Dolphins and Manatees of Amazonia

No, these aren’t mermaids. They’re sirenians!

Sirenians or ‘sea cows’ are aquatic mammals found in four places in the world. The three places shown here are home to three species called ‘manatees’:

For example, the sirenians shown above are West Indian manatees, Trichechus manatus, which live in the Caribbean. There’s also a big region stretching from the western Pacific Ocean to the eastern coast of Africa that’s home to the ‘dugong’.

Right now there’s one different species of sirenian in each place. But once there were many more species, and it’s just been discovered that there often used to be several species living in the same place:

• Multiple species of sea cows once coexisted, Science Daily, 8 March 2012.

The closest living relatives of the sirenians are elephants! They kind of look similar, no? More importantly, they share some unusual features. They keep growing new teeth throughout their life, molars that slowly move to the front of the mouth as the teeth in front wear out. And quite unlike cows, say, the females have two teats—located between their front limbs.

Here’s an evolutionary tree of sirenians:

You’ll see they got their start about 50 million years ago and blossomed in the late Oligocene, about 25 million years ago. Later the Earth got colder, and they gradually retreated to their present ranges.

You’ll also notice that three branches of the tree seem to reach the present day:

• Trichechus, which includes all the manatees,

• Dugong, which (surprise!) is the dugong… and

• Hypodamilis, which is another name for Steller’s sea cow.

Steller’s sea cow was discovered in the North Pacific in 1741, and hunted to extinction shortly thereafter. Ouch! It took 24 million years of evolution to refine and polish the information in that species, and it was wiped out without trace in just 27 years.

The Amazonian manatee, Trichechus inunguis, is of special interest to me today because it lives in many branches of the Amazon river:

How did it get there? Why does it live in rivers? Its nearest living neighbor, the West Indian Manatee, likes coastal waters but can also go up rivers. Another clue might be the wonderful Amazon river dolphin, Inia geoffrensis.

It’s also called a pink dolphin. Here’s why:

Their are some interesting myths about it… one of which connects it with the manatee!

In traditional Amazon River folklore, at night, an Amazon river dolphin becomes a handsome young man who seduces girls, impregnates them, and then returns to the river in the morning to become a dolphin again. This dolphin shapeshifter is called an encantado. It has been suggested that the myth arose partly because dolphin genitalia bear a resemblance to those of humans. Others believe the myth served (and still serves) as a way of hiding the incestuous relations which are quite common in some small, isolated communities along the river. In the area, there are tales that it is bad luck to kill a dolphin. Legend also states that if a person makes eye contact with an Amazon river dolphin, he or she will have lifelong nightmares. Local legends also state that the dolphin is the guardian of the Amazonian manatee, and that, should one wish to find a manatee, one must first make peace with the dolphin.”

Indeed, the range of the Amazon river dolphin, shown here, is similar to that of the Amazonian manatee:

Dolphins and other cetaceans are not closely related to sirenians. Dolphins are carnivores, but sirenians only eat plants. But they both started as land-dwelling mammals, and both took to the seas at roughly the same time. And it seems the Amazon river dolphin became a river dweller around 15 million years ago. Why? As sea levels dropped, what once was an inland ocean in South America gradually turned into what’s now the Amazon! According to the Wikipedia article:

It seems this species separated from its oceanic relatives during the Miocene epoch. Sea levels were higher at that time, says biologist Healy Hamilton of the California Academy of Sciences in San Francisco, and large parts of South America, including the Amazon Basin, may have been flooded by shallow, more or less brackish water. When this inland sea retreated, Hamilton hypothesizes, the Amazon dolphins remained in the river basin…

So maybe the manatees did the same thing. I don’t know. But I find the idea of an inland sea gradually turning into a river-filled jungle, and life adapting to this change, very intriguing and romantic!

This shows what South America may have looked like during the early-middle Miocene, when the Amazon river dolphin was just getting its start. The upper Amazon Basin drained into the Orinoco Basin at left, while the the lower Amazon Basin drained directly to the Atlantic Ocean at fight. This is from a paper on megafans, which are huge regions covered with river sediment:

• M. Justin Wilkinson, Larry G. Marshall, and John G. Lundberg, River behavior on megafans and potential influences on diversification and distribution of aquatic organisms, Journal of South American Earth Sciences 21 (2006), 151–172.

Almost needless to say, we’ll need to work a bit to protect the dolphins and manatees of Amazonia if we want them to survive. Check out this Amazon river dolphin in action:

This guy is swimming in the Rio Negro, a large tributary of the Amazon. But there are also Amazon river dolphins in the Orinoco, another huge river in South America, not connected to the Amazon! You can see it just north of the Rio Negro:

Was it ever connected to the Amazon? If not, what’s the story about how the same species of dolphins live in both river basins?

By the way, my joke about mermaids comes from the etymology of the word ‘sirenian’. There’s a legend that lonely sailors—very lonely, it seems—mistook sea cows for mermaids, also known as ‘sirens’.