Mercury is a non-essential, globally distributed toxic metal with no biological function, and its behavior in marine food webs makes it a particular concern for top predators. In aquatic ecosystems, inorganic mercury released from industrial sources, coal combustion, and artisanal gold mining is converted by microbes into methylmercury, an organic form that is readily absorbed by living tissue. Unlike many contaminants that dilute as they move up the food chain, methylmercury bioaccumulates in individual organisms and biomagnifies across trophic levels, meaning that animals at the top of the web, such as marine mammals, can carry body burdens orders of magnitude higher than the water they swim in. Because methylmercury crosses the placenta and the blood-brain barrier, developing fetuses are especially vulnerable to its neurotoxic effects.

The research team, led by Anaclara Roselli-Laclau of the Universidad de la República in Uruguay together with colleagues from Uruguay and Brazil, focused on two sympatric otariid species that breed side by side at Isla de Lobos but occupy different ecological niches: the South American sea lion (Otaria flavescens) and the South American fur seal (Arctocephalus australis). The researchers quantified total mercury, or THg, in two complementary matrices: hair from adult females and lanugo, the fine natal fur that pups grow in utero and shed in the weeks after birth. Sampling covered 11 mother-pup pairs of sea lions and 46 mother-pup pairs of fur seals, providing an unusually direct way to examine maternal transfer because both members of each pair were analyzed.

The results revealed a striking contrast between the two species. Adult female sea lions averaged 30.5 micrograms of mercury per gram of hair on a dry weight basis, with a standard deviation of 9.3, while female fur seals averaged just 5.7 micrograms per gram, with a standard deviation of 3.4. The pattern was mirrored in their offspring: sea lion pups averaged 25.3 micrograms per gram of lanugo, compared with only 4.4 micrograms per gram in fur seal pups. In other words, sea lions at this colony carried roughly five to six times more mercury than their fur seal neighbors, and the concentrations measured in sea lion pups rank among the highest ever recorded for pinnipeds anywhere in the world.

This interspecies gap is best explained by differences in diet and foraging ecology. Sea lions from Uruguayan colonies are known to feed at higher trophic positions and in waters closer to the Río de la Plata estuary, a system influenced by both natural and anthropogenic metal inputs, while fur seals tend to forage further offshore on prey lower in the food web. Because methylmercury biomagnifies with each step in the food chain, a predator eating larger, older, more predatory fish accumulates substantially more of the metal than one feeding on small pelagic species. Previous work by the same group had already linked trophic habits to bioaccumulation in these two otariids, and the new mother-pup data extend that picture to the earliest stage of life.

Perhaps the most consequential finding is that mercury was detectable in lanugo at all. Lanugo is formed entirely before birth, so any mercury it contains must have been transferred across the placenta from the mother during gestation. The presence of THg in this prenatal fur therefore provides direct evidence of prenatal mercury exposure in both species, confirming that placental transfer, and not only lactation, is a significant route by which these animals hand their contaminant burden to the next generation. This aligns with earlier studies on Baltic grey seals and northern elephant seals, which documented transplacental mercury transfer, but the Uruguayan data add a South Atlantic baseline for two species that had been comparatively understudied.

Statistical modeling showed that pup mercury concentrations were positively associated with maternal concentrations in both species, which is exactly what one would expect if the pup’s prenatal fur faithfully reflects the mother’s body burden during pregnancy. However, the strength of that relationship differed dramatically between the species. In sea lions, the final model explained 75 percent of the variance in pup THg, a remarkably tight fit that suggests maternal levels are the dominant driver of pup exposure in this species. In fur seals, by contrast, the model explained only 20 percent of the variance, indicating that other factors, such as individual variation in foraging behavior, gestation timing, or physiological processes governing mercury mobilization, play a much larger role in shaping how much of the metal reaches each fetus.

The models also uncovered a sex effect that was present in one species but absent in the other. Among sea lions, male pups had lower mercury concentrations than female pups for a given maternal mercury level, a pattern that could reflect sex-specific differences in prenatal growth, body composition, or maternal investment, since male otariid pups are typically larger and may dilute their contaminant burden across more tissue mass. No such sex effect was detected in fur seals. The authors note that this kind of species-specific nuance matters for anyone hoping to use pup fur as a shortcut for monitoring adult contamination, because the relationship between mother and pup is not uniform across species or even between the sexes within a species.

That leads to the study’s central practical message: pup lanugo is not a reliable quantitative proxy for the mercury concentration of an individual mother. The variability in the mother-pup relationship, especially in fur seals, is too great to back-calculate a specific female’s burden from her pup’s fur. But the researchers argue that lanugo remains valuable in a different way. Because it can be collected from pups with minimal disturbance, without immobilizing or even closely approaching adult females, it offers a minimally invasive matrix for characterizing prenatal mercury exposure and for tracking broader species-level and population-level patterns over time and across geography. For colonies that are difficult to sample intensively, this could transform monitoring programs that would otherwise require invasive procedures on protected animals.

