POPULATION CYCLES I N SMALL MAMMALS
353
of patchy habitat than in the tundra. Whether this is the case we
do not know. It is not possible to sample lightly from a number of arem
to determine if populations are fluctuating together. Each population
must be studied over time. Pruitt (1968) sampled populations across
the North American Arctic and while he could not verify synchronous
cycles in small mammal population densities, he claimed to fhd a
cycle in the small mammal biomass. The sizes of most samples were
quite small. Haynes and Thompson (1965) found positive relationships
in the amount of vole activity among areas within 20 miles of each
other, but it is not possible from these data to determine whether
asynchronous populations were observed.
Out-of-phase populations are hard to locate and hence rarely
studied. Until we know what weather conditions are most important
to microtines, we shall be unable to determine how synchrony is
brought about between scattered rodent populations.
In summary, weather must be important to microtine populations if
synchrony occurs. We do not know how weather acts to synchronize
cycles. Few studies have been made of arctic microtines during the
winter, and we have found no simple associations between weather and
population events. Weather effects do not seem to explain vole and
lemming cycles, and the main driving forces must be sought elsewhere.
D. S T R E S S H Y P O T H E S I S
High animal densities increase the probability that individuals will
interact. If these interactions are disturbing to the animals, ‘‘social
stress” may be expected to rise with increasing population densities.
The stress hypothesis of Christian (1950) is probably one of the most
widely known theories of population regulation, even reaching the
every-day world of analogies with human populations.
The stress hypothesis is an outcrop of the work of Selye (1946) on the
response of the pituitary and adrenal to stress. In the extreme case the
increased activities of the pituitary and adrenals cause exhaustion,
low resistance and general susceptibility of the individual to a variety
of potential mortality factors. In addition a corresponding inhibition
of the pituitary-gonadal function can decrease reproduction (Christian
the rate of increase in an expanding population. Part C plots data taken from
Chitty and Chitty (1962) and shows the number of Microtus agrestis trapped
per hundred trap nights. Because of favorable weather in the winter of 1956-67
one population remained at peak densities while the other increased to peak
densities. The two populations declined in synchrony. The two curves represent
two different populations. Bee discussion in Chitty (1967).
353
of patchy habitat than in the tundra. Whether this is the case we
do not know. It is not possible to sample lightly from a number of arem
to determine if populations are fluctuating together. Each population
must be studied over time. Pruitt (1968) sampled populations across
the North American Arctic and while he could not verify synchronous
cycles in small mammal population densities, he claimed to fhd a
cycle in the small mammal biomass. The sizes of most samples were
quite small. Haynes and Thompson (1965) found positive relationships
in the amount of vole activity among areas within 20 miles of each
other, but it is not possible from these data to determine whether
asynchronous populations were observed.
Out-of-phase populations are hard to locate and hence rarely
studied. Until we know what weather conditions are most important
to microtines, we shall be unable to determine how synchrony is
brought about between scattered rodent populations.
In summary, weather must be important to microtine populations if
synchrony occurs. We do not know how weather acts to synchronize
cycles. Few studies have been made of arctic microtines during the
winter, and we have found no simple associations between weather and
population events. Weather effects do not seem to explain vole and
lemming cycles, and the main driving forces must be sought elsewhere.
D. S T R E S S H Y P O T H E S I S
High animal densities increase the probability that individuals will
interact. If these interactions are disturbing to the animals, ‘‘social
stress” may be expected to rise with increasing population densities.
The stress hypothesis of Christian (1950) is probably one of the most
widely known theories of population regulation, even reaching the
every-day world of analogies with human populations.
The stress hypothesis is an outcrop of the work of Selye (1946) on the
response of the pituitary and adrenal to stress. In the extreme case the
increased activities of the pituitary and adrenals cause exhaustion,
low resistance and general susceptibility of the individual to a variety
of potential mortality factors. In addition a corresponding inhibition
of the pituitary-gonadal function can decrease reproduction (Christian
the rate of increase in an expanding population. Part C plots data taken from
Chitty and Chitty (1962) and shows the number of Microtus agrestis trapped
per hundred trap nights. Because of favorable weather in the winter of 1956-67
one population remained at peak densities while the other increased to peak
densities. The two populations declined in synchrony. The two curves represent
two different populations. Bee discussion in Chitty (1967).