Isotopic variation between adult female guadalupe fur seals and their offspring: Implications for the use of neonates as proxies for maternal foraging

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 elemen tal analyzer coupled to a ThermoFinnigan Delta Plus XP isotope ratio mass spectrometer to determine the δ¹³C and δ¹⁵ N. For each GFS pair, we calculated the isotopic variation using the SIBER (Stable Isotope Bayesian Ellipses in R) routine included in the SIARpackage for Rsoftware. 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.