The Amazon River dolphin, known locally as the boto or the pink river dolphin, is a unique freshwater cetacean inhabiting the vast river systems of the Amazon and Orinoco basins in South America. This species has adapted to life in the complex, often murky waters of these river networks and their tributaries. The boto is the largest species of river dolphin, with males sometimes reaching lengths of 2.5 meters and weights up to 185 kilograms. Its feeding ecology directly reflects the immense biodiversity found within its aquatic home.
Primary Prey Categories
The Amazon River dolphin is an opportunistic and generalist feeder, possessing one of the most varied diets among all toothed whales. Analyses show they consume at least 43 to 53 different species of prey from up to 19 different families. The size of the prey they target is wide-ranging, typically averaging around 20 centimeters in length, but sometimes including fish as small as 5 centimeters or as large as 80 centimeters.
Fish constitute the vast majority of their diet, with a preference for species from the croaker (Sciaenidae), cichlid (Cichlidae), and characin (Characidae) families. They consume challenging prey like piranhas and armored catfish, which many other predators avoid. The ability to crush and consume these tougher fish is a significant factor in their dietary success.
Beyond fish, the boto’s diet regularly includes other aquatic life. They feed on crustaceans, such as freshwater crabs and shrimp, especially during certain seasons. Their powerful dentition also allows them to crush the shells of smaller river turtles, such as Podocnemis sextuberculata.
Specialized Hunting Techniques
To successfully capture such a diverse range of prey in the low-visibility environment of the Amazon, the boto employs several physical and behavioral adaptations. Their primary tool is echolocation, a biological sonar system they use to navigate and locate prey by emitting clicks and interpreting the returning echoes. This allows the dolphins to create an acoustic map of their surroundings, effectively “seeing” through the turbid water where eyesight is limited.
A unique anatomical feature that aids hunting is the lack of fused cervical vertebrae in their neck, allowing their head to turn up to 90 degrees. This flexibility is crucial for maneuvering through dense habitats like flooded forests and capturing agile prey hiding among submerged tree roots. They also use their elongated snout to probe the riverbed and crevices for bottom-dwelling prey.
Their dentition is highly specialized, unlike the uniform conical teeth of oceanic dolphins. The boto possesses heterodont teeth, which means they have both sharp, conical teeth in the front for grasping prey, and flatter, molar-like teeth in the back for grinding and crushing. This adaptation enables them to pulverize the shells of crabs and turtles and break up the tough armor of certain catfish species. Although often solitary feeders, they sometimes coordinate with other species like giant otters or take advantage of disturbances created by boats to catch disoriented prey.
Habitat and Seasonal Influence on Diet
The feeding habits of the Amazon River dolphin are influenced by the annual fluctuations in the water level of the Amazon and Orinoco basins. This hydrological cycle dictates the availability and concentration of their prey, leading to seasonal shifts in their diet.
During the high water or flood season, the river spills into the surrounding floodplain, known as igapós or várzea forests. The dolphins follow this expanding habitat, gaining access to fish dispersed into the newly flooded areas. The diet during this period tends to be more varied because fish are spread out and more difficult to catch.
As water levels drop during the dry season, the dolphins are restricted to the main river channels, lakes, and deeper areas. This concentration of water also concentrates the fish population, making hunting more efficient. Prey density is higher, and the dolphin’s diet becomes more selective, focusing on the most abundant or easily accessible species. Diet composition can also vary depending on the geographical location, such as whether the dolphin is in the Amazon or Orinoco basin.