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Whisker growth dynamics in two North Pacific pinnipeds: implications for determining foraging ecology from stable isotope analysis McHuron, EA. 1* , Walcott, S.M. 2,3 , Zeligs, J. 4 , Skrovan, S. 4 , Costa, D.P 1 , Reichmuth, C. 2 Department of Ecology & Evolutionary Biology, University of California, Santa Cruz, CA 95060, USA Institute of Marine Sciences, Long Marine Laboratory, Santa Cruz, CA 95060, USA Department of Biological Sciences, University of Alaska, Anchorage, AK 99508, USA Moss Landing Marine Laboratories, Moss Landing, CA 95039, USA *Corresponding author, emchuron@ucsc.edu Abstract Stable isotope analysis (SIA) of whiskers is increasingly used to investigate the foraging ecology of pinnipeds. An understanding of whisker growth dynamics is lacking for most species, yet is necessary for study design and interpretation of isotope data. Here we present measurements of whisker growth in five California sea lions (Zalophus californianus) and two spotted seals (Phoca largha) obtained using photogrammetry. Data were collected from captive individuals for at least one year, resulting in serial measurements of 321 sea lion and 153 spotted seal whiskers. The sea lion whiskers exhibited linear growth, with growth rates that ranged from <0.01 to 0.18 mm day -1 . In contrast, spotted seal whiskers exhibited asymptotic growth characterized by rapid initial growth of up to 1.40 mm day -1 ; whiskers reached 75% and 95% of their asymptotic length after an average of 48 and 105 days, respectively. Over half of the spotted seal whiskers were lost annually during a period that coincided with the annual pelage molt, whereas the estimated lifespan of sea lion whiskers was 10+ years. Our data indicate that sea lion whisker growth rates can be used to reliably determine time periods of tissue deposition and link isotope values with ecological events over multiple years. In contrast, spotted seal whiskers archive dietary information over a period of months, and interpretation of isotope values is complicated by growth and shedding patterns of whiskers, and physiological changes associated with the annual pelage molt. Keywords: Vibrissae, Phocid, Otariid, Growth rate, Foraging behavior