AUTHOR=Hobson Keith A. , Kardynal Kevin J. , Koehler Geoff 

TITLE=Expanding the Isotopic Toolbox to Track Monarch Butterfly (Danaus plexippus) Origins and Migration: On the Utility of Stable Oxygen Isotope (δ18O) Measurements

JOURNAL=Frontiers in Ecology and Evolution

VOLUME=7

YEAR=2019

URL=https://www.frontiersin.org/journals/ecology-and-evolution/articles/10.3389/fevo.2019.00224

DOI=10.3389/fevo.2019.00224

ISSN=2296-701X

ABSTRACT=<p>The measurement of naturally occurring stable hydrogen (δ<sup>2</sup>H) and carbon (δ<sup>13</sup>C) isotopes in wings of the eastern North American monarch butterflies (<italic>Danaus plexippus</italic>) have proven useful to infer natal origins of individuals overwintering in Mexico. This approach has provided a breakthrough for monarch conservation because it is the only viable means of inferring origins at continental scales. Recently, routine simultaneous analyses of tissue δ<sup>2</sup>H and δ<sup>18</sup>O of organic materials has emerged leading to questions of whether the dual measurement of these isotopes could be used to more accurately infer spatial origins even though the two isotopes are expected to be coupled due to the meteoric relationship. Such refinement would potentially increase the accuracy of isotopic assignment of wintering monarchs to natal origin. We measured a sample of 150 known natal-origin monarchs from throughout their eastern range simultaneously for both δ<sup>2</sup>H and δ<sup>18</sup>O wing values. Wing δ<sup>2</sup>H and δ<sup>18</sup>O values were correlated (<italic>r</italic><sup>2</sup> = 0.42). We found that wing δ<sup>2</sup>H values were more closely correlated with amount-weighted growing season average precipitation δ<sup>2</sup>H values predicted for natal sites (<italic>r</italic><sup>2</sup> = 0.61) compared to the relationship between wing δ<sup>18</sup>O values and amount-weighted growing season average precipitation δ<sup>18</sup>O values (<italic>r</italic><sup>2</sup> = 0.30). This suggests that monarch wing δ<sup>2</sup>H values will be generally more useful in natal assignments than δ<sup>18</sup>O values. Spatial information related to the use of deuterium excess in environmental waters was similarly found to be not useful when applied to monarch wings likely due to the considerable variance in wing δ<sup>18</sup>O values. Nonetheless, we recommend further testing of monarch wing δ<sup>2</sup>H and δ<sup>18</sup>O values from known natal sites with an emphasis on field data across a strong gradient in precipitation deuterium excess.</p>