The biggest freshwater predator in South America is the mighty black caiman, which vies with the American alligator as the largest member of the alligatorid branch of the crocodilian family tree. Reaching lengths of at least 4.8 metres (16 feet), it sits pretty at the top of this Neotropical aquatic food web, and presumably inspires plenty of respect from all fellow dwellers of the rivers, oxbow lakes, and wetlands it prowls.
This drone footage taken last November from Brazil’s Itapará River, therefore, might be deemed a bit surprising:
It shows a bouto—an Amazon River dolphin, known for the pinkish hide it develops in adulthood—brazenly nipping the tail tip of a good-sized—and comparatively good-mannered—black caiman.
The bouto is the largest of the world’s freshwater dolphins, reaching some 1.8 to 2.7 metres (six to nine feet) in length and weighing up to 158 kilograms (350 pounds). Inhabiting a broad range in northern and central South America, it cruises both main river channels as well as smaller tributaries and seasonally flooded forest. Huge flippers help it navigate shallows and maze-like floodwaters, and echolocation allows it to zero in on mostly bottom-dwelling prey—catfish and other piscivorous fare as well as crustaceans—amid the murk.
Not much is known about predation on the bouto, although the black caiman has certainly been suggested as a potential threat, as has the jaguar. There is, in fact, this footage showing an attack by a black caiman on a river dolphin:
As such, perhaps the bouto in the Itapará River drone video—which also has at least one companion—was engaging in the sort of predator-mobbing behaviour we see in many different organisms, from songbirds harassing owls to pilot whales ganging up on orcas.
Dr. Adam Rosenblatt of the University of North Florida, who’s studied both American alligators and black caimans, told me by email that occasional caiman predation on boutos occurs, but doubts that it's common. While the bouto in this footage may indeed be harassing a potential predator—or, hey, simply goofing around—it’s possible there was another impetus. Rosenblatt pointed out that the caiman has a tag on its tail, suggesting it’s been captured and marked in a scientific study. “The dolphin appears to nip the tail at the exact point where the tag is,” he said, “so maybe the dolphin was being curious about this foreign item it doesn’t normally see.”
He also noted that black caimans, formidable as they are, can be spooked by gangs of giant river otters, rather feisty top-level cooperative hunters that may confront caimans to protect their kits and, perhaps, when compelled by competition. Boutos are usually seen solo or in pairs—river dolphins overall tend to be less sociable than their oceanic relatives, including those that also range into freshwater habitats—but may gather to a dozen or more in the low-water season.
It’s not clear whether multiple dolphins might represent at least a “pestering” threat to a caiman. “It’s well known that even large caiman will actively avoid packs of river otters,” he said, “so maybe the same is true for groups of dolphins.”
Regardless of the significance of the interspecific behaviour captured in the drone video, it’s certainly a fantastic bird’s-eye look at two of the most charismatic freshwater beasts in the Neotropics. While populations of the black caiman were majorly hammered in the 20th century by the hide (and, to a lesser extent, meat) trade, the crocodilian has shown a cheering recovery in many parts of its range, though information gaps as to its status in some areas definitely remain. The IUCN Red List, meanwhile, classifies the bouto as Endangered and considers numbers to likely be on the downslide, with significant threats including intentional killing of river dolphins for use as bait as well as bycatch mortality.
Header image: Allan Hopkins
Whales and Dolphins
Animal Behaviour
Whales and Dolphins
