Evidence for dietary time series in layers of cetacean skin using stable carbon and nitrogen isotope ratios.

RATIONALE

Stable isotope analysis integrates diet information over a time period specific to the type of tissue sampled. For metabolically active skin of free-ranging cetaceans, cells are generated at the basal layer of the skin and migrate outward until they eventually slough off, suggesting potential for a dietary time series.

METHODS

Skin samples from cetaceans were analyzed using continuous-flow elemental analyzer isotope ratio mass spectrometry. We used ANOVAs to compare the variability of δ C and δ N values within and among layers and columns ("cores") of the skin of a fin, humpback, and sperm whale. We then used mixed-effects models to analyze isotopic variability among layers of 28 sperm whale skin samples, over the course of a season and among years.

RESULTS

We found layer to be a significant predictor of δ C values in the sperm whale's skin, and δ N values in the humpback whale's skin. There was no evidence for significant differences in δ N or δ C values among cores for any species. Mixed-effects models selected layer and day of the year as significant predictors of δ C and δ N values in sperm whale skin across individuals sampled during the summer months in the Gulf of Alaska.

CONCLUSIONS

These results suggest that skin samples from cetaceans may be subsampled to reflect diet during a narrower time period; specifically different layers of skin may contain a dietary time series. This underscores the importance of selecting an appropriate portion of skin to analyze based on the species and objectives of the study.