Old Skulls in a Murky New World

River Dolphins surfacing.  Photo by Cody Sullivan
River Dolphins surfacing. Photo by Cody Sullivan

By Cody Sullivan
BU News Service

While this topic is not in the news, nor is it a particularly large area of scientific study, I have been fascinated by the pink river dolphin’s skull for five years and counting.

My obsession started in the summer of 2009 when I worked in the Amazon as a research assistant on a pink river dolphin study. I loved these dolphins from the moment I first saw one. My skull morphology craze started in 2010 when I learned that pink river dolphins have primitive skulls.

First, I will present a brief introduction about why the shape of a dolphin’s skull matters. Echolocation is a well-known aspect of dolphin life. Dolphins send out bursts of high frequency sound that bounce off objects around them; the timing and directionality of these sound rebounds describe the surrounding environment. Most dolphins tell which direction a sound is coming from because the bones in their skulls are asymmetric. The sounds hit the skull on side sooner than the other, allowing a dolphin to determine which direction it came from. But like I said, only most dolphins have this skull asymmetry, and guess which dolphin doesn’t have an asymmetric skull, Pink River Dolphins!

Pink River Dolphins, or botos, are almost completely blind and live in the muddy Amazon River, and the Amazon’s flooded forests during the wet season – which add another level of navigational complexity with trees, roots, and submerged vegetation. It is plausible to assume that botos navigate through the flooded forests and murky waters by using echolocation as most dolphins do, but their skulls are almost entirely symmetric. By having a uniform skull, botos receive echolocation sound waves at the same time. They shouldn’t be able to tell direction with that skull shape.

The symmetric skull is a very primitive trait compared to marine dolphins and their highfalutin asymmetry. I realize that the two dolphins have been geographically isolated for years upon years, and they have taken divergent evolutionary paths from each other, but why has no parallel evolution occurred? Birds and bats evolved flight separately. It would stand to reason that two closer related species could evolve the same skull shape. River dolphins also have a different ecologic role and environment than marine dolphins, but still, an orienting mechanism is needed. I am so confused by this. I am also confused, confounded and annoyed that there is no answer. What else could help a blind dolphin swim through a flooded forest? I need to know!

There have been two recent studies (this one here, and this one here) on skulls, sound generation and reception, and echolocation in dolphins, both marine and riverine, but neither satisfies me. How does a boto tell left from right?
Someone please answer this question for me!

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