Grazing animals like cattle and sheep may be quietly reshaping the mountain slopes, according to a new study led by the University of Colorado Boulder. The animals' simple foraging behaviors can lead to the formation of elaborate terracettes, the staircase-like ridges that stripe hillsides around the world.
The finding, published July 1 in the Journal of the Royal Society Interface, challenges a long-held assumption that wandering livestock cannot restructure landscapes on such a large scale.
"It's just another example that animals, even the humble and mundane cow, can change our environments in ways we don't realize," said Saad Bhamla , associate professor in the Department of Chemical and Biological Engineering and the BioFrontiers Institute.
Terracettes first piqued Bhamla's interest when his former graduate student, Ben Seleb , described to him the large clusters of steps he encountered during a trip in the Swiss countryside.
For decades, scientists have debated how these patterns arise. Although terracettes are already colloquially known as "cow trails" or "sheep tracks," the prevailing scientific theory has argued that they are actually a result of gravity slowly pulling soil downhill.
Scientists assumed animals couldn't create these remarkably regular patterns alone, because they move randomly and independently across the land— or so it seemed.
As a cow lover and someone who aspires to their peaceful pastoral lifestyle, Seleb, now a postdoctoral engineer at the Rockefeller University in New York, suspected the animals deserved more credit.
"There has been increasing research in animals creating neat patterns in nature, from honeycombs to spider webs. It makes sense to me that cows can do it too. So I took it upon myself to defend the cows' agency," Seleb said.
To test the idea, the team built a computer model to simulate how grazing animals move across a slope.
Just like a real cow, the simulated animal chose its foraging path by constantly weighing two competing priorities: reaching grass while spending the least amount of energy.
The simulation showed the animal naturally preferred to travel on paths with minimal elevation change on a slope, because going uphill requires additional energy, and walking downhill is uncomfortable. The easiest and most efficient route, it turned out, was to walk horizontally across the hillside.
As the cow ate up the grass along one path, it would shift slightly up or down to reach fresh vegetation, carving another path parallel to the previous one.
"Imagine I have to mow a grassy hill by pushing a lawn mower," Seleb said. "I'd rather take these horizontal paths. But once I've already cut one path, I don't need to cut it again. So I'll take the smallest step up or down the slope to mow the next strip."
Over time, these individual but similar decisions made by many cows result in horizontal bands stretching across hillsides seen around the world without any intentional coordination among the animals, the study suggests.
Seleb said that gravity may still play a role in terracettes formation. But grazing animals alone can lead to these natural staircases.
"Sometimes the mundane turns out to be fascinating. We just have to keep an open mind and spend a little time looking," Bhamla said.
