The sea otter (Enhydra lutris) is a marine mammal hunted to near extinction during the 1800s. Despite their well-known importance as a keystone species, we know little about historical sea otter ecology. Here, we characterize the ecological niche of ancient southern sea otters (E. lutris nereis) using d¹³C and d¹⁵N analysis of bones recovered from archaeological sites spanning ~7,000 to 350 years before present (N=112 individuals) at five regions along the coast of California. These data are compared with previously published data on modern animals (N=165) and potential modern prey items. In addition, we analyze the d¹⁵N of individual amino acids for 23 individuals to test for differences in sea otter trophic ecology through time. After correcting for tissue-specific and temporal isotopic effects, we employ nonparametric statistics and Bayesian niche models to quantify differences among ancient and modern animals. We find ancient otters occupied a larger isotopic niche than nearly all modern localities; this likely reflects broader habitat and prey use in pre-fur trade populations. In addition, ancient sea otters at the most southerly ­sites occupied an isotopic niche that was more than twice as large as ancient otters from northerly regions. The latter likely reflects greater invertebrate prey diversity in southern California relative to northern California. Thus, we suggest the potential dietary niche of sea otters in southern California could be larger than in central and northern California. At two sites, Año Nuevo and Monterey Bay, ancient otters had significantly higher d¹⁵N values than modern populations. Amino acid d¹⁵N data indicated this resulted from shifting baseline isotope values, rather than a change in sea otter trophic ecology. Our results help in better understanding the contemporary ecological role of sea otters and exemplify the strength of combing zooarchaeological and biological information to provide baseline data for conservation efforts.

LIST OF SUPPLEMENTARY DATA TABLES

Table S1. Isotopic results of paired tissues from stranded sea otters in central California.

Table S2. Ancient and modern sea otter samples.

Table S3. ANOVA and Tukey’s HSD results for pairwise comparisons of sea otter bulk tissue d¹³C and d ¹⁵N values.

Table S4. ANOVA and Tukey’s HSD results for pairwise comparisons of ancient and modern sea otter d¹³C and d ¹⁵N values.