Abstract
<title>Abstract</title> <p>Mercury (Hg) is a non-essential, globally distributed toxic metal that bioaccumulates in aquatic organisms and biomagnifies across trophic levels, posing a threat to top predators such as marine mammals. Our aim was to evaluate total mercury (THg) relationships in mother-pup pairs of two pinniped species, South American sea lions (Otaria flavescens) and South American fur seals (Arctocephalus australis), from Isla de Lobos, Uruguay. We quantified THg in the hair of 11 O. flavescens and 46 A. australis mother-pup pairs. THg concentrations were significantly higher in O. flavescens than in A. australis for both adult females (30.5 ± 9.3 μg/g vs. 5.7 ± 3.4 μg/g, dry weight) and pups (25.3 ± 13.2 μg/g vs. 4.4 ± 2.5 μg/g, dry weight). Hg levels in O. flavescens pups are among the highest recorded for pinnipeds. A significant positive relationship between female and pup THg (O. flavescens p=0.003; A. australis p=0.002) supported transplacental Hg transfer. In O. flavescens, pup sex significantly influenced this relationship, with males showing lower THg for a given maternal value, whereas no sex effect was detected in A. australis. These finding support the use of pups as effective minimally invasive proxies for maternal Hg levels. This study highlights the relevance of early-life exposure in environmental toxicology and supports integrating pup-based sampling in long-term contaminant monitoring programs.</p>