Joanna Macur, Institute of Nature Conservation Polish Academy of Science

Non-essential trace elements (TEs) are well-known environmental contaminants that lack a defined biological function in mammals. Although TEs occur naturally, anthropogenic activity remains the primary driver of this pollution. Elements such as mercury (Hg) and lead (Pb) exert adverse effects on living organisms, targeting the nervous and endocrine systems, among others. For omnivores like bears, the risk of TEs toxicity is caused by the bioaccumulation of inorganic toxicants from plant- and animal-based diets. Despite the established neurotoxicity of TEs in wildlife, data regarding their accumulation in the brain and subsequent effects on large terrestrial mammals remain limited. Thus, results are often interpreted using toxicity thresholds from domestic species, laboratory models, or humans. Our aim was to identify all TEs’ neurotoxicity thresholds for wild terrestrial mammals by conducting a systematic literature review in accordance with PRISMA 2020 guidelines. The search yielded 24 articles providing neurotoxicity thresholds exclusively for Pb and Hg in wild mammals. Subsequently, we searched available published data on Pb and Hg levels in brain tissue across all bear species to evaluate their exposure relative to these neurotoxic thresholds. Concentrations in brain tissue were reported only for polar bears (Ursus maritimus; Hg) and brown bears (Ursus arctos; Hg and Pb). Toxicity thresholds for Hg were exceeded in polar bears, with values indicative of possible neurochemical and neurobehavioural impairments, as well as changes in electrical potential and genetic damages. Although Pb and Hg levels in brown bears’ brains did not exceed threshold values, the finding of the highest Pb concentrations in young individuals was concerning due to particularly high risk of adverse effects on developing brain. With threshold data limited to two bear species and two TEs, future effect studies including other contaminants would help assess neurotoxicological risk in bears.