Isotopic analysis of serially sampled vibrissae (whiskers) is a powerful method to investigate changes in an individual's resource and habitat use over time, which is difficult or impossible to accomplish using traditional dietary proxies such as observation or scat analysis. A vibrissae-based isotopic approach is limited by knowledge of vibrissae growth rates, which are required to determine the time period represented by each subsampled segment. Likewise, determining the magnitude of, and variation in, isotopic differences between a consumer and its diet, commonly referred to as trophic discrimination factors (TDFs), is a crucial step in quantifying diet composition using stable isotopes. TDF estimates are available only for a few mammalian taxa. We measured vibrissae growth rates and δ13C and δ15N TDFs in captive southern sea otters (Enhydra lutris nereis). Sea otters were administered 15N-enriched glycine intravenously and vibrissae were collected periodically and serially sampled for carbon (δ13C) and nitrogen (δ15N) isotope analysis. Growth of adult sea otter vibrissae was linear with a mean (±SD) rate of 7.7 (± 1.2) cm/year. Mean (±SD) whole diet–vibrissae TDFs were 2.8‰ (±0.2‰) for δ13C and 5.5‰ (±0.2‰) for δ15N. Mean (±SD) lipid-extracted diet–vibrissae TDFs were 2.4‰ (±0.2‰) for δ13C and 4.9‰ (±0.3‰) for δ15N. δ13C TDFs were similar to previously reported values for mammalian carnivores, but δ15N TDFs were higher than expected. These results will increase the accuracy of isotopic diet analyses of mustelids and other carnivores for which there are few estimates of TDFs and no estimates of vibrissae growth rates.
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Vol. 94 • No. 2