ot indicative of actual oestrus, as it persisted through
the preceding and following stages of an oestrus cycle. In anoestrus the
orifice is sealed, the genitalia are reduced in size and the skin in the
genital region is white. Immature females, and adults during most of the
winter, are in this quiescent condition. Onset of the breeding season in
late winter is relatively abrupt, and seemingly is a photoperiodic
response. Breeding may begin in late January, and most females are in
breeding condition within the first half of February. In oestrus the
genitalia are enlarged and discolored and the vaginal orifice is
prominent and gaping. By February most females born the previous season
have matured, and breeding involves the entire population, except
possibly for retarded young and individuals suffering from disease,
injury or malnutrition. Rainey (1956) recorded an average of 2.3 young
per litter.

Number of litters normally produced in the course of a season by an
adult female is unknown, but most mature females examined within the
period February to September inclusive were in some stage of the
breeding cycle. It is obvious that the females which are successful in
rearing their litters produce at least two litters annually, and
probably some produce three litters. When entire litters are lost at an
early age, to predation, or other causes, productivity is much
increased, with perhaps only short intervals between pregnancies.

The smallest female having a vaginal orifice weighed 160 grams, but in
most instances somewhat larger size is attained before the onset of
oestrus. Judging from the average growth rate of immature females (Fig.
3), most probably attain sexual maturity at an age of five to six months
unless this age is reached in the winter period of sexual quiescence.
Rainey (_op. cit._) found no clear cut instances of young maturing in
time to breed before their first winter. He concluded, tentatively,
that in most instances sexual maturity is not attained until the spring
of the year following that in which the rat is born. However, the
evidence was inconclusive because few of the young marked survived to
maturity. In late summer and early autumn, the latter third of the
breeding season, newly matured young of the year, born in early spring,
may be the most productive group. Young conceived at the beginning of
the breeding season, and born in early March, would normally reach adult
size and breeding maturity in