1. Strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) have shown promise for tracing the geographic origin of animal tissues because they have high-resolution and show discrete spatial patterns independent and complementary to those of light isotopes. In this study, we provide a complete quantitative framework to apply ⁸⁷Sr/⁸⁶Sr for tracking migratory animals using the eastern North American population of monarch butterflies Danaus plexippus as a case study.2. To enable continuous-surface geographic assignment using ⁸⁷Sr/⁸⁶Sr, we recommend following five key steps: (a) assessing feasibility, (b) sample collection, (c) laboratory analysis, (d) modelling the isoscape and (e) geographic assignment.We provide a detailed outline of these steps and then focus on steps 3-5 for the case study. For monarchs, using an extensive plant ⁸⁷Sr/⁸⁶Sr dataset (n = 400), geospatial data and a machine learning approach, we first calibrate a regional, high-resolution ⁸⁷Sr/⁸⁶Sr isoscape (i.e. a baseline for ⁸⁷Sr/⁸⁶Sr assignment) over their eastern North American summer breeding range. We then use the ⁸⁷Sr/⁸⁶Sr isoscape to estimate the posterior probability surface of natal origin for 100 monarchs of unknown origin.3. Our results demonstrate that ⁸⁷Sr/⁸⁶Sr can greatly improve the precision of isotope-based geographic assignment. Furthermore, combining δ 2 H and ⁸⁷Sr/⁸⁶Sr into a dual assignment provides the most constrained area of natal origin.4. We provide a framework for ecologists and palaeoecologists to apply ⁸⁷Sr/⁸⁶Srbased geographic assignments for animal movement studies using contemporary or archived samples. The addition of the ⁸⁷Sr/⁸⁶Sr assignment tool will enhance our ability to study migration and dispersal in a wide variety of animals.
Forensic practitioners, archeologists, and ecologists increasingly use hair isotope profiles and isotope databases and maps to reconstruct the life history of unidentified individuals. Relationships between hair isotope profiles with travel history have been primarily investigated through controlled laboratory experiments. However, those controlled studies do not reflect the complex life history of modern individuals who often travel between multiple locations over different periods. Here, we collect one bundle of hairs from a volunteer whose primary residence is in Ottawa (Canada) but who traveled to multiple destinations over a period of 18 months. Those travels include multi-week trips to distant locations and multi-day trips to more local areas. We use multi-isotope profiles on the individual's hairs coupled with isotope databases across the world to reconstruct travel history at sub-monthly temporal resolution. We compare the isotopic interpretation of mobility with the recorded travel history. A prominent shift in δ 2 H values is interpreted as a westward movement toward central Canada, which corresponds to a month-long road-trip to the Prairie. We observed a marked negative excursion in δ 13 C values along the hairs profile, which was related to a multi-week-long trip to several countries in Europe. Except for an exceptionally variable interval likely driven by health issues, δ 15 N values show very little fluctuation across the entire profile, indicating consumption of consistent amounts of animal or marine-based protein at different locations. The isotopic shifts in the proximal part of the hair reveal a clearer picture of traveling destinations than the shifts in the distal part, which have larger uncertainty in terms of timing and amplitude. Except for the last couple of months before collection, 87 Sr/ 86 Sr ratios show little variation throughout the profile likely due to recent exchange with Ottawa tap water during bathing or showering. The different 87 Sr/ 86 Sr ratios in the month preceding collection appear to partially preserve the 87 Sr/ 86 Sr ratio of the volunteer's diet. This study demonstrates the interest of using multi-isotope systems and large isotopic databases to reconstruct individual mobility. This study also underlines the challenges in linking isotope data to mobility, particularly in rapidly growing keratinous tissues.
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