354
CHARLES J. KREBS AND JUDITH H. MYERS
et al., 1965). Aggressive behavior of individuals is an intimate part of
this theory, with stress and aggressiveness increasing in a spiral.
We will consider behavioral characteristics of cycling rodents in another
section of this article, but here we want to consider evidence which has
been gathered relevant to the search for characteristics of physiological
stress associated with high microtine densities.
First let us reiterate the characteristics of cycling microtines which
must be explained if we wish to support the stress hypothesis. The
impairment of reproduction occurs at peak density in the form of a
shortened reproductive season, but other factors of natality such as
litter size and prenatal loss do not consistently vary with microtine
density. Therefore, we might expect stress to shorten the reproduction
season but not to cause increased prenatal deaths. Mortality is higher
during the population decline and is particularly severe in very young
animals. Males and females may suffer poor survival at different times.
From this we might predict that the social climate of males and females
and of young animals is different.
One of the &st steps toward testing the stress hypothesis is to
look for evidence of hyperactivity of the adrenals among rodents of
high density populations. Secondly, we would look for poor physiological
conditions among animals from declining populations. Christian et al.
(1965) cite a number of examples of rodents under abnormally high
densities in the laboratory which show characteristics consistent with
what would be predicted from the stress hypothesis. An extensive
review of work using caged laboratory animals to elucidate the effects
of isolation and grouping on brain chemistry and the functioning of
the endocrine glands is available in the review by Brain (1971a) and will
not be dealt with here.
For years adrenal function has been assayed by the weight of the
adrenal glands. Several attempts at finding relations between adrenal
weights and microtine population densities have failed (Christian, 1961;
H. Chitty, 1961; Krebs, 1964a). A primary drawback has been the
analysis of adrenal weight data. The adrenal weight changes with the
body weight but it also varies with reproductive condition, age and sex
of the individuals and season of the year. Furthermore, the relation
between adrenal weight and body weight is most likely not linear
(Krebs, 1964a), and it is not valid to compare adrenal weights by using
values which are given in mg adrenal wt/gm body wt. Chitty (1961)
and Krebs (1964a) used the technique of standardized means (Hill,
1959) to correct for body weight so that comparisons of adrenal weights
could be made independent of body weight.
While adrenal weight may not be a good measure of adrenal activity
(Christian and Davis, 1964; Andrews and Strohbehn, 1971), this was
CHARLES J. KREBS AND JUDITH H. MYERS
et al., 1965). Aggressive behavior of individuals is an intimate part of
this theory, with stress and aggressiveness increasing in a spiral.
We will consider behavioral characteristics of cycling rodents in another
section of this article, but here we want to consider evidence which has
been gathered relevant to the search for characteristics of physiological
stress associated with high microtine densities.
First let us reiterate the characteristics of cycling microtines which
must be explained if we wish to support the stress hypothesis. The
impairment of reproduction occurs at peak density in the form of a
shortened reproductive season, but other factors of natality such as
litter size and prenatal loss do not consistently vary with microtine
density. Therefore, we might expect stress to shorten the reproduction
season but not to cause increased prenatal deaths. Mortality is higher
during the population decline and is particularly severe in very young
animals. Males and females may suffer poor survival at different times.
From this we might predict that the social climate of males and females
and of young animals is different.
One of the &st steps toward testing the stress hypothesis is to
look for evidence of hyperactivity of the adrenals among rodents of
high density populations. Secondly, we would look for poor physiological
conditions among animals from declining populations. Christian et al.
(1965) cite a number of examples of rodents under abnormally high
densities in the laboratory which show characteristics consistent with
what would be predicted from the stress hypothesis. An extensive
review of work using caged laboratory animals to elucidate the effects
of isolation and grouping on brain chemistry and the functioning of
the endocrine glands is available in the review by Brain (1971a) and will
not be dealt with here.
For years adrenal function has been assayed by the weight of the
adrenal glands. Several attempts at finding relations between adrenal
weights and microtine population densities have failed (Christian, 1961;
H. Chitty, 1961; Krebs, 1964a). A primary drawback has been the
analysis of adrenal weight data. The adrenal weight changes with the
body weight but it also varies with reproductive condition, age and sex
of the individuals and season of the year. Furthermore, the relation
between adrenal weight and body weight is most likely not linear
(Krebs, 1964a), and it is not valid to compare adrenal weights by using
values which are given in mg adrenal wt/gm body wt. Chitty (1961)
and Krebs (1964a) used the technique of standardized means (Hill,
1959) to correct for body weight so that comparisons of adrenal weights
could be made independent of body weight.
While adrenal weight may not be a good measure of adrenal activity
(Christian and Davis, 1964; Andrews and Strohbehn, 1971), this was