IN SMALL MAMMALS
357
are not significantly different. However, the respiratory rate of adrenals
from animals captured in July 1965 and analyzed approximately one
month later was significantly less than those of lemmings caught in
July 1965 and analyzed in February 1966, seven months after capture.
While ACTH stimulated steriod production in adrenals of lemmings
maintained in the laboratory for seven months, it decreased steriod
production of adrenals from lemmings collected and analyzed during
the summer of 1965. This is interpreted as an indication that adrenals
of animals collected from populations of peak densities were secreting
at the maximum rate.
According to Andrews’ description, the second year of his work
(1969) was one of low lemming density. Sample sizes were quite small
but the following trends were observed during the summer: (1) The
content of ACTH in male pituitaries remained high through the summer
but decreased in female pituitaries from early July through August.
(2) The responsiveness of adrenals to exogenous ACTH measured as
corticosteroid production was similar between males and females and
seemed to increase in the August sample. (3) Andrews and Strohbehn
claim a decreased sex ratio a t the end of the summer, but there are no
statistics to verify this and the sex ratio is 60% females (N = 13).
This does not indicate strong differential mortality between males and
females as stated by these authors, and leaves in doubt their conclusion
that higher male mortality is associated with higher male adrenocortical and pituitary function.
Comparisons of data from the 1969 study of low population density
to earlier data on peak populations were not made by Andrews and
Strohbehn (197 1). However, adrenals from low-density populations
were stimulated by ACTH while those from a peak-density population
were not.
Analyses of adrenal activity obtained by Andrews using the more
elaborate methods are going to be even more complicated than were
analyses which utilized adrenal weight. Along with the variations
associated with body weight, sex, reproductive condition, and season
of the year (similar to those observed in earlier studies of adrenal
weights), there are circadian rhythms of adrenal secretion and respiration, and complex pictures which result from measuring steroid production with several techniques.
For any progress to be made in determining the role of the adrenal
in affecting population processes, it will be necessary for proper
demographic, physiological, and statistical procedures to be employed.
The behavior of microtines is most certainly mediated through the
endocrine system. So far no progress has been made toward correlating
endocrinological changes to the early termination of the breeding
357
are not significantly different. However, the respiratory rate of adrenals
from animals captured in July 1965 and analyzed approximately one
month later was significantly less than those of lemmings caught in
July 1965 and analyzed in February 1966, seven months after capture.
While ACTH stimulated steriod production in adrenals of lemmings
maintained in the laboratory for seven months, it decreased steriod
production of adrenals from lemmings collected and analyzed during
the summer of 1965. This is interpreted as an indication that adrenals
of animals collected from populations of peak densities were secreting
at the maximum rate.
According to Andrews’ description, the second year of his work
(1969) was one of low lemming density. Sample sizes were quite small
but the following trends were observed during the summer: (1) The
content of ACTH in male pituitaries remained high through the summer
but decreased in female pituitaries from early July through August.
(2) The responsiveness of adrenals to exogenous ACTH measured as
corticosteroid production was similar between males and females and
seemed to increase in the August sample. (3) Andrews and Strohbehn
claim a decreased sex ratio a t the end of the summer, but there are no
statistics to verify this and the sex ratio is 60% females (N = 13).
This does not indicate strong differential mortality between males and
females as stated by these authors, and leaves in doubt their conclusion
that higher male mortality is associated with higher male adrenocortical and pituitary function.
Comparisons of data from the 1969 study of low population density
to earlier data on peak populations were not made by Andrews and
Strohbehn (197 1). However, adrenals from low-density populations
were stimulated by ACTH while those from a peak-density population
were not.
Analyses of adrenal activity obtained by Andrews using the more
elaborate methods are going to be even more complicated than were
analyses which utilized adrenal weight. Along with the variations
associated with body weight, sex, reproductive condition, and season
of the year (similar to those observed in earlier studies of adrenal
weights), there are circadian rhythms of adrenal secretion and respiration, and complex pictures which result from measuring steroid production with several techniques.
For any progress to be made in determining the role of the adrenal
in affecting population processes, it will be necessary for proper
demographic, physiological, and statistical procedures to be employed.
The behavior of microtines is most certainly mediated through the
endocrine system. So far no progress has been made toward correlating
endocrinological changes to the early termination of the breeding