Monarchs lay their eggs on milkweed (swan plants), a member of the genus Asclepias. As the caterpillars eat the milkweed leaves, they ingest chemicals called cardiac glycosides. Birds or other animals that eat the caterpillars (or milkweed itself) become sick and vomit. The caterpillars sequester (hold on to) this toxin as they pupate, and the toxins are transferred to the adult butterflies. Birds or other creatures that eat the monarchs become sick, so they learn to leave both the butterflies and larvae alone. Not all monarch butterflies are equally poisonous. The concentration of cardiac glycosides decreases as individual butterflies age, and male monarchs have lower concentrations of toxins than females. Predators recognise the aposematic colouration and tend to avoid all monarchs.