Upon maturity, monarch butterflies travel hundreds of miles from their places of origin and lay their eggs on milkweed. The caterpillars that emerge feed on milkweed and absorb the glycosides in milkweed sap. The specific glycosides present in milkweed differ from region to region within the monarch butterfly's range. Since mature butterflies retain the glycosides they absorb as caterpillars, the glycosides in a mature monarch butterfly could be used to determine its place of origin.