Soil ingestion: An important contamination pathway for toxic element exposure in wild herbivores

Human activities have increased pollution and toxic element concentrations in soils across the Earth's surface. These activities could have profound implications for wildlife that directly or incidentally ingest soil during foraging, and for humans that consume animals as a primary source of protein. To date, levels of soil ingestion in large herbivores and species-specific vulnerability to toxic element exposure via this pathway have not yet been assessed. Here, we tested the hypothesis that animals feeding closer to the ground ingest more soil than their browsing counterparts. Across a community of 16 African herbivore species, we found that soil ingestion was highest in grazing (blue wildebeest, white rhino and buffalo) and fossorial (warthog and porcupine) animals. This translated to 20-60 % higher total element intake for chromium, cobalt, tin, lead, vanadium and arsenic in these groups. Faecal and fur concentrations were similarly elevated in these species, highlighting that the toxic elements ingested from soil are assimilated into body tissues with potential repercussions for animal and human health. In ungulates, soil ingestion rates align with species-level hypsodonty (height of the tooth crown). We propose that hypsodonty can be used as a proxy for identifying species-specific vulnerability to toxic-element exposure at broad scales. The non-invasive framework developed here, may be applied to monitoring wild herbivore exposure to toxic elements and may be particularly relevant for ongoing conservation and rewilding efforts in areas with high levels of anthropogenic pollution.