Microplastics found in brains of endangered island foxes

On a windswept, Navy-run island roughly 110 kilometers off the coast of Southern California, a tiny fox found nowhere else on Earth is carrying a hidden burden in its brain. A team of researchers has found microplastics and nanoplastics — collectively known as MNPs — embedded in the brain tissue of San Nicolas Island foxes, a threatened subspecies whose entire global population lives on a single, remote island. The discovery, published in the journal Environmental Advances, marks the first time microplastic bioaccumulation has been documented in the brain of a wild island endemic mammal, and it delivers a sobering message: even the most isolated wildlife populations on the planet are not beyond the reach of plastic pollution.

The foxes of the Channel Islands are icons of conservation biology. The San Nicolas Island fox (Urocyon littoralis dickeyi) is among the most genetically uniform mammalian populations ever studied, the result of a severe population bottleneck in the 1970s that left the animals with low genetic diversity and a heavy load of deleterious alleles. The subspecies was listed as Threatened by the State of California in 2025, joining a long roster of pressures that include climate-driven shifts in precipitation, disease risk, and the lingering effects of human infrastructure on the island. Because insular endemics represent 37 percent of all critically endangered species worldwide, and because 61 percent of documented extinctions over the past four centuries have occurred on islands, understanding novel threats to these animals carries urgency far beyond one small patch of California coastline.

The research team, led by Eve N. Rowland and Marcus A. Garcia of the University of New Mexico, working with Matthew J. Campen’s laboratory at the UNM College of Pharmacy, turned to an unexpected scientific resource: museum drawers. The study drew on 54 fox brain specimens archived at the Museum of Southwestern Biology, originally collected between 2003 and 2023 after the animals died, mostly from vehicular trauma, and necropsied at the University of California, Davis. Of those, 34 samples produced usable data spanning the full 20-year window, with even representation across sexes and age groups, from juveniles under one year old to adults older than five.

Extracting plastic from brain tissue is no simple task. The researchers adapted a protocol previously used to quantify microplastics in human placental tissue. Roughly 500 milligrams of brain tissue from each animal was digested in 10 percent potassium hydroxide at 60 degrees Celsius for 72 hours, a saponification process that dissolves the biological matrix. The digested material was then spun in an ultracentrifuge at 100,000 times gravity for four hours, forcing dense plastic particles into a tiny pellet at the bottom of the tube. Because residual lipids can interfere with downstream chemical analysis, the pellets underwent an additional wash in cyclohexane, an organic solvent that strips away fatty residues before filtration onto quartz filters. This lipid-removal step proved critical: the team’s filtration procedure achieved an average digestion efficiency of 99.092 percent, and running samples both with and without the wash demonstrated that lipid contamination had been inflating apparent polymer readings, particularly for polyethylene.

The cleaned pellets were then subjected to pyrolysis gas chromatography mass spectrometry, or Py-GC/MS, a technique in which samples are rapidly heated to 600 degrees Celsius, causing polymers to break apart into characteristic fragments. Mass spectrometry separates these fragments by their mass-to-charge ratio, and comparing the resulting chromatograms against calibration curves built from a standard containing 12 target polymers allows researchers to identify and quantify specific plastics. The calibration curves were highly linear across the analytical range, with coefficients of determination ranging from 0.9826 to 0.9997. Rigorous quality control — including unused “true” blanks, field blanks wiped across the weigh station, and spike recoveries — confirmed that blank samples fell below the limit of quantification, ruling out laboratory contamination as the source of the signal.

The results were striking. Across the 34 analyzed brains, the average total polymer concentration reached 3,034.95 micrograms per gram of tissue — roughly 0.3 percent of the tissue mass by weight. Polyethylene, the world’s most common plastic, dominated at 1,137.25 micrograms per gram, followed by poly(methyl methacrylate), or PMMA — better known as acrylic — at 682.82 micrograms per gram, nylon 66 at 620.72, nylon 6 at 480.16, and lower levels of polyethylene terephthalate, the plastic of beverage bottles. The prominence of nylon is notable: synthetic fibers from fishing gear and textiles are a major source of environmental microplastics, and Channel Island foxes are known to scavenge marine carrion on beaches, where wave-cast debris accumulates. The islands sit in a convergence zone of ocean currents, making them natural traps for drifting plastic even in the absence of dense local human activity.

