<p>Iron is a critical micronutrient underpinning aerobic capacity in diving mammals, yet its transfer and utilization during early development remain poorly understood. Weddell seals provide a remarkable study system, as females mobilize large iron stores to produce milk with iron concentrations far higher than those found in terrestrial mammals. Weddell seal adult female-pup pairs were sampled during early lactation, late lactation, and post-weaning to quantify iron mobilization in adult females and iron acquisition in their pups using physiological measurements and iron isotope analysis. Pups displayed exceptionally high transferrin saturation (&gt; 90%), rapid development of blood oxygen stores, and modest increases in muscle myoglobin. Females that maintained substantial muscle and circulating iron stores through lactation had pups with more robust iron stores prior to weaning. Additional archived seal tissue samples and prey items were analyzed for iron isotopic composition to characterize the full iron landscape. Iron isotope signatures revealed trophic enrichment of lighter isotopes, while pup blood became isotopically lighter over lactation, reflecting incorporation of milk-derived iron. Shifts in organ isotope values indicated iron mobilization and depletion, while blood isotope composition correlated with maternal iron stores and mobilization, highlighting its potential as an indicator of iron status. This study demonstrates how iron transfer underpins the aerobic development of a free-living marine mammal and establishes stable iron isotopes as a powerful tool for tracing nutrient allocation and maternal investment in the wild.</p>