DEER MOUSE REPRODUCTION AND ITS RELATIONSHIP TO THE TREE SEED CROP

Deer mice (P. maniculatus rubidus) averaged 57% males in a sample of 3833 collected in the Douglas fir (Pseudotsuga menziesii) belt in western Oregon [USA] from 1952-1966. The percentage of mice captured in autumn and recaptured in spring was significantly greater in good (2.66 kg/ha or more) than in poor (0.45 kg/ha or less) Douglas fir and western hemlock (Tsuga heterophylla) seed years. The autumn-spring population estimates for good and poor seed years were not significantly different, but by the following autumn the good seed years had significantly greater populations. There was a significantly larger number of young female (and, by inference, male) deer mice in the population during the good seed years. Individuals of both sexes were fecund each month of the year; yearly fecundity periods varied and the longest were 11 mo. Males were most active sexually from Feb. through Nov. and females from March through Oct. Males reached peak fecundity in May, 1-2 mo. earlier than the females. The average male and female fecundity percentage from Sept.-March, when tree seed was most abundant, was significantly greater for good seed years. Average yearlong fecundity for both sexes was not significantly larger in the good seed years, but for the females there was a significant difference. The percentage of pregnancies from Sept.-March was significantly greater (8 times) in good years, but during April-Aug. was only 1/2 of those for the poor, a significant difference. On a seed year basis the average percentage of pregnancies was 2.9% greater for good years but was not significantly different. Average litter size was not significantly affected by the size of the seed fall, being 4.6 for corpora lutea and 4.4 for both the embryos and placental scars. Smallest average embryo litters were in winter and spring and larger ones were tallied in summer and autumn. Calculated average number of litters per year was 2.9 for good seed years and 2.5 for poor, a significant difference. Positive factors contributed to higher populations during and/or following a good seed crop, at least by the following autumn.