Cardenolide content of Danaus chrysippus butterflies from three areas of East Africa

The African butterfly Danaus chrysippus , like other members of the family Danaidae, feeds as a larva on species of milkweeds (Asclepiadaceae). It has been demonstrated in a sample from a West African population that only a minority of adult D. chrysippus accumulated detectable amounts of poisonous and presumably emetic cardenolides from their larval foodplants. It has also been shown in D. chrysippus and the related monarch butterfly ( D. plexippus ) that the cardenolide content of adults varies with that of their respective milkweed foodplants.
Three population samples of D. chrysippus from sites in East Africa are analysed for cardenolides using the spectrophotometric technique refined by Brower, Edmunds & Moffitt (1975), which allows the assay of individual butterflies. The East African samples all have a palatability spectrum with a considerably higher percentage of cardenolide-containing individuals than the West African one.
This is discussed in terms of its effect on potential bird predators and the 'automimicry' of emetic by palatable individuals. In East Africa D. chrysippus is polymorphic and appears to act as a model in a complex mimicry ring, whereas in West Africa it is monomorphic and may have 'shed' most of its mimics. This latter hypothesis is discussed in the light of our results.