No freshwater baleen whales

Odontoceti (toothed whales) have successfully colonized freshwater: Yangtze River Dolphin, South Asian Dolphin, and several South American river dolphins. We also see a transitional form, the Irrawaddy Dolphin, which lives in coastal waters and also ascends into rivers.

No Mississippi Dolphin, oddly enough. Considering the habitat of the other river dolphins, the Mississippi would seem to be a suitable river.

But the Mysticeti (baleen whales) are entirely marine. Could it be a size constraint? The largest of the river dolphins is the Boto, which at 8 feet is bigger than some of the marine dolphins. Whereas the smallest baleen whale is the Pygmy Right Whale, which becomes sexually mature at 16 feet and full-grown at 20 feet – more than twice the size of the Boto. The next-smallest, the Minke Whale, is 27 feet (males) or 29 feet (females). It may be that this is simply too big to live in even big rivers like the Amazon.

So the question is, could a baleen whale smaller than the Pygmy Right Whale be biologically viable? Is a baleen whale as small as a dolphin possible?

Or, perhaps it is a food contstraint. River dolphins and Baikal Seals are fish eaters, as are the Bull Sharks regularly found in Lake Nicaragua; is this a necessary prerequisite for them to live in freshwater? Suppose a baleen whale could enter freshwater. Could a freshwater environment produce enough plankton for a filter feeder of that size?

In short, are there environmental barriers to the evolution of a freshwater baleen whale, or is their absence merely an accident of evolution?


Feeding strategies employed by rorquals are more energetically efficient in larger animals. A recent study by researchers at UC Santa Cruz looked at apects of this question and determined that a minke whale is at the lower limit of size at which a lunge feeding whale could make a living.

I expect that even large rivers would have a hard time producing enough plankton to sustain a viable population.


What would they eat? Rivers don’t have krill equivalent to feed even a dolphin-sized animal.

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