Energy reserves and metabolic expenditures of monarch butterflies overwintering in southern California

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Abstract
1. Energetic expenditure and predicted requirements for overwintering metabolism were determined for monarch butterflies (Danaus plexippus L.) in southern California.2. Fat content of butterflies declined steadily from a maximum of 71% lean dry weight in late November to a minimum of 36% lean dry weight in late January. The energy expenditure measured by fat depletion over a 61 day period from 24 November to 25 January was 26.05 joules per day.3. Butterflies were generally the same temperature as the air at any time they were inactive, whether they were part of a large cluster or roosting solitarily.4. Oxygen consumption of butterflies increased in a curvilinear fashion with increasing air temperature. Thus, the predicted metabolic requirements for an inactive butterfly during their quiescent period from late November to late January was 30.32 joules per day.5. In contrast to the steady depletion of fat reserves during their quiescent period in December and January, low and stable fat levels of butterflies in late February coincide with high levels of flight activity, mating and emigration of females from the colony.6. Progressive tightening of the energy balance due to increasing nocturnal temperatures from northern to southern California coastal areas may limit the southern distribution and duration of overwintering aggregations.