Overwintering ecology of the monarch butterfly, Danaus plexippus L., in California

Abstract. 1. Population and physiological measurements were made on monarch butterflies in three overwintering colonies along the California coast during the 1975–76 season. 2. Mark, release and recapture studies indicated that two northern colonies (Muir Beach and Santa Cruz) stabilized at maxima of about 40000 and 95 000 individuals with little movement in or out of colonies, whereas the southern one at Santa Barbara appeared dynamic with a maximum of about 45 000. 3. Initially low, sexual activity builds to a massive mating ceremony in February, followed by rapid colony dispersal. The butterflies deteriorate gradually during the overwintering period, but males do so to a greater extent because of their involvement in frenzied courting flights. 4. Changes in several parameters at the Muir Beach colony were measured between 1 November and 2 February. Right forewing length did not change with time, indicating no differential mortality with respect to size. However, both sexes lost about 25% of their total dry weight, and 49–60% of their lipid contents. Minimum fat levels were about 20 mg and some butterflies were as lean on arrival as were the most depleted overwintering survivors. 5. The importance of nectaring in restoring lipid reserves is potentially great but remains uninvestigated. The hypothesis is put forward that the distance spring migrants are capable of flying is dependent largely upon their lipid reserves following the overwintering period. This and the intracolony mass‐mating ceremony are seen as randomizing factors which prevent geographic differentiation and allow the maintenance of the generalized, nature of the migration, which in turn enables the monarch to exploit the vast milkweed flora across the North American continent. 6. Cardenolides also diminished during the wintering, but to a substantially lesser extent than the lipids. There was no evidence for a change in the proportion of cardenolide‐positive butterflies, suggesting no differential elimination by predation of the poison‐free palatable individuals. This is the first field evidence in support of the theory of automimicry. 7. The overwintering behaviour of the monarch butterfly is seen as an integrative force in optimizing foodplant exploitation, reproduction, anti‐predator, and migration strategies.