Isotopic Variation Between Adult Female Guadalupe Fur Seals and Their Offspring: Implications for the Use of Neonates as Proxies for Maternal Foraging

Abstract: Pinniped neonates are indirect indicators of their mothers’ foraging areas and the prey they consume. However, the isotopic difference between neonates and their mothers is often unknown, presenting a substantial obstacle to our understanding of foraging behavior. In this study, we assessed the isotopic differences between Guadalupe fur seal (GFS) neonates and their mothers. Fur was sampled from ten 1-mo-old GFS neonates and their adult female GFS mothers on Guadalupe Island, Mexico. We used a Carlo Erba 1108 elemental analyzer coupled to a ThermoFinnigan Delta Plus XP isotope ratio mass spectrometer to determine the δ13C and δ15N. For each GFS pair, we calculated the isotopic variation using the SIBER (Stable Isotope Bayesian Ellipses in R) routine included in the SIAR package for R software. For neonates, the mean values were 17.8 ± 0.4‰ for δ15N and -17.6 ± 0.4‰ for δ13C, while the values for their mothers were 16.8 ± 0.1‰ for δ15N and -17.4 ± 0.5‰ for δ13C. The mean variation in δ15N between age groups was +1.0 ± 0.4‰, with most values falling between +0.5 and +1.0‰. The mean variation in δ13C was -0.2 ± 0.6‰, with the majority ranging from -0.5 to -0.1‰. We identified isotopic segregation between the two groups. The variation observed indicated that GFS neonates are effective proxies for their mothers’ δ15N, with values consistently higher in neonates than their mothers. Differences in δ13C between the two groups did not follow any apparent pattern. Our results serve as a reference for other studies using fur from offspring to infer values for adult females and can be used in conjunction with new or existing data. The mother-neonate relationship should be considered with caution as variations may arise from different factors, some of which can be controlled.
Key Words: Guadalupe fur seal, Arctocephalus philippii townsendi, stable isotopes, mothers, offspring, isotopic variation
Document Type: Research Article
Page Numbers: 268-276

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