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Larger clutches of chemically defended butterflies reduce egg mortality: evidence from Battus philenor

Many toxic butterflies lay eggs in clusters and their eggs are aposematically coloured to warn predators. The pipevine swallowtail, Battus philenor (L.) (Papilionidae), is a specialist herbivore on plants in the genus Aristolochia, from which it sequesters toxic alkaloids (aristolochic acids, AAs). Eggs of this group of butterflies are laid singly or in clusters of different sizes, are aposematic and can possess AAs. A field study was conducted during three consecutive summers in Tennessee where we manipulated the exposure of B. philenor eggs of different clutch sizes to predators to assess the defensive role of egg clustering. It was found that larger egg clutches suffer less predation compared with small clutches. No relationship between clutch size and AA content in the eggs was detected. Crawling predators seem to play the most important role in egg mortality. This study suggests that for toxic eggs there is a clear benefit in laying eggs in large clusters in areas where there are high levels of predator threat.