Microplastic bioaccumulation in brain tissue of an island endemic mammal of conservation concern, the San Nicolas Island fox (Urocyon littoralis dickeyi)

Microplastics and nanoplastics (MNPs) are pervasive contaminants that have been detected across a wide range of environments worldwide. Assessing the extent of MNP contamination in animals is critical for understanding and mitigating potential health risks to both wildlife and humans. Although MNPs are expected to accumulate in multiple tissues of wild and domestic mammals through various exposure pathways, their occurrence, distribution, and biological effects in mammalian species remain poorly understood. Samples of brain tissue from island foxes ( Urocyon littoralis dickeyi ) endemic to San Nicolas Island (SNI) were archived over multiple decades (2003-2023) and provide a temporal record of exposure to environmental contaminants, making this an ideal group for testing MNP bioaccumulation. Fox specimens (n = 34) were assessed using pyrolysis-gas chromatography/mass spectrometry and transmission electron microscopy to document MNP bioaccumulation. We found nylon, polyethylene, and poly(methyl methacrylate) in high concentrations (114 - 1137 µg of polymer/g tissue) in brain tissue. However, there were no significant differences observed across sexes, age classes, or years, suggesting regular exposure to these contaminants over several decades in these insular foxes. Potential anthropogenic sources include beach cast plastic and military infrastructure. Our findings emphasize that MNP accumulation may be widespread, even in remote insular wildlife, and should be monitored as a potential stressor, especially in species of conservation concern.