Dear colleagues, My co-authors and I are pleased to announce the publication of the following paper online in Rapid Communications in Mass Spectrometry What's in a whisker? Disentangling ecological and physiological isotopic signals. McHuron, EA, Holser, RR, and DP Costa. https://doi.org/10.1002/rcm.8312 *Abstract:* *Rationale:* Stable isotope analysis of keratinized tissues is an informative tool to quantify foraging ecology that can address questions related to niche specialization and temporal variation in behavior. Application of this approach relies on an understanding of tissue growth and how isotope ratios relate to physiological and ecological processes, data that are lacking for many species. *Methods:* We collected paired whisker length measurements from northern elephant seals to estimate growth and shedding patterns (n = 16). A subset of seals (n = 5) carried a satellite tag and time-depth recorder across the 7+ month foraging trip following the annual pelage molt. Stable isotopes of carbon and nitrogen were measured in segments grown across the 6+ week fasting on land and the subsequent foraging trip; profiles were combined with growth parameters to timestamp each segment and investigate relationships with foraging behavior. *Results:* Whisker loss and initial regrowth primarily occurred during the annual pelage molt, but newly grown whiskers exhibited active, nonlinear growth across the foraging trip. δ13C and δ15N values were higher in segments grown on land than at sea and exhibited a characteristic decline upon departure from the rookery. There was a relationship between latitude and longitude and δ15N values, and individual whisker segments grown at sea could be classified to the correct ecoregion with 81% accuracy. *Conclusions: *Fasting affected both δ13C and δ15N values and the ability to exclude these values from ecological investigations is crucial given the temporal overlap with tissue growth. The rapid decline in isotope ratios upon departure can be used to isolate portions of the whisker with a strong physiological signal, even for whiskers with unknown growth histories. The active growth across the foraging trip combined with the ability to identify differences in foraging behavior validate the utility of this approach for addressing ecological questions. Please don't hesitate to contact me (emchuron@ucsc.edu) with any questions or for a copy of the manuscript if you do not have access to the journal. -- Elizabeth McHuron, Ph.D. Research Scientist University of Washington JISAO 115 McAllister Way Santa Cruz, CA 95060 emchuron@uw.edu <emchuron@ucsc.edu> emchuron@ucsc.edu