Stable isotopes (delta D and delta C-13) are geographic indicators of natal origins of monarch butterflies in eastern North America

Wing membranes of laboratory and field-reared monarch butterflies (Danaus p lexippus) were analyzed for their stable-hydrogen (delta D) and carbon (del ta(13)C) isotope ratios to determine whether this technique could be used t o identify their natal origins. We hypothesized that the hydrogen isotopic composition of monarch butterfly wing keratin would reflect the hydrogen is otope patterns of rainfall in areas of natal origin where wings were formed . Monarchs were reared in the laboratory on milkweed plants (Asclepias sp.) grown with water of known deuterium content, and, with the assistance of v olunteers, on native milkweeds throughout eastern North America. The result s show that the stable hydrogen isotopic composition of monarch butterflies is highly correlated with the isotopic composition of the milkweed host pl ants, which in turn corresponds closely with the long-term geographic patte rns of deuterium in rainfall. Stable-carbon isotope values in milkweed host plants were similarly correlated with those values in monarch butterflies and showed a general pattern of enrichment along a southwest to northeast g radient bisecting the Great Lakes. These findings indicate that natal origi ns of migratory and wintering monarchs in Mexico can be inferred from the c ombined delta D and delta(13)C isotopic signatures in their wings. This rel ationship establishes that analysis of hydrogen and carbon isotopes can be used to answer questions concerning the biology of migratory monarch butter flies and provides a new approach to tracking similar migratory movements o f other organisms.