Understanding early ontogeny and whisker growth dynamics of Weddell seal (<i>Leptonychotes weddellii</i>) pups through stable isotope analysis

Abstract Stable isotope analysis of animal tissues is a valuable tool for understanding foraging ecology, habitat use, and developmental changes throughout an animal's life. Stable isotope values of whisker segments offer long‐term data on mammalian foraging, as whisker growth incorporates isotopic signals from the diet and reflects dietary shifts during events like birth and weaning. We estimated birth dates of 7‐week‐old Weddell seal ( Leptonychotes weddellii ) pups ( n = 17) by analyzing stable carbon ( δ 13 C) and nitrogen ( δ 15 N) isotope values measured from sequentially sampled whisker segments to evaluate the technique's accuracy in predicting early developmental events. We also estimated prenatal and postnatal whisker growth rates to construct a timeline for each pup using the stable isotope values measured longitudinally along each whisker. All pups showed consistent trends in δ 13 C and δ 15 N values along the length of their whiskers, with no evidence of a weaning signal. Post‐birth, δ 15 N values across the sequentially sampled segments of vibrissae steadily increased, likely reflecting the pups' dependence on maternal milk. The δ 13 C values remained mostly constant, suggesting that mothers did not forage far from their breeding colonies, and the δ 15 N values were similar across individual timelines, indicating that mothers occupied a narrow trophic range.