296
CHARLES J. KREBS AND JUDITH H. MYERS
breeding early during the peak year, and Thompson (1955a) also
observed this in the brown lemming. Krebs (1964a) observed an early
stop to summer breeding in a peak year for the brown lemming and the
varying lemming. Kalela (1957) found a shortened summer breeding
period in both the peak year and in the decline year for Clethrionomys
rufocanus (Fig. 15). Zejda (1967) pointed out that a very short reproductive season was a characteristic feature of the peak year. He observed
a peak population of C. glareolus that stopped breeding in June.
Koshkina (1966) states that the summer breeding season is one month
shorter in the peak year for C. rufocanus.
I n declining populations the breeding season often starts later than
usual and may also end early. Godfrey (1955) observed a three-week
delay in onset of summer breeding in Microtus agrestis. Chitty (1952)
also reported a delay in the summer breedingseason in declining populations of M . agrestis. Declining populations of M . calijornicus may delay
breeding for one to two months (Krebs, 1966). A slight delay in the
start of summer breeding was observed for M. ochrogaster and M .
pennsylvanicus by Keller and Krebs (1970). There are few data available
for northern species with respect to possible delays in the start of
summer breeding. Kalela’s (1 957) observation on Clethriomys
rufocanus is one which has been noted: a declining population started
breeding late in 1955 even though the spring came early. I n northern
species which typically begin breeding in spring when the snow melts,
it may be difficult to detect any delay independent of spring weather
variations. Further consideration of the interaction of weather and
breeding season is discussed later.
We conclude that the phase of increase in many voles and lemmings is
associated with an extended summer breeding season and possibly
winter breeding. In the peak phase the summer breeding season is
shortened and winter breeding is absent. The decline phase often
resembles the peak phase, and may show a delay in the onset of summer
reproduction.
4. Age at sexual maturity
The age at which an organism reaches sexual maturity has a critical
impact on its potential for population growth (Cole, 1954). At present
there are no good ways of aging living small rodents and we must rely
on weight as an index of age. Several methods have been suggested
for aging dead microtines. For Clethrionomys wear of the rooted
molars can be taken as an indication of age, and Lidicker and MacLean
(1969) suggest an aging method for Microtus californicus based on
relative cranial and body measurements. However, both of these
techniques are influenced by the environmental conditions to which the
CHARLES J. KREBS AND JUDITH H. MYERS
breeding early during the peak year, and Thompson (1955a) also
observed this in the brown lemming. Krebs (1964a) observed an early
stop to summer breeding in a peak year for the brown lemming and the
varying lemming. Kalela (1957) found a shortened summer breeding
period in both the peak year and in the decline year for Clethrionomys
rufocanus (Fig. 15). Zejda (1967) pointed out that a very short reproductive season was a characteristic feature of the peak year. He observed
a peak population of C. glareolus that stopped breeding in June.
Koshkina (1966) states that the summer breeding season is one month
shorter in the peak year for C. rufocanus.
I n declining populations the breeding season often starts later than
usual and may also end early. Godfrey (1955) observed a three-week
delay in onset of summer breeding in Microtus agrestis. Chitty (1952)
also reported a delay in the summer breedingseason in declining populations of M . agrestis. Declining populations of M . calijornicus may delay
breeding for one to two months (Krebs, 1966). A slight delay in the
start of summer breeding was observed for M. ochrogaster and M .
pennsylvanicus by Keller and Krebs (1970). There are few data available
for northern species with respect to possible delays in the start of
summer breeding. Kalela’s (1 957) observation on Clethriomys
rufocanus is one which has been noted: a declining population started
breeding late in 1955 even though the spring came early. I n northern
species which typically begin breeding in spring when the snow melts,
it may be difficult to detect any delay independent of spring weather
variations. Further consideration of the interaction of weather and
breeding season is discussed later.
We conclude that the phase of increase in many voles and lemmings is
associated with an extended summer breeding season and possibly
winter breeding. In the peak phase the summer breeding season is
shortened and winter breeding is absent. The decline phase often
resembles the peak phase, and may show a delay in the onset of summer
reproduction.
4. Age at sexual maturity
The age at which an organism reaches sexual maturity has a critical
impact on its potential for population growth (Cole, 1954). At present
there are no good ways of aging living small rodents and we must rely
on weight as an index of age. Several methods have been suggested
for aging dead microtines. For Clethrionomys wear of the rooted
molars can be taken as an indication of age, and Lidicker and MacLean
(1969) suggest an aging method for Microtus californicus based on
relative cranial and body measurements. However, both of these
techniques are influenced by the environmental conditions to which the