. The Canadian field-naturalist. 1986 HOYLE AND BOONSTRA: LiFE HISTORY OF MEADOW JUMPING MOUSE 541. Figure 2. Mean weekly body weights of adult Z. hudsonius. Pregnant females were excluded. period (Table 2). Since our survival rates do measure whether an animal alive in the fall reappears the next spring, the low survival rates seen in late August and September suggest high overwinter losses. An estimate of overwinter survival was determined by calculating the proportion of those animals caught in the last week of August or later in the fall that were caught at some future time. More males (39
. The Canadian field-naturalist. 1986 HOYLE AND BOONSTRA: LiFE HISTORY OF MEADOW JUMPING MOUSE 541. Figure 2. Mean weekly body weights of adult Z. hudsonius. Pregnant females were excluded. period (Table 2). Since our survival rates do measure whether an animal alive in the fall reappears the next spring, the low survival rates seen in late August and September suggest high overwinter losses. An estimate of overwinter survival was determined by calculating the proportion of those animals caught in the last week of August or later in the fall that were caught at some future time. More males (39%, N = 23) than females (18%, N = 18) survived the winter, but this difference is not significant (x^ = ). For all animals caught in more than one trapping session, we calculated a survivorship curve and an expectation of further life (Leslie et al. 1955) for each sex. Males had an average life expectancy that was eight weeks longer than females. Three successive plateaus occurred in females and two in males, corresponding to overwinter periods. Approximately 60% of the males lived one year and 20% lived two years, compared with 40% and 10%, respectively, of the females (Figure 3). Animals were inferred to have been at least one year old if they were caught for the first time in the spring before breeding had occurred and thus must have been born sometime previously. Six individuals, two males and four females, lived through at least three winters and hence were approximately three years old. Two of these females were caught only twice: one was caught on 22 September 1978 and again on 25 June 1981 on grid A; and one was caught on 14 June 1979 on grid D and again on 25 June 1981 on grid E. Discussion This study's regular trapping regime and high densities over five years allowed for a more detailed picture of the demography of Z. hudsonius than has previously been reported. Adler et al. (1984) also trapped for five years, but they had very low densities ( pen ha) and on
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