The broader implications extend beyond the South Atlantic. Marine mammals are widely regarded as sentinel species for ocean and human health, and elevated mercury in newborn sea lions signals that contamination in the southwestern Atlantic food web is reaching the most sensitive life stages of long-lived apex predators. Mercury exposure in young mammals has been linked in laboratory and field studies to impaired immune function, altered lymphocyte proliferation, and neurodevelopmental deficits, and similar concerns have been raised for harbor seal pups and Steller sea lion calves elsewhere. As mercury continues to enter the ocean from atmospheric deposition and from gold mining in the Amazon basin and beyond, studies like this one provide the baseline data needed to detect change, identify the species and populations most at risk, and inform international efforts, such as the Minamata Convention, to curb the flow of this persistent neurotoxin into marine ecosystems.

Subject of Research: Maternal-to-pup transfer of total mercury in South American sea lions and fur seals measured through maternal hair and pup lanugo

Article Title: Maternal–Pup Mercury Relationship in South American Sea Lions and Fur Seals

Article References: Roselli-Laclau, A., García-Alonso, J., Valdés, A., Freitas-Souza, M., Almeida, M. G., de Rezende, C. E., & Franco-Trecu, V. (2026). Maternal–Pup Mercury Relationship in South American Sea Lions and Fur Seals. Archives of Environmental Contamination and Toxicology, 91(3), Article 23. https://doi.org/10.1007/s00244-026-01227-z

Image Credits: AI Generated

DOI: 10.1007/s00244-026-01227-z

Keywords: mercury, South American sea lion, South American fur seal, pinnipeds, lanugo, prenatal exposure, biomagnification, ecotoxicology, marine mammals, Uruguay, maternal transfer, heavy metals

The review, led by Kenneth N. Kikanme and senior author Todd A. Anderson, synthesizes tissue-residue data from a series of chronic toxicity studies in which northern bobwhite, a popular gamebird and a standard avian toxicology model, were exposed through drinking water to either perfluorooctane sulfonic acid (PFOS) alone or a binary mixture of PFOS and perfluorohexane sulfonic acid (PFHxS). The birds received an analytically verified dose of 20 nanograms per milliliter over 60 days, after which the researchers measured PFAS concentrations in adult liver and brain, whole eggs, the chorioallantoic membrane, and the liver and brain of 21-day-old hatchlings. In total, 263 samples were collected across the experimental groups, all analyzed with QuEChERS-based extraction, isotopically labeled surrogate standards, and liquid chromatography tandem mass spectrometry.

The clearest signal to emerge from the synthesis is that the liver is the dominant reservoir for PFAS in both adult and hatchling quail. Across all exposure groups, the tissue ranking followed a consistent pattern: adult liver accumulated the most, followed by adult brain, then the post-hatch chorioallantoic membrane, whole egg, hatchling liver, and finally hatchling brain. In males exposed only to PFOS, liver concentrations reached 565 nanograms per gram wet weight, while in the mixture exposure males carried 463 nanograms per gram in liver and females 325 nanograms per gram. The researchers attribute this hepatic dominance to the strong affinity of PFOS for liver-binding fatty acid proteins, which act as molecular docking stations for the long-chain sulfonic acids.

Sex emerged as a surprisingly important variable. Male quail accumulated significantly more PFOS in both liver and brain than females, and bioaccumulation factors calculated from measured water intake were consistently higher in males. Because females can shed a portion of their chemical burden into eggs, the researchers had expected them to show lower tissue concentrations, but the magnitude of the difference was notable. A liver bioaccumulation factor of exactly 1.0 in females under PFOS-only exposure suggests a state of equilibrium between intake and clearance, effectively making the female liver a bioindicator of exposure, whereas the male value of 2.6 indicates genuine net accumulation. The authors suggest that females may also drink more water during egg production, raising their actual daily intake and making the male-skewed pattern even more striking.

Perhaps the most novel contribution of the review concerns the chorioallantoic membrane, or CAM, the highly vascularized structure that forms during embryonic development and mediates gas exchange, nutrient transfer, and waste sequestration between the embryo and the egg. After hatching, remnants of the CAM typically remain attached to the shell, allowing researchers to sample it non-lethally. The data showed that PFOS concentrations in post-hatch CAMs, at 142 nanograms per gram wet weight under PFOS-only exposure, were actually slightly higher than in whole eggs, which averaged 92 nanograms per gram. Because the CAM receives a direct blood supply from the embryo, the authors argue it reflects the PFAS burden genuinely available to the developing chick, whereas whole-egg measurements capture only the total amount the mother deposited.

