Understanding early ontogeny and whisker growth dynamics of Weddell seal (Leptonychotes weddellii) 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 ¹³C) and nitrogen (δ ¹⁵N) 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 ¹³C and δ ¹⁵N values along the length of their whiskers, with no evidence of a weaning signal. Post-birth, δ ¹⁵N values across the sequentially sampled segments of vibrissae steadily increased, likely reflecting the pups' dependence on maternal milk. The δ ¹³C values remained mostly constant, suggesting that mothers did not forage far from their breeding colonies, and the δ ¹⁵N values were similar across individual timelines, indicating that mothers occupied a narrow trophic range.