Why Do Animals Walk Miles to Eat Dirt? Something Is Missing From the Grass
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Herbivores travel long distances to particular spots of exposed earth and eat the soil there. The behaviour looks strange and solves a genuine shortage in their diet.
What the animals are short of
Plants take up the minerals they need and largely ignore the rest, so an animal eating nothing but plants receives a diet that reflects plant requirements rather than its own. Sodium is the clearest case, since plants need very little and animals need a great deal for nerve signalling, muscle function and maintaining the fluid balance of the body, and a diet of leaves and grass supplies far less than a mammal requires. Calcium, magnesium, phosphorus and several trace elements can also fall short depending on the local rock. The animal has no way to manufacture any of them and must find them somewhere.
Where the deficiency is worst
The need varies enormously by place and by season:
- •Inland areas far from sea spray, which supplies sodium near coasts
- •Ground on heavily weathered rock, where soluble minerals have leached away
- •Wet tropical regions, where rainfall removes minerals from the soil
- •Spring growth, which is watery and low in minerals
- •Females in late pregnancy and while feeding young, who need far more
- •Males growing antlers, which takes a large quantity of calcium
What the animals actually do
The behaviour goes well beyond licking exposed rock and takes forms that surprise people. Elephants in Kenya enter caves on Mount Elgon in darkness and use their tusks to break mineral-rich rock from the walls, and the caves have been enlarged substantially over a very long period by exactly that. Parrots and pigeons across the Amazon gather in large numbers at exposed riverbank clay. Mountain goats climb dam walls to reach mineral deposits. Deer chew bones and shed antlers, which supplies calcium and phosphorus directly. Gorillas and other primates eat soil, and analysis of the soils chosen shows they are selected rather than taken at random.
The artificial versions
Providing the minerals deliberately is routine in farming and in wildlife management, and the practice raises issues the natural version does not. Livestock are supplied with compressed blocks containing salt and trace elements, which is standard husbandry and prevents deficiency diseases that were once common. Deer managers put out blocks to improve antler growth and body condition, and hunters use them to draw animals to a chosen spot, which is illegal in a number of jurisdictions on fairness grounds. The concentration of animals at any such site also concentrates disease transmission, and blocks have been implicated in the spread of several wildlife diseases, which is why some agencies discourage them.
The other explanation
Mineral shortage is not the only reason animals eat earth, and the competing account has good evidence behind it. Clays of certain types bind toxic compounds in the gut and carry them out, which matters to animals eating plants defended by bitter and poisonous chemicals or seeds containing them. Parrots at Amazon clay banks eat seeds that other birds avoid, and the clays they choose are effective binders, which supports that reading. The two explanations are not exclusive, since a clay bank can supply sodium and bind toxins at once, and current work generally tests which effect dominates at a particular site rather than treating them as rival theories.
The takeaway
Plants take up what plants need, so a herbivore diet is short of sodium and can be short of calcium and other minerals, especially inland, on weathered rock and during pregnancy or antler growth. Elephants mine cave walls with their tusks and parrots gather at river clay. Certain clays also bind plant toxins in the gut, and the two explanations frequently apply at the same site.