POPULATION CYCLES IN SMALL MAMMALS
373
determine the mechanisms behind the behavioral shifts. On the one
hand, behavioral changes can be caused by physiological shifts in
brain chemistry or hormonal balances caused by isolation or grouping
(reviewed by Brain, 1971a, b). These changes are usually considered as
phenotypic and are often tied in with the stress hypothesis. On the
other hand, behavioral changes could have a genetic basis and be tied
in with the genetic hypothesis of Chitty (1967). Obviously behavioral
changes will still have physiological and endocrinological mechanisms
even if they are genetically influenced. No one knows whether the
behavioral changes described above are phenotypic or genotypic, and
we suggest that these questions should be tackled in the logical order:
(1) does spacing behavior change over the population cycle? (2) can the
behavioral changes be shown to be heritable by the standard techniques
of quantitative genetics? and (3) what are the physiological pathways
by which the relevant spacing behaviors are iduenced? At present
most effort is being expended on question (3), even though we have
little data on question (1) and no data at all on question (2).
In summary, the behavior hypothesis seems to hold the best possibility for explainjng the changes in reproduction, mortality, dispersal,
and growth which drive the population cycle. Spacing behavior, or
hostility, seems to produce a ccsurplus” population of animals which
move into vacant areas. If spacing behavior causes the population
fluctuations, the aggressive behavior of individuals must change with
density. This hypothesis has been verified both by examining skin
wounds and by paired-encounters of males in arenas. Male voles and
lemmings are most aggressive in peak populations. Little work has
been done on female aggressive behavior, and some workers suggest
that females may be even more aggressive than males. It is not clear
how behavioral interactions can account for the mortality changes
found in declining populations, since fighting itself rarely leads to
deaths. No one knows whether the behavioral changes that have been
found in microtine rodents are phenotypic or genotypic, and the
behavioral hypothesis could be subsumed under either the stress hypothesis or the genetic hypothesis once this is known.
F. G E N E T I C S
When Chitty (1958, 1960) first proposed his hypothesis that the
quality of microtines changed with density as a result of selection on
genetically determined behavioral types, two immediate objections
arose. The first of these was that the hypothesis was overly complex.
As stated by Pitelka (1958), “it may be a strain on Occam’s razor to
N
373
determine the mechanisms behind the behavioral shifts. On the one
hand, behavioral changes can be caused by physiological shifts in
brain chemistry or hormonal balances caused by isolation or grouping
(reviewed by Brain, 1971a, b). These changes are usually considered as
phenotypic and are often tied in with the stress hypothesis. On the
other hand, behavioral changes could have a genetic basis and be tied
in with the genetic hypothesis of Chitty (1967). Obviously behavioral
changes will still have physiological and endocrinological mechanisms
even if they are genetically influenced. No one knows whether the
behavioral changes described above are phenotypic or genotypic, and
we suggest that these questions should be tackled in the logical order:
(1) does spacing behavior change over the population cycle? (2) can the
behavioral changes be shown to be heritable by the standard techniques
of quantitative genetics? and (3) what are the physiological pathways
by which the relevant spacing behaviors are iduenced? At present
most effort is being expended on question (3), even though we have
little data on question (1) and no data at all on question (2).
In summary, the behavior hypothesis seems to hold the best possibility for explainjng the changes in reproduction, mortality, dispersal,
and growth which drive the population cycle. Spacing behavior, or
hostility, seems to produce a ccsurplus” population of animals which
move into vacant areas. If spacing behavior causes the population
fluctuations, the aggressive behavior of individuals must change with
density. This hypothesis has been verified both by examining skin
wounds and by paired-encounters of males in arenas. Male voles and
lemmings are most aggressive in peak populations. Little work has
been done on female aggressive behavior, and some workers suggest
that females may be even more aggressive than males. It is not clear
how behavioral interactions can account for the mortality changes
found in declining populations, since fighting itself rarely leads to
deaths. No one knows whether the behavioral changes that have been
found in microtine rodents are phenotypic or genotypic, and the
behavioral hypothesis could be subsumed under either the stress hypothesis or the genetic hypothesis once this is known.
F. G E N E T I C S
When Chitty (1958, 1960) first proposed his hypothesis that the
quality of microtines changed with density as a result of selection on
genetically determined behavioral types, two immediate objections
arose. The first of these was that the hypothesis was overly complex.
As stated by Pitelka (1958), “it may be a strain on Occam’s razor to
N