UC San Diego

Marine mammals spend much of their lives migrating across the ocean and are notoriously difficult to track over long periods of time. Stable isotope analysis (SIA) of animal tissues is an important tool used to determine foraging ecology and the timing of important developmental changes that occur throughout a marine mammal’s lifetime. Stable isotope values from whisker segments provide long-term information of an animal’s foraging, as whisker growth incorporates isotopic signals from their diet, such as dietary shifts that occur during birth and weaning events. We estimated the birth dates of 7–8-week-old Weddell seal (Leptonychotes weddellii) pups using the δ15N and δ13C values measured from sequentially sampled segments of their vibrissae to gain further insight into the utility of this technique to accurately predict important early ontogenetic events. Results showed that all pups shared similar stable carbon and nitrogen isotope trends. We designated a drop in nitrogen isotope values to pinpoint each individuals’ birth event and, using prenatal and postnatal whisker growth rates, found that estimated birth dates aligned with the individuals’ observed birth dates. Reconstructed timelines showed no evidence of a weaning signal. Post-birth, the δ15N values steadily increased, likely reflecting their dependency on their mothers’ milk, and the δ13C values remained mostly constant, suggesting that mothers did not forage far from their breeding colonies.