Dolphins are aquatic mammals belonging to the order Cetacea, specifically the toothed whales (Odontoceti). While the vast majority of dolphin species live exclusively in the oceans, a small, specialized group has adapted to live permanently in river systems. This difference in habitat reflects a profound difference in their evolutionary history and physiological adaptations.
The Marine Majority
The family Delphinidae, commonly known as the oceanic dolphins, accounts for nearly forty species and represents the overwhelming majority of the dolphin population globally. These species are found in saltwater environments across all the world’s oceans and seas, ranging from polar to tropical zones. Most prefer the warmer, shallower waters of the continental shelves, though some inhabit the open ocean.
The common bottlenose dolphin (Tursiops truncatus) is perhaps the best-known example, thriving in temperate and tropical coastal waters worldwide. Other species, such as the common dolphin (Delphinus delphis), are found in large groups across the Atlantic, Pacific, and Indian Oceans. Their dependence on the high salinity of the marine ecosystem shapes their existence, including their diet of fish and squid and their specialized internal biology.
Riverine Specialists
River dolphins are the exceptions that reside solely in freshwater or brackish environments. These species are not a single taxonomic group but are ancient lineages that have evolved unique characteristics to survive in their muddy, flowing habitats. They are restricted to specific river systems across South America and Asia.
The Amazon River Dolphin, or Boto (Inia geoffrensis), found in the Amazon and Orinoco river basins, is known for its flexible neck, which allows it to maneuver through flooded forests and tight river bends. The South Asian river dolphins, which include the Indus River Dolphin and the Ganges River Dolphin, are functionally blind due to the turbid water. These animals compensate by relying almost entirely on echolocation to navigate and hunt for prey. This sensory adaptation is necessary for communication and survival in their opaque surroundings.
Navigating Salinity
The fundamental difference between these two habitats is the osmotic challenge they present, which dolphins overcome through osmoregulation. Marine dolphins live in a hyperosmotic environment, meaning the surrounding seawater has a much higher salt concentration than their body fluids. To prevent dehydration, they do not typically drink seawater. Instead, they obtain the necessary water from the food they eat and from the metabolic breakdown of fat and protein.
Their specialized kidneys are capable of concentrating urine to a salinity level higher than that of seawater, allowing them to excrete excess salt without losing freshwater. Conversely, freshwater dolphins face a hypo-osmotic environment where water constantly tries to enter their bodies while salts attempt to diffuse out. Their kidneys must manage water retention and conserve electrolytes to maintain a stable internal balance. This ability to rapidly adjust urine electrolyte output allows some coastal marine dolphins to temporarily enter river estuaries, though only the riverine specialists can live there permanently.