Fruit-feeding butterfly populations respond to variation in adult food availability: evidence from longitudinal body mass and abundance data

The degree to which variation in adult food availability affects the population dynamics of a species depends on its position on the capital-income breeding continuum. The long-lived butterflies that feed on fruits as adults constitute an example of Lepidoptera with a high degree of income breeding. For three species of fruit-feeding butterflies in Uganda, we assessed the contribution of the income to breeding in the wild, and the consequences of variation in fruit availability for body mass and population dynamics. We interpreted body mass loss within individuals as well as younger individuals having higher body mass than older ones as evidence for the depletion of capital reserves. Despite large sample sizes, we were able to show only modest body mass loss in one species, indicating that large-bodied fruit-feeding butterflies are functionally income breeders in the wild. Butterfly body mass was sensitive to environmental factors, although the responses to fruit availability and weather parameters were dominated by interactive effects. In all three species, periods of higher availability of fruit were followed by periods of higher adult abundance three to five months later, fitting the egg-to-adult time. Our results suggest that adult food is rapidly used for reproduction so that body mass remains stable and population size responds to adult food availability. For these income breeding species, the frequent periods of low adult food availability may select for extended adult longevity for the purpose of postponing reproduction to the onset of more favorable conditions.