POPULATION CYCLES I N SMALL MAMMALS
295
trimucronatus in the Canadian Arctic during a period of increase.
Krebs (1964a) found winter breeding in both these lemming species
during a phase of increase and no winter breeding during a decline
phase. Mullen (1965) shows the same result for Lemmus at Barrow,
Alaska. Soviet workers have recognized for many years the importance
of winter breeding in lemmings. Dunaeva and Kucheruk (1941) found
winter breeding in both Dicrostonyx torquatus and Lernrnus sibiricus
during a period of increase. Nasimovich et al. (1948) believed that
winter breeding of the Norwegian lemming was limited to the phase of
increase. Koshkina and Khalansky (1962) review winter breeding in
the Norwegian lemming and conclude that it plays a significant role in
the rapid population growth of this species.
Winter breeding has been noted in many vole species but there is
conflicting evidence of its relation to cyclic phases. After a favorable
summer and autumn Khlebnikov (1970) observed winter reproduction
in Clethrionomys rutilus. He interpreted this as being related to the
increase phase. Zejda (1962) analyzed winter breeding in the bank vole,
C. glareolus, and observed two successive winters of breeding. Neither
of these episodes of winter breeding led to population growth, and he
concluded that winter breeding was affected by the availability of food
(principally acorns) but did not lead to an outbreak. Smyth (1966)
argued that the relationship between winter breeding in voles and acorn
crops is not a simple one. A good food supply, such as a heavy acorn
crop, may be necessary for winter breeding but not sufficient. Newson
(1963), for example, found C . glareolus breeding in the winter of 19581959 (phase of increase) but not breeding in the winters of 1957-1958
or 1959-1960. There was a good acorn crop in fall 1958, but none in
1957 or 1959. But Newson noted that voles in grassland where there were
no acorns also bred during the winter of 1958-1959. Some aspect of
population density may interact with the available food supply and this
question awaits an experimental attack.
Winter breeding has been noted in Nicrotus by many workers, and
in many cases it occurs during the increase phase of the cycle and is
absent in the winter following the peak (reviewed in Keller and Krebs,
1970). This association, however, is not perfect. Chitty (personal
communication) has recorded winter breeding in Microtus agrestis in
the increase phase of the cycle, but some cycles occurred in which no
winter breeding was evident.
More evidence is available on the length of the summer breeding
period. In the phase of increase the summer breeding often starts
early and ends late (or carries on through the winter), while in the peak
year the breeding season often ends abnormally early. Koshkina and
Khalansky (1962) pointed out that the Norwegian lemming stops
295
trimucronatus in the Canadian Arctic during a period of increase.
Krebs (1964a) found winter breeding in both these lemming species
during a phase of increase and no winter breeding during a decline
phase. Mullen (1965) shows the same result for Lemmus at Barrow,
Alaska. Soviet workers have recognized for many years the importance
of winter breeding in lemmings. Dunaeva and Kucheruk (1941) found
winter breeding in both Dicrostonyx torquatus and Lernrnus sibiricus
during a period of increase. Nasimovich et al. (1948) believed that
winter breeding of the Norwegian lemming was limited to the phase of
increase. Koshkina and Khalansky (1962) review winter breeding in
the Norwegian lemming and conclude that it plays a significant role in
the rapid population growth of this species.
Winter breeding has been noted in many vole species but there is
conflicting evidence of its relation to cyclic phases. After a favorable
summer and autumn Khlebnikov (1970) observed winter reproduction
in Clethrionomys rutilus. He interpreted this as being related to the
increase phase. Zejda (1962) analyzed winter breeding in the bank vole,
C. glareolus, and observed two successive winters of breeding. Neither
of these episodes of winter breeding led to population growth, and he
concluded that winter breeding was affected by the availability of food
(principally acorns) but did not lead to an outbreak. Smyth (1966)
argued that the relationship between winter breeding in voles and acorn
crops is not a simple one. A good food supply, such as a heavy acorn
crop, may be necessary for winter breeding but not sufficient. Newson
(1963), for example, found C . glareolus breeding in the winter of 19581959 (phase of increase) but not breeding in the winters of 1957-1958
or 1959-1960. There was a good acorn crop in fall 1958, but none in
1957 or 1959. But Newson noted that voles in grassland where there were
no acorns also bred during the winter of 1958-1959. Some aspect of
population density may interact with the available food supply and this
question awaits an experimental attack.
Winter breeding has been noted in Nicrotus by many workers, and
in many cases it occurs during the increase phase of the cycle and is
absent in the winter following the peak (reviewed in Keller and Krebs,
1970). This association, however, is not perfect. Chitty (personal
communication) has recorded winter breeding in Microtus agrestis in
the increase phase of the cycle, but some cycles occurred in which no
winter breeding was evident.
More evidence is available on the length of the summer breeding
period. In the phase of increase the summer breeding often starts
early and ends late (or carries on through the winter), while in the peak
year the breeding season often ends abnormally early. Koshkina and
Khalansky (1962) pointed out that the Norwegian lemming stops