Stable isotopes (δD and δ 13 C) are geographic indicators of natal origins of monarch butterflies in eastern North America

Abstract

Wing membranes of laboratory and field-reared monarch butterflies (Danaus plexippus) were analyzed for their stable-hydrogen (δD) and carbon (δC) isotope ratios to determine whether this technique could be used to identify their natal origins. We hypothesized that the hydrogen isotopic composition of monarch butterfly wing keratin would reflect the hydrogen isotope 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 volunteers, on native milkweeds throughout eastern North America. The results show that the stable hydrogen isotopic composition of monarch butterflies is highly correlated with the isotopic composition of the milkweed host plants, which in turn corresponds closely with the long-term geographic patterns 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 gradient bisecting the Great Lakes. These findings indicate that natal origins of migratory and wintering monarchs in Mexico can be inferred from the combined δD and δC isotopic signatures in their wings. This relationship establishes that analysis of hydrogen and carbon isotopes can be used to answer questions concerning the biology of migratory monarch butterflies and provides a new approach to tracking similar migratory movements of other organisms.

Stable isotope signature of philopatry and dispersal in a migratory songbird

“…Interestingly, ␦ 13 C values appear to be inversely correlated with latitude on the breeding range, with the highest values occurring in high-altitude populations from the southern Appalachian Mountains (Ͻ36°N) and the lowest values occurring north of 44°N at relatively low altitudes. This pattern runs counter to the latitudinal trends in ␦ 13 C values observed at low altitudes among many species of C 3 plants (21) and in monarch butterflies (22), whose larvae feed exclusively on C 3 milkweeds. The cause of this empirical paradox is unknown, but it may be due to strong altitudinal influences on photosynthesis (23-28) transmitted through C 3 -based food webs.…”

Tracking multi-generational colonization of the breeding grounds by monarch butterflies in eastern North America

“…For each butterfly, we were interested in calculating a probability based on the correspondence between the measured d C. Here, S was assumed to be constant across the entire isoscape and was estimated based on all values from known-location butterflies from data in [33]. Explicitly incorporating the variance-covariance in our models acknowledges the inherent variation in isotopic measurements that influence conditional probability of origin and allows us to draw more robust inference [35].…”

Monarch butterflies cross the Appalachians from the west to recolonize the east coast of North America

“…We examined both hypotheses by collecting monarchs at 17 sites in the eastern United States (figure 1) and used stable-hydrogen (dD) and -carbon (d 13 C) isotopes to estimate natal origins [8].…”

“…Field-and laboratory-rearing studies have shown that both dD p and d 13 C L are directly reflected in the isotopic composition of wing tissue of monarch larvae that feed on milkweed and in emergent adults [8]. dD p is correlated with dD values in monarch wing chitin (r 2 ¼ 0.69) according to the linear function: dD m ¼ 0.62dD p 279‰ and isotope discrimination between d 13 C L and monarch wing chitin is minimal, with d 13 C values 0.5‰ lower in wings compared with milkweed [8].…”

“…Stable-carbon isotopes (d 13 C) in milkweed leaf tissue varies longitudinally ( [8]; electronic supplementary material, figure A1). Isotopic values in milkweed are transferred up the food web to monarch wings, providing a natal geospatial fingerprint that is fixed for the duration of the individual's lifespan [8].…”