Darwin's 150‑year‑old hillside steps mystery may have a new answer from virtual grazing animals
Steep hillsides and mountainsides in many regions worldwide are often covered in characteristic step-like patterns, also known as terracettes. These repeating landforms have fascinated scientists for more than a century, yet the factors contributing to their formation had not been clearly confirmed until now.
Past studies proposed two distinct hypotheses. The first is that they result from the slow downhill movement of soil and rock. The second is that they are formed from the repeated trampling of grazing animals, such as sheep, cattle and other hoofed mammals.
Researchers at Georgia Institute of Technology and BioFrontiers Institute at the University of Colorado Boulder recently tried to solve this century-old mystery by simulating the movements of animals in natural landscapes and their effects on the shape of hillsides. Their paper, published in Journal of the Royal Society Interface, suggests that even the simplest animal behaviors, such as seeking food while avoiding unnecessary energy expenditure, could produce large-scale landscape patterns like terracettes.
"I first noticed the step-like ridges on a hike in the Alps, while I was staying nearby for the 2024 Konstanz School of Collective Behavior," said Benjamin Seleb, the study's lead author. "I suspected the numerous grazing cows—hard to miss with their clanking bells—had something to do with it, and was already sketching out how I would model their movement before the hike was over. It wasn't until I got back to my hotel and started digging into the topic that I learned about the contested origins of these terracettes. With my curiosity piqued, I got to work building a simple model to ask whether grazing animals really could reshape mountains."
Tackling an unsolved mystery that also intrigued Darwin
Interestingly, the question of how terracettes are formed also fascinated renowned naturalist Charles Darwin more than a century ago. In a book titled "The Formation of Vegetable Mould, through the Action of Worms, with Observations on their Habits", published in 1881, Darwin describes the same step-like ledges investigated by Seleb and his colleagues.
"Little horizontal ledges, one above another, have been observed on steep grassy slopes in many parts of the world," Darwin wrote in his book. "Their formation has been attributed to animals traveling repeatedly along the slope in the same horizontal lines while grazing, and that they do thus move and use the ledges is certain; but Professor Henslow (a most careful observer) told Sir J. Hooker that he was convinced that this was not the sole cause of their formation."
The puzzling ledges that Darwin pondered also attracted the attention of Seleb, a Ph.D. student supervised by Saad Bhamla. In collaboration with Louis González and Atanu Chatterjee, Seleb and Bhamla set out to further explore the possibility that terracettes could be formed by the movement of hoofed animals.
"Darwin described these same horizontal ledges on steep grassy slopes, and he was certain grazing animals used them, though his old Cambridge mentor John Henslow wasn't convinced animals were the whole story," said Bhamla.
"Darwin had his son Francis measure a chalk slope near Lewes (southern England), where Francis counted about 30 parallel ledges running more than 100 yards (91 meters) down a 40-degree slope. Steep ground like that is coincidentally where our model produces its most regular terracettes. So, in a way, we spent a fair bit of simulations confirming something Darwin was already puzzling about nearly 150 years ago. I find that so cool."
Simulating how animals move across changing landscapes
To explore the possibility that animals prompt the formation of terracettes, Seleb, Bhamla and their colleagues used computational tools to simulate the movement of individual animals. In their simulated model, the movement of animals followed a series of simple mathematical rules.
"Each virtual grazer wants two things: to eat, and to spend as little energy as possible getting around," explained Bhamla. "Climbing burns energy, so on a steep slope the energetically efficient strategy is to walk across the hill instead of straight up it or down it. Then there is the feedback. Each footstep packs the soil down and eats the grass a bit, which changes the ground and tips where the next animal is likely to step. Nobody's coordinating these footsteps, so there is no blueprint."
The researchers let the simulated animals loose and simply observed how their movements affected the shape of hillsides. They found that after the animals took thousands of steps, their movements tended to shift more along sideways paths. As a result, slopes gradually changed shape and started exhibiting stair-like patterns.
"We then came up with a simple 'coherence' score to put a number on how ordered the pattern was and stacked our simulated slopes up against real photos of cow and goat paths from around the world," said Bhamla.
When simple behaviors create complex natural patterns
The model created by this research team suggests that collective movements of animals can, in fact, produce step-like patterns on hillsides. It also indicates that the formation of these patterns occurs via a process known as stigmergic feedback. This process occurs when living organisms indirectly influence each other's behavior by modifying their shared environment over time.
"People have argued about terracettes for over a century," said Bhamla. "Some say they're purely geological, just slow soil creep and slumping. Others say animals create them, and the standard criticism for the animal-origin camp is that the animals wander around randomly, so how could they ever carve something this regular? What we show is that they can and that this regularity emerges out of that chaotic, random movement, not in spite of it, through the back-and-forth between the animals and the ground they're chewing up with their mouths and packing down with their feet."
The insights gathered by Seleb, Bhamla and their colleagues could potentially inspire new research exploring how terracettes are formed. In addition, other researchers could use similar computational modeling approaches to study how animals or other living organisms influence the shape of other natural landscapes.
"As far as we know, this is the first model that ties what happens under a single hoof to the pattern across a whole hillside. It's also a good reminder that animals are constantly reshaping the ground under them," said Bhamla. "The same basic physics turns up in ant trails and the footpaths people wear across a lawn—I see this every day on my CU Boulder campus as I walk home after work."
Written for you by our author Ingrid Fadelli, edited by Gaby Clark, and fact-checked and reviewed by Andrew Zinin—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you.
Publication details
Benjamin Seleb et al, Moo-ving mountains: grazing agents drive terracette formation on steep hillslopes, Journal of the Royal Society Interface (2026). DOI: 10.1098/rsif.2025.0451.
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Citation: Darwin's 150‑year‑old hillside steps mystery may have a new answer from virtual grazing animals (2026, July 10) retrieved 12 July 2026 from https://phys.org/news/2026-07-darwin-150yearold-hillside-mystery-virtual.html
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