If you've watched a giraffe browsing in the tree canopy, a white rhino meandering across open grassland or a warthog shuffling around on its knees in South Africa's Kalahari desert, you know what they eat: leaves, grass, shoots and roots. With every mouthful, they swallow something less obvious - soil.
Some ingest more soil than others, but all are exposed to toxic elements, such as arsenic, lead or chromium.
I study how environmental pollutants affect wildlife health. I led a team that investigated how and why soil ingestion varied between 16 African herbivore species at Tswalu Kalahari Reserve, in South Africa's southern Kalahari desert.
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Ultimately we wanted to identify which species are most vulnerable to toxic elements in the soil - insights that can guide wildlife species reintroductions to re-establish ecological balance at rehabilitation, restoration and rewilding sites.
Over two years we collected surface sediments and vegetation around 25 waterpoints across the reserve to identify which toxic elements were more concentrated in soil. From there, we moved on to the animals themselves. Faeces from each species allowed us to estimate their exposure - what was passing through the digestive system. Fur samples provided a longer-term record of which elements were actually taken up into the body over time.
Using a unique combination of established non-invasive approaches allowed us to trace how toxic elements move from soil, to plants and into wildlife.
Our findings showed that some toxic elements were more concentrated in soils than plants. We also found that animals that eat leaves from higher up in the tree canopy may be less vulnerable to toxic element exposure than those that eat grass or roots and feed closer to the ground. So where they eat, not just what they eat, strongly influences how herbivores are exposed to toxic elements.
Knowing how herbivores interact with soil and soil-borne toxins in their environment allows conservationists to better understand how the natural low presence of toxic elements differs from new contamination levels caused by human activities such as mining, farming, or other land-use changes.
How animals feed determines how much soil they swallow
One of the clearest patterns to emerge was that soil ingestion is different across species.
Burrowing species such as warthogs and porcupines ingest the most soil during digging, burrowing and rooting for food.
Grazing animals, particularly blue wildebeest, white rhinoceros and African buffalo, also stood out. This makes sense because these species feed close to the ground, often uprooting grass or cropping vegetation coated in dust, especially in dry environments like the Kalahari.
Browsers like giraffe that eat leaves, buds and pods from the top of trees naturally ingested much lower amounts of soil.
When eating soil becomes a problem
Soil isn't inert. Its characteristics - including its mineral makeup, how acidic it is, how much decomposing plant and animal material there is and how easily water moves through it - influence toxic element levels in the soil. These factors also influence how toxic elements and nutrients - some of which are necessary for survival - are transferred to plants and animals.
Human activities such as mining, agriculture, industrial emissions, and water management can increase metal concentrations. This puts animals that ingest large amounts of soil, particularly grazers and burrowing species, at greater risk.
In the soils we studied, vanadium, aluminium, lead, chromium, tin, cobalt and arsenic were concentrated. Browsing species, including black rhino, showed low exposure and retention compared to other herbivores. Eland and springbok, which graze in the wet season and browse during the dry season, are known as mixed feeders. Because they don't only eat leaves, they ingest more soil when grazing on grass. This means their exposure to soil-borne elements is higher than that of black rhinos.
Because animals have evolved to cope with natural levels they ingest from soil, the presence of toxic elements in wildlife tissues does not necessarily indicate pollution or ecological harm.
Teeth reveal which species are at risk from toxins in the soil
Grasses naturally contain high levels of abrasive silica, a hard, sand-like mineral that wears down teeth. In dry environments, grass is often coated with fine soil and dust. So herbivores that feed on grass or soil-covered food items, like bulbs and roots, experience greater tooth wear.
Read more: Elephant teeth: how they evolved to cope with climate change-driven dietary shifts
To cope with high abrasion, some species have evolved teeth with a high enamel crown, which reduces wear over time. This specialised tooth structure has been linked to levels of soil ingestion, making it a potentially useful indicator to assess which species are vulnerable to toxic element intake.
When toxic elements are highly concentrated in soil, we found a strong relationship between tooth structure and toxic element levels in faeces and fur. This means conservation managers can use tooth structure to better identify species that are at risk and monitor them more closely.
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Why this matters
Animals are not passive recipients of toxic elements. Their behaviour, anatomy, evolutionary history and environment shape what they encounter and how they cope. Our study shows that soil ingestion is a natural part of herbivore ecology, but not all species face the same level of risk.
Read more: Madagascar: giant tortoises have returned 600 years after they were wiped out
Conservation increasingly focuses on ecosystem restoration and reintroducing large herbivores such as rhino and elephants. Our study sets up a framework that will help prioritise monitoring, flag species most likely to be affected by landscape change, and avoid misinterpreting contamination as natural exposure.
Recognising these differences helps conservation managers make informed decisions grounded in the realities of how animals interact with their environment, right down to the soil beneath their feet.
Andrea Webster, Snr research fellow, University of Pretoria
