. Bulletin. Science; Natural history; Natural history. MONARCH BUTTERFLY 100 LIPID % LEAN 60 DRY WEIGHT 20. 40 "Tl JAN. I 80 120 TIME (DAYS) Fig. 2. Least squares linear regression performed separately on male and female data. Each point represents 25 pooled individuals (all male or all female). Regression analyses were performed on pre- mating samples (solid points). Male and female regressions are signficantly different. Only the female post-mating sample (o, male A) was significantly different than predicted by the 95% prediction limits based on the regression analyses (from Wells et a
. Bulletin. Science; Natural history; Natural history. MONARCH BUTTERFLY 100 LIPID % LEAN 60 DRY WEIGHT 20. 40 "Tl JAN. I 80 120 TIME (DAYS) Fig. 2. Least squares linear regression performed separately on male and female data. Each point represents 25 pooled individuals (all male or all female). Regression analyses were performed on pre- mating samples (solid points). Male and female regressions are signficantly different. Only the female post-mating sample (o, male A) was significantly different than predicted by the 95% prediction limits based on the regression analyses (from Wells et al. 1992). shaped the life history of D. plexippus, however, has only gradually become ap- parent over the past ten years. Winter presents the monarch butterfly with acute energetics problems not nor- mally faced by most species. Specifically, energy resources for both monarch adults (nectar) and larvae (milkweed) are scarce or absent during winter throughout the West Coast geographic range of Z). plexippus (Tuskes and Brower 1978; Chaplin and Wells 1982). The scant nectar resources that may be present (Brower 1977) are insignificant in terms of monarch energetics (Ackery and Vane-Wright 1984). Similar conditions exist seasonally for Mexican and Australian aggregating mon- archs (James 1984; Masters et al. 1988). Monarch butterflies must overwinter on stored lipid reserves (Chaplin and Wells 1982; James 1984; Masters et al. 1988). Particularly important in this light is the fact that metabolic rates of ectotherms are dependent upon temperature of the environment; ectotherm metabolism ex- ponentally increases as temperature is raised (Gordon 1968). This correlation dictates that the overwinter range of the monarch butterfly must be consistently cool. Otherwise, death from starvation would occur prior to the availability of spring oviposition sites (Chaplin and Wells 1982). Even though North American D. plexippus is very widely distributed throughout summer, the physiological limi
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