δ15N and δ13C values in dental collagen as a proxy for age- and sex-related variation in foraging strategies of California sea lions

We assessed the foraging habits of California sea lions, Zalophus californianus, from Isla Santa Margarita, BCS, Mexico, by analyzing delta C-13 and delta N-15 values of dentin collagen. Since dentin is deposited annually in growth layer groups (GLGs), it can be subsampled to construct ontogenetic isotopic profiles at the individual level. We drilled 20 canine teeth and obtained 141 samples for isotopic analysis that were assigned to age-specific categories from GLG-based estimated ages. Pups' GLGs had the highest mean delta N-15 values and the lowest mean delta C-13 values, a pattern likely driven by the consumption of milk. Juveniles had delta N-15 values between those of pups and adult females, which may reflect continued nursing into the second year or preferential consumption of coastal benthic versus pelagic prey. Significant differences were observed between the sexes of adults; adult females had lower mean delta C-13 and delta N-15 values than adult males. Higher isotope values in adult males relative to females may reflect a higher trophic position, but differences in foraging grounds cannot be excluded as a potential explanation because tracking data are not available at this time. Evidence of intra-specific foraging diversification may be related to a strategy to reduce competition within and among age and sex categories.