. Il campo Canadese-naturalista. 30 10 O) 0 D. I I I I I I I I I I TTTT femmine ex= 32,63 + 5,68 N=39 maschi ^ Ex = 40,62 + 5,52 N = 37. ] 1 I I I I I I M I I I I I I I I I I I I I I I F^TTI I I I I I I I I I I I I I I I I E 1 R-T 1 I I I rI I I I I 1 1 I I I I 1 r 1-I-I-I-I-[-I I I I-I--I-1- 0 8 16 24 32 40 48 56 64 72 80 88 96 104 112 120 128 136 144 settimane figura 3. Maschio e femmina le curve di sopravvivenza di Z. hudsonius per tutti gli animali catturati più di una volta sulla area di studio e la loro aspettativa di vita (^x) + 1 SE. il quadro generale di Z. hudsonius demografia è corretta. Abbiamo trovato che i maschi pre

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. The Canadian field-naturalist. 30 10 O) 0 Q. I I I I I I I I I I TTTT FEMALES Ex= 32.63 + 5.68 N=39 MALES ^ Ex = 40.62 + 5.52 N = 37. ] 1 I I I I I I M I I I I I I' I I I I I I I I I F^TTI I I I I I I I I I I I I I I I I E 1 r-T 1 I I I r~i I I I I 1 1 I I I I 1 r 1—I—I—I—I—[—I I I I—I——I—1— 0 8 16 24 32 40 48 56 64 72 80 88 96 104 112 120 128 136 144 Weeks Figure 3. Male and female survivorship curves of Z. hudsonius for all animals caught more than once on the study area and their life expectancy (^x) + 1 SE. the broad picture of Z. hudsonius demography is correct. We found that males preceded females in emergence dates by about two weeks (Table 1). Michener (1983) suggested that a similar pattern in Richardson's Ground Squirrels {Spermophilus richardsonii richardsonii) was related to competition for mates. In these ground squirrels, females mate soon (1-4 days) after emerging from hibernation. Thus, males which had emerged after most females had reduced opportunity to mate. Cranford (1983) reported that male Z. princeps (Western Jumping Mouse) preceded females in emergence (3-9 days) only at altitudes above 2200 m, whereas at lower elevations both sexes emerged synchronously. Cranford (1983) found that both sexes were in breeding condition at emergence in late spring years. Whitaker (1963) reported that Z. hudsonius were in good condition and ready to breed immediately after hibernation. Female emergence time may be more subject to selection by environmental factors which influence their survival and that of their offspring (Michener 1983). Such selection would favour continued hibernation until environmental conditions are optimal. We also found males to precede females in entry into hibernation, with old animals entering hibernation before young. Michener (1983) reported a similar pattern for Richardson's Ground Squirrels, except that young males entered hibernation after young females. Entry dates may be dictated by critical fat accumu

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