This finding challenges the conventional focus on yolk and albumen as the primary routes of maternal transfer. Previous work, including studies in mallards, cormorants, and chickens, has emphasized the association of PFOS with very low-density lipoproteins and yolk proteins as the main vehicle carrying the chemical into the egg. The quail data suggest that the CAM, with its dense network of proteinaceous binding sites such as integrins and histones and its continuous exposure to embryonic circulation, may act as an underappreciated reservoir. If confirmed across species, this would make the discarded eggshell membrane a cheap and non-lethal biomonitor for embryonic PFAS exposure in wild bird populations, a tool that could be deployed simply by collecting hatched shells from nests.

The mixture experiments added another layer of complexity. When PFHxS was present alongside PFOS, PFOS concentrations in whole eggs and post-hatch CAMs dropped significantly compared with PFOS-only exposure, falling to 50 and 93 nanograms per gram respectively. PFHxS itself accumulated at lower levels than PFOS in nearly every tissue, consistent with the shorter perfluorinated chain of PFHxS and its weaker hydrophobic interactions with tissue proteins. The researchers propose competitive binding as the likely mechanism: both compounds compete for the same high-affinity sites on albumin, liver fatty acid-binding proteins, and organic anion transporters, and the longer-chain PFOS generally wins, but the presence of PFHxS measurably reshapes how PFOS partitions across organs.

Maternal transfer was confirmed but diluted. Hatchling livers carried PFOS concentrations roughly 4.5 times lower than those of exposed adult females under PFOS-only exposure, and 7 and 14 times lower for PFOS and PFHxS respectively under the mixture. Hatchling brains showed similar dilution, with PFOS about 3.8 times lower than in adult female brains. Because hatchlings received no exposure after hatching, these residues represent the pure signature of in ovo transfer. Notably, PFHxS in hatchling liver was statistically far lower than PFOS, at just 9.5 nanograms per gram, underscoring how selectively the longer-chain compound is mobilized from maternal stores into eggs and embryonic tissues during the breeding cycle.

The brain findings carry their own ecological weight. Birds depend on brain regions such as the dorsal ventricular ridge, which lies outside the blood-brain barrier, for foraging and communication behaviors, making PFAS accumulation in neural tissue a potential route to neurobehavioral disruption. Earlier field studies in glaucous gulls and juvenile seabirds have documented PFOS in avian brain tissue, and the quail data now provide controlled-exposure confirmation that maternally transferred PFOS reaches the hatchling brain. The authors caution that their synthesis relies on relative distribution patterns rather than strict quantitative equivalence across matrices, since extraction protocols varied slightly between studies, but the consistency of the tissue hierarchy across independent trials lends confidence to the overall picture.

The broader implications extend well beyond one gamebird species. PFOS remains the dominant PFAS contaminant at firefighting-foam-affected sites, accounting for roughly 82 percent of targeted PFAS measurements in soils at U.S. Air Force installations, and European soils far from point sources still frequently exceed thresholds that threaten drinking water. As regulators push PFOS and PFHxS toward global elimination under the Stockholm Convention, understanding how residual body burdens translate into next-generation exposure becomes critical for ecological risk assessment. The Texas Tech team argues that risk models should incorporate sex-specific toxicokinetics, since males appear to bear a higher burden of organ-level accumulation, and that the chorioallantoic membrane deserves a formal place in avian monitoring programs. Future work, they note, should dissect the mechanistic drivers of PFAS competition at protein binding sites and further characterize the membrane’s biochemical capacity, steps that could transform a discarded eggshell into one of the most informative and least invasive windows into the invisible chemical inheritance passed from mother birds to their young.

Subject of Research: Bioaccumulation and maternal transfer of PFOS and PFHxS in northern bobwhite quail

Article Title: Bioaccumulation dynamics of perfluorooctane sulfonic acid (PFOS) and a binary mixture of PFOS and perfluorohexane sulfonic acid (PFHxS) in northern bobwhite (Colinus virginianus): a mini review

Article References: Kikanme, K. N., Karnjanapiboonwong, A., Dennis, N. M., Angappan, R., Isukuru, E. J., Salice, C. J., & Anderson, T. A. (2026). Bioaccumulation dynamics of perfluorooctane sulfonic acid (PFOS) and a binary mixture of PFOS and perfluorohexane sulfonic acid (PFHxS) in northern bobwhite (Colinus virginianus): a mini review. Ecotoxicology, 35(8), Article 176. https://doi.org/10.1007/s10646-026-03160-2

Image Credits: AI Generated

DOI: 10.1007/s10646-026-03160-2

Keywords: PFAS, PFOS, PFHxS, bioaccumulation, maternal transfer, northern bobwhite, chorioallantoic membrane, ecotoxicology, avian toxicology, liver, brain, forever chemicals