Most people associate whales with the vast, open ocean. While this holds true for many species, some cetaceans have adapted to life in rivers. Though large ocean whales in rivers seem improbable, a unique group of mammals thrives in inland waters, expanding our understanding of where these creatures can live.
The Freshwater Whales of Our Rivers
While not whales in the conventional sense of massive marine animals, “freshwater whales” exist in rivers and are known as river dolphins. These creatures are a distinct group of toothed whales (Odontoceti), generally smaller than marine dolphins, with slender bodies adapted for riverine habitats.
Five recognized extant species of river dolphins are found in major river systems across South America and Asia. Examples include the Amazon River Dolphin (boto or pink river dolphin) in the Amazon and Orinoco basins, the Ganges River Dolphin (susu) in the Ganges and Brahmaputra systems, and the Indus River Dolphin (bhulan) in Pakistan’s Indus River.
The Yangtze River Dolphin (baiji) from China’s Yangtze River is considered functionally extinct. These animals represent ancient lineages that ventured from marine environments into freshwater, evolving distinct characteristics over millions of years.
How River Dolphins Adapt to Inland Waters
River dolphins have adaptations enabling them to thrive in unique freshwater environments, often featuring murky waters and complex underwater landscapes. Unlike oceanic relatives, many river dolphin species have small eyes or reduced functional eyesight, suited for low-visibility conditions. To compensate, they rely on highly developed echolocation, emitting sound waves to navigate, locate prey, and perceive surroundings.
Their physical structure also shows specific adaptations for river life. Many river dolphins feature long, slender beaks with numerous teeth, ideal for probing river bottoms and catching fish or crustaceans. An adaptation is their unfused neck vertebrae, granting them exceptional neck flexibility. This allows them to turn their heads nearly 90 degrees, crucial for maneuvering through submerged trees and winding river channels. Species like the Ganges River Dolphin are known to swim on their sides, using a flipper to trail along the riverbed as they search for food.
Conservation Status and Habitats
River dolphins inhabit some of the world’s most densely populated and anthropogenically impacted river basins. The close proximity of human populations to these freshwater environments poses significant challenges to the survival of river dolphins.
These unique cetaceans face numerous threats that have severely impacted their populations. Habitat degradation due to human activities is a major concern, encompassing issues such as pollution from industrial and agricultural runoff, including mercury contamination. The construction of dams and barrages fragments river systems, isolating dolphin populations and disrupting their movements and access to food sources.
Additionally, river dolphins are highly vulnerable to bycatch, becoming accidentally entangled in fishing nets, which leads to mortality. Increased shipping traffic also poses a threat through collisions and noise pollution. Consequently, most river dolphin species are currently classified as endangered or critically endangered by conservation organizations.