Predation and the Elevation of Thoracic Temperature in Brightly Colored Neotropical Butterflies

Abstract

We observed a positive partial correlation between (1) the mean thoracic temperatures of 55 brightly colored Neotropical butterfly species and (2) their palatabilities to rufous-tailed jacamars (a specialized insectivorous bird) following adjustment for mean ambient temperature and lineage membership. The higher thoracic temperature of palatable butterflies suggests that predation is a strong selective force on the thermal physiology of these butterflies. Activity patterns differ between palatable and unpalatable butterflies. Palatable butterflies were active (and were captured) toward late morning, during sunny weather, and in open microhabitats, whereas unpalatable butterflies were not so constrained and were captured more evenly across time and microhabitat categories. These differences result in positive correlations between a species' palatability and the mean ambient temperature, as well as between butterfly palatability and its mean thermal excess (thoracic temperature minus ambient temperature). Within the same microhabitat, palatable butterflies have a higher mean thermal excess than unpalatable species. The higher thermal excess of palatable species could result from (1) greater net influx of radiative energy, (2) greater production of metabolic heat, or (3) lower conductive and convective heat loss. The ambient radiation in which the butterflies are active tends to be maximum when and where the palatable species are active. Palatable butterflies tend to fly more quickly and erratically-which may mean that they require more metabolic energy to fly the same distance than do unpalatable ones-but may suffer greater convective heat loss. They also possess wider thoraxes, which may retain more metabolic heat. Palatability is associated with divergences within phylogenetic lineages of the flight temperatures of brightly colored palatable and unpalatable species. As a result of its association with coloration and flight mimicry, palatability is also associated with convergences in flight temperatures of distantly related lineages. Correlation does not prove causation; however, palatability is a measure for predation, which is a selective force. The association of flight temperature and palatability within monophyletic lineages and the convergence of flight temperatures and color patterns among those lineages provide additional evidence that natural selection by predation has resulted in the evolutionary association of palatability and thoracic temperatures of butterflies in a Neotropical rain forest.