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Growth rates and patterns of vibrissae (whiskers) in captive harbor seals Phoca vitulina were examined by intravenous infusion of 15 N-labeled amino acid tracers to mark their keratinous tis- sues. The use of vibrissa segmental isotopic analysis as diet indicators was evaluated during con- trolled feeding trials. Harbor seals shed their vibrissae annually. Replacement of new vibrissae started in May or June, depending on individual seals. Growth rates of new vibrissae were very fast at up to 0.78 mm d -1 during summer and fall, and then changed to a much slower growth rate throughout winter and early spring. An average growth rate of 0.075 mm d -1 was obtained from 1 vib- rissa from December to May. δ 13 C and δ 15 N values in vibrissae co-varied and reflected temporal vari- ations of diet or habitat changes of seals, particularly over a rapid growth period from late spring to fall. Compared with other tissues such as blood components, vibrissae can be sampled less invasively and archive ecological records over a longer period. Vibrissa segmental isotopic analysis provides a more flexible tool for studying foraging ecology of wild seals, despite the varying seasonal growth rates and annual replacements.

Stable isotope ratios in harbor seal Phoca vitulina vibrissae: effects of growth patterns on ecological records