388
CHARLES J. KREBS AND JUDITH H. MYERS
15. Prenatal mortality may vary slightly over the cycle but is not a
serious loss even in declining populations.
16. Dispersal is most frequent from increasing populations, and
relatively infrequent from declining populations.
17. Two closely related species may live in the same habitat and
reach peak numbers in the same year, but declines in one
species may occur while the other species in the same area
remains a t high numbers for several months.
18. Voles brought into the laboratory from field populations which
are declining will live for a very long time as isolated pairs or
individuals.
19. Populations kept in small cages in the laboratory or room-sized
cages outdoors increase to unnaturally high densities, several
to many times that found in nature, and do not cycle.
20. Populations in two-acre enclosures in the field increase to
unusually high numbers and may reach the limit of their food
supply and starve.
21. Adult animals in peak populations are typically larger (by
20-50%) than those at other times in the cycle. This increase in
body size is reflected in changed skeletal proportions. These
large animals may occur in the late increase phase and early in
the decline phase when the decline is very gradual.
22. Changes in gene frequency at marker loci occur in association
with density fluctuations.
23. Aggressiveness of male and female microtines increases and
home range size decreases with increasing population density.
We have attempted throughout this article to point out experiments
which might be done to test various hypotheses and elucidate certain
aspects of microtine cycles. Two of the most important questions which
remain to be answered are: (1) what permits longer reproductive seasons
in some years? and (2) what is the nature of the mortality occurring
during the decline? Studies of either of these questions should not
overlook the quality of the individuals in the population. While it may be
possible to show what nutritional factors permit microtines to reproduce, the crucial aspect to relate nutrition to population cycles might
be to demonstrate that individuals in increasing populations have lower
nutritional thresholds or are more efficient in their use of nutritional
factors. Similarly, there might be a number of causes of mortality
CHARLES J. KREBS AND JUDITH H. MYERS
15. Prenatal mortality may vary slightly over the cycle but is not a
serious loss even in declining populations.
16. Dispersal is most frequent from increasing populations, and
relatively infrequent from declining populations.
17. Two closely related species may live in the same habitat and
reach peak numbers in the same year, but declines in one
species may occur while the other species in the same area
remains a t high numbers for several months.
18. Voles brought into the laboratory from field populations which
are declining will live for a very long time as isolated pairs or
individuals.
19. Populations kept in small cages in the laboratory or room-sized
cages outdoors increase to unnaturally high densities, several
to many times that found in nature, and do not cycle.
20. Populations in two-acre enclosures in the field increase to
unusually high numbers and may reach the limit of their food
supply and starve.
21. Adult animals in peak populations are typically larger (by
20-50%) than those at other times in the cycle. This increase in
body size is reflected in changed skeletal proportions. These
large animals may occur in the late increase phase and early in
the decline phase when the decline is very gradual.
22. Changes in gene frequency at marker loci occur in association
with density fluctuations.
23. Aggressiveness of male and female microtines increases and
home range size decreases with increasing population density.
We have attempted throughout this article to point out experiments
which might be done to test various hypotheses and elucidate certain
aspects of microtine cycles. Two of the most important questions which
remain to be answered are: (1) what permits longer reproductive seasons
in some years? and (2) what is the nature of the mortality occurring
during the decline? Studies of either of these questions should not
overlook the quality of the individuals in the population. While it may be
possible to show what nutritional factors permit microtines to reproduce, the crucial aspect to relate nutrition to population cycles might
be to demonstrate that individuals in increasing populations have lower
nutritional thresholds or are more efficient in their use of nutritional
factors. Similarly, there might be a number of causes of mortality