Abstract
The measurement of naturally occurring stable hydrogen (δ2H) and carbon (δ13C) isotopes in wings of the eastern North American monarch butterflies (Danaus plexippus) 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 δ2H and δ18O 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 δ2H and δ18O wing values. Wing δ2H and δ18O values were correlated (r2=0.42). We found that wing δ2H values were more closely correlated with amount-weighted growing season average precipitation δ2H values predicted for natal sites (r2=0.61) compared to the relationship between wing δ18O values and amount-weighted growing season average precipitation δ18O values (r2=0.30). This suggests that monarch wing δ2H values will be generally more useful in natal assignments than δ18O 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 δ18O values. Nonetheless, we recommend further testing of monarch wing δ2H and δ18O values from known natal sites with an emphasis on field data across a strong gradient in precipitation deuterium excess.
Highlights
The monarch butterfly (Danaus plexippus) is an iconic migratory insect that navigates over thousands of kilometers between natal sites in the eastern USA and Canada and overwintering sites in the Oyamel fir (Abies religiosa) forests of central Mexico (Urquhart and Urquhart, 1976; Ries et al, 2015; but see Vander Zanden et al, 2018)
Our results confirm a strong relationship between wing chitin δ2H values and precipitation δ2H values at known natal sites
The different analytical approaches resulted in different calibrations between wing chitin δ2H (δ2Hw) and predicted amount-weighted growing season δ2Hp and we recommend that on authors use the recent calibration relationship reported here
Summary
The monarch butterfly (Danaus plexippus) is an iconic migratory insect that navigates over thousands of kilometers between natal sites in the eastern USA and Canada and overwintering sites in the Oyamel fir (Abies religiosa) forests of central Mexico (Urquhart and Urquhart, 1976; Ries et al, 2015; but see Vander Zanden et al, 2018). This outstanding migration involves multiple generations whereby those individuals returning to wellestablished, long-term overwintering sites do so without any previous experience of their locations. Other applications have involved the role of wing coloration in flight distance (Hanley et al, 2013) and general conservation concerns related to where most individuals are being produced (Flockhart et al, 2017)
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