A preliminary assessment of lipid content and body mass of the monarch butterfly (Danaus plexippus) (Lepidoptera: Danaidae) during the fall migration in Texas

Background/Question/Methods

Monarch butterflies are widely distributed throughout North America and have well-documented annual migrations.  During the fall, a single generation flies up to 4,000 km from their breeding areas in temperate regions of North America through Texas to mountainous areas of central Mexico.  This species undergoes a reproductive diapause during an overwinter in Mexico and subsequently returns north mating and depositing eggs en route.  Because lipids are the most efficient energy source for long-distance flight and provide energy stores during the 5-month inactive overwintering period, they play a vital role in the success of the annual monarch migration.  There are conflicting hypotheses that suggest either lipids primarily fuel the long-distance flight during migration or primarily function as the energy source during the overwintering period.  The former hypothesis predicts that fall butterflies rapidly accumulate lipid content in their northern breeding areas which they deplete during the migration to Mexico, and the latter predicts lipid accumulations that occur in Texas and northern Mexico just before they reach their overwintering sites.  We monitored the migration and randomly sampled monarchs during the fall migration through central Texas in 2010.  Samples were collected to assess overall body size (determined by wing surface area, cm², N=87), body mass (mg, N=87), and total thoracic lipid content (mg lipid/mg thorax, N=17).  Also, daily population counts were made during October and November to assess migration variability. 

Results/Conclusions

The migration peaked at the end of October when more than 15% of the total numbers of migrants were present.  Males and females did not differ in mean body size (male=12.8 cm², female=12.9 cm²;  P > 0.75), mass (male=55.9 mg, female=59.5 mg; P > 0.31), or thoracic lipid content (male=7.49 mg lipid mg^-1, female=8.80 mg lipid mg^-1; P > 0.54), therefore they were combined for analysis.  Body size did not differ during the migration (mean=12.8 cm² ± 1.2) but body mass decreased through time – an average of 30% during the sampling period.  Interestingly, while not significant, a trend was evident that indicates increased total thoracic lipid content during this time.  While our sample size may limit interpretation, this may reflect an increased foraging behavior in Texas resulting in lipid accumulation primarily for post-migration overwintering survival.