Submitted to: Functional Ecology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/25/2007
Publication Date: 4/1/2008
Citation: Dudley, R., Srygley, R.B. 2008. Airspeed adjustment and lipid reserves in migratory Neotropical butterflies. Functional Ecology. 22(2):264-270. Interpretive Summary: In order to maximize their migratory distance, animals should fly more slowly as they lose weight. This energy-conserving adjustment in airspeed has been demonstrated in migrating Brent geese but it has not been investigated in any migratory insects. We measured airspeeds of butterflies as they flew across the Panama Canal by pacing them with a boat and using an anemometer to measure the speed of the boat through the air at the same height as the insects. We captured the same butterflies and measured their body weight and lipid content. For females of three migratory species and males of two other species, we sampled greater than 14 individuals. In all but one species, individuals of the same sex flew more slowly when they had less lipid reserves. Therefore migratory butterflies adjust their airspeed even while flying over water, demonstrating sophisticated long-distance optimization of their fuel expenditure for the distance flown. By coupling energetic and aerodynamic analyses of migratory insects, we have demonstrated behavioral adjustments to maximize their capacity to migrate long-distances. This approach can be used to improve our ability to predict insect movement between agricultural patches.
Technical Abstract: Aerodynamic theory predicts that migrant fliers should reduce their speed of flight as endogenous energy reserves are gradually consumed. This prediction was tested for butterfly species that engage in annual rainy season migrations through central Panama. Direct airspeed measurements together with chloroform:methanol extractions of abdominal lipids from the same insects were used to determine that, among individuals within particular species/gender subsets following adjustment for body mass, flight speed was higher with greater lipid content. Although it was not possible to measure lipid content repeatedly on a single insect, these comparisons among individuals for five migratory species suggest that butterflies reduce their flight speed as lipid reserves are progressively depleted. Together with previously demonstrated strategies of wind drift compensation in these taxa, Neotropical butterflies flying within the boundary layer exhibit sophisticated long-distance orientation and optimization strategies during migration.