Perhaps the most surprising finding was PMMA. Acrylic had never before been detected at such concentrations in any mammalian brain studied. The authors speculate that the island’s heavy military footprint may be relevant: San Nicolas Island is owned and operated by the U.S. Navy, hosting an airfield, missile-testing facilities, radar, optics, and telemetry installations, all of which can incorporate acrylic components. The island’s history also includes decades of use as a waste disposal site, culminating in a massive Navy cleanup of tons of refuse in the 1990s. Yet the researchers caution that pinpointing exposure routes is difficult; mapping of individual fox collection sites against roads and military infrastructure revealed no locations associated with systematically higher contamination, and these territorial animals, with home ranges as small as half a square kilometer, likely traverse much of the 5,770-hectare island during their lives.

Transmission electron microscopy added visual confirmation. Examining pellets from three individuals spanning the sampling period — one from 2003, one from 2016, and one from 2023 — the team identified shard-like particles measuring roughly 200 nanometers or less in length, morphologically consistent with the putative micro- and nanoplastics previously imaged in human brain tissue by the same research group. While TEM cannot determine chemical composition, and spectroscopic confirmation of such nanoscale particles is not technically feasible, the images lend weight to the conclusion that genuine plastic particles, not merely chemical artifacts, reside within the foxes’ brains.

Statistical analysis revealed another intriguing pattern — or rather, the absence of one. After a cube-root transformation to meet assumptions of normality, a linear regression testing sex, age class, and year of death as predictors of total polymer burden found no significant effects for any factor. Contamination was equally high in juveniles and adults, males and females, and — critically — showed no upward or downward trend over two decades. This temporal flatness contrasts with human studies, and the authors interpret it as consistent with a plateau model of bioaccumulation, in which uptake and elimination processes reach a steady state within individuals. It also implies that plastic contamination on San Nicolas Island has been substantial and stable since at least 2003, despite the island’s tiny human population of only 100 to 200 people.

The comparison with humans is impossible to ignore. Previous work from the same laboratories documented microplastic accumulation in decedent human brains, with polyethylene the dominant polymer but relatively little nylon and almost no PMMA. The foxes’ polymer profile differs in ways that likely reflect their distinct environment — more fiber-derived nylon, more acrylic — yet the broader message converges: plastic bioaccumulation in brain tissue is not uniquely human but a feature of mammalian biology across ecosystems. Because polymer types and concentrations have been found to be similar between marine mammals and humans, the authors propose that island endemics could serve as sentinels, proxy indicators of contaminant levels that humans themselves experience. Museum collections, they argue, are uniquely positioned for this role, offering temporally deep, geographically broad sampling that has already been used to trace plastics back to 1950 in archived fish and to document decades of microfiber accumulation in caddisfly casings.

The study is not without limitations, which the authors address candidly. Ultracentrifugation cannot capture every nanoparticle or less dense polymer, potentially underestimating total burden; Py-GC/MS is an indirect method that depends on representative pyrolyzates, and weathering, oxidation, and residual biological matrix can complicate spectra. The team’s mitigation strategy — KOH digestion, ultracentrifugation, cyclohexane lipid extraction, filtration, procedural blanks, and spike recoveries — represents the current state of the art, but uncertainty remains, particularly for nanoscale particles whose polymer identity cannot be spectroscopically confirmed.

What the findings make unambiguously clear is that environmental contamination now belongs on the conservation agenda for island species, alongside habitat loss, invasive species, and disease. For a population already squeezed by genetic monotony, climate variability, and a long history of human activity, microplastic accumulation adds a novel and poorly understood risk — one whose health consequences, from neuroinflammation to possible interference with the blood-brain barrier, remain active questions. The team plans to expand the work to other island systems and remote inland locations, to test for co-contaminants such as heavy metals and PFAS, and to explore whether parasite loads, disease prevalence, traumatic brain injury, or infections like H5N1 avian influenza might alter how plastics move into the brain. In the meantime, the image lingers: a fox alone on a far-off island, its isolation no protection at all from a pollutant that has reached the summit of Everest and the floor of the Mariana Trench — and now, the brain of one of Earth’s rarest carnivores.