Virtual Herd of Cows Solves Century-Old Mystery of Terracettes on Slopes That Puzzled Darwin

Stepped stripes on steep grassy slopes can be found in the Alps, on pastures in the United Kingdom, and in many other regions worldwide. These rows of narrow horizontal ledges stretch one above another and resemble miniature terraces. Scientists have debated their origin for more than a century. A new computer model has now shown that a regular pattern can be created by grazing animals, even though no one lays out routes or directs the herd’s movement.

Such landforms are called terracettes. They typically appear on steep slopes as repeating steps that run almost parallel to the horizontal. Researchers have proposed two main explanations. The first suggests that the surface changes due to slow downhill creep of soil and rock fragments. The second links the ledges to sheep, cows, goats, and other ungulates that repeatedly walk along the same areas during grazing over many years.

Neither hypothesis had long provided a complete answer. Soil movement can indeed alter slope shape but does not always explain the regularity of the stripes. Animals, by contrast, constantly leave trails, yet their movements appear too random to produce dozens of nearly parallel lines.

A computer model that simulates the behavior of individual animals on a changing slope helped clarify the mechanism. The authors did not attempt to pre-define a ready-made network of paths. Virtual cows or sheep were given only two simple tasks: find food and expend as little energy as possible on movement.

On a steep slope, climbing requires more effort than moving across the slope. Descending can also be unstable, especially on loose soil. Therefore, the virtual animals gradually selected routes along the slope rather than straight up or down. This direction reduced energy costs and allowed them to stay longer near available grass.

Every step simultaneously modified the surface. Hooves compacted the soil, and animals consumed part of the vegetation. After one animal passed, the next was more likely to choose the already trampled section because movement there required less effort. Repeated passages further reinforced the path and increased the probability that others would use it.

After several thousand steps, the initially random movements began to form stable horizontal routes. First, isolated trampled patches appeared; then the lines lengthened and leveled out. The surface gradually acquired a stepped shape resembling real terracettes.

The animals did not coordinate their actions or follow any common plan. Order emerged through continuous feedback between the herd and the surface. Each individual altered the environment, and the changed environment influenced the route choices of the next animal.

This mechanism is known as stigmergy. The term describes indirect interaction through traces left in the environment. An ant, for example, lays a route using chemical markers, and subsequent individuals reinforce the most convenient path. On a pasture, the role of markers is played by trampled soil, grazed grass, and small relief irregularities.

The authors developed a separate metric to measure the orderliness of the resulting lines. They then compared the virtual slopes with photographs of cow and goat trails from various countries. The model produced the most regular steps on the steepest sections, where moving vertically was especially costly for the animals.

The connection between grazing and horizontal ledges was already discussed by Charles Darwin. In his 1881 book on the role of earthworms in soil formation, the naturalist described small horizontal platforms arranged one above another on steep grassy slopes. Darwin was convinced that animals walked along these ledges while feeding, but he allowed that trampling alone might not fully explain their formation.

To verify his observations, Darwin enlisted his son Francis. Francis studied a chalk slope near the English town of Lewes and counted approximately 30 parallel ledges. The stripes extended more than 91 meters along a slope with an incline of about 40 degrees.

Nearly 150 years later, computer calculations have shown that precisely on such steep sections, simple behavioral rules produce the most regular pattern. The more energy required for climbing, the more frequently animals move across the slope. Repeated passages gradually turn a convenient route into a stable ledge.

The study does not claim that all terracettes result exclusively from grazing. Slow soil creep, moisture, soil composition, vegetation, and slope angle can also influence surface shape. The model addresses a different question: it demonstrates that the chaotic movement of ungulates by itself can generate a large-scale, ordered pattern without pre-existing trails.

The results answer the main objection to the animal-origin hypothesis of terracettes. Random movements do not prevent the formation of even stripes. On the contrary, repeated decisions by individual animals gradually strengthen some routes and weaken others until only a few stable lines remain on the slope.

The authors link the same principle to other landscape changes. Ants create paths through forest litter, humans trample shortcuts across lawns, and herds compact soil on pastures. In all cases, a single step is almost imperceptible, yet thousands of repetitions alter the surface over a large area.