What's in a whisker? Disentangling ecological and physiological isotopic signals.

ABSTRACT

RATIONALE Stable isotope analysis of keratinized tissues is an informative tool for quantifying 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 whisker 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. The δ13 C and δ15 N 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 δ15 N values, and individual whisker segments grown at sea could be classified to the correct ecoregion with 81% accuracy. CONCLUSIONS: Fasting affected both δ13 C and δ15 N 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 validates the utility of this approach for addressing ecological questions.