Nick Garbutt was leading a photography tour in the tropical wetlands and flooded savannas of Brazil’s Pantanal region when he spotted a dark shadow in the brush. It was a jaguar (Panthera onca), and the cat was stalking a family of capybaras (Hydrochaeris hydrochaeris) munching plants at the edge of the Paraguay River. Although jaguars are agile swimmers and have been known to prey on turtles, crocodiles, and even dolphins, one of their favorite foods is capybara—the world’s largest rodent, about the size of a full-grown gray wolf.

In places like the Pantanal where jaguar populations are particularly dense, capybaras tend to stick close to rivers and other bodies of water. Even though jaguars don’t mind getting wet, capybaras have a greater chance of swimming to safety than scurrying away on land. The semi-aquatic rodents have webbed feet to propel them through water, and—as German explorer Hans Staden described in 1557—“when anything alarms them, they flee into the water toward the bottom.” Capybaras can hold their breath for up to five minutes, and are so well adapted to their aqueous habitat that they’ve been filmed trotting along river bottoms.

As Garbutt watched from a boat, the jaguar crept closer to the capybaras. About 60 feet (20 meters) away, the predator paused. “Suddenly,” Garbutt recalls, “the jaguar rushed, but the capybara were fractionally quicker, leaping immediately into the water and diving beneath the surface.”

The jaguar didn’t follow, and the capybaras eventually resurfaced in a neat line, the mother loudly barking her alarm. Capybaras’ eyes, ears, and nose are all perched high on their heads, allowing them to stay mostly submerged with just the crowns sticking out like periscopes. These capybaras stayed in the water, while the jaguar crouched on land, watching.

“It was an amazing little snippet of behavior to witness,” Garbutt says. It was also an example of the ways in which predator-prey relationships shape the ecosystems of the Pantanal and beyond. In places where jaguars have been extirpated, for example, capybaras venture further from waterways to forage, and have been known to reach such high population densities that they’re sometimes considered a nuisance. The lack of jaguars is partially to blame—or thank—for the hundreds of capybaras that have “invaded” a gated community and torn up manicured lawns near Buenos Aires, Argentina. Only there, when capybaras face off against residents’ pet dogs, there are few places left where the herbivores can dive to safety.