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’.

12 Responses to Dolphins and Manatees of Amazonia

  1. lukas says:

    John, the Orinoco is currently connected to the Amazon through the Casiquiare distributary of the upper Orinoco, which flows into the Rio Negro. You can actually see it in the last map you included with your post.

    • John Baez says:

      Okay, wow! I thought the Orinoco and Amazon were truly separate river basins. And I don’t think I’ve ever heard of a ‘distributary’ before! So in theory you could row a canoe up the Orinoco and then back down the Amazon? I didn’t know rivers did that. That might explain how the pink dolphins got from one to the other.

      Another possibility is that both once flowed in the reverse direction, into the South American inland sea… but if the dolphin populations were separate for so long, you’d think they’d be noticeably different by now.

      Another possibility is that sometimes they swim through the Atlantic.

    • John Baez says:

      Hmm, it seems the Orinoco River dolphin is a separate subspecies, and there’s another one separated from the main one by waterfalls:

      I’m just reading a novel about someone who studies the Ganges dolphin! All this is from:

      • Healy Hamilton, Susana Caballero, Allen G. Collins and Robert L. Brownell, Jr, Evolution of river dolphins, Proc. Roy. Soc. Lond. B 268 (2001), 549–556.

      Free online for dolphin fans!

  2. John Baez says:

    It seems my guess about manatees getting trapped in the Amazon may be right:

    Sirenians probably reached South America during the Eocene. According to Domning (1982), the Late Miocene Andean orogeny blocked the oceanic entrances to western Amazonia and manatees trapped in lowland Amazonia subsequenly adapted to Amazonian aquatic habitats.

    This is from:

    • Francisco Ricardo Negri et al, A review of Tertiary mammal fauna and birds from western Amazonia, in Amazonia, Landscape and Species Evolution: A Look Into the Past, Carina Hoorn and Frank Wesselingh, eds., John Wiley & Sons, New York, 2011.

  3. The Amazon river dolphin reminds me of the Yangtze river dolphin or Baiji, “Goddess of the Yangtze”. They seem to have the same long snout, but Baiji was almost blind. The Baiji is functionally extinct since 2006. What a sad damn shame.

    • John Baez says:

      I’ve heard rumors they might still exist… but yes, a shame! From the Wikipedia article you linked to:

      In the 1950s, the population was estimated at 6,000 animals, but declined rapidly over the subsequent five decades. Only a few hundred were left by 1970. Then the number dropped down to 400 by the 1980s and then to 13 in 1997 when a full-fledged search was conducted. Now the most endangered cetacean in the world, according to the Guinness Book of World Records, the Baiji was last sighted in August 2004, though there was a possible sighting in 2007.

      Look at my comment up above for a map of river dolphins.

  4. Jeremy says:

    Here’s something on how manatees spread across the Atlantic, for anyone interested.

    • John Baez says:

      That’s neat! Manatees have been spotted as far north as Rhode Island and Connecticut.

      This post on the same blog is also intereresting. It discusses some newly discovered types of dolphins and manatees in Amazonia:

      Amazonia is home not only to large terrestrial mammals, but to amphibious and aquatic ones too. Most of us are familiar with the fact that freshwater dolphins swim through the flooded forests, and there is also an Amazonian manatee Trichechus inunguis, and the Giant otter Pteronura brasiliensis, the world’s longest (though not heaviest) otter. Marc has three new amphibious/aquatic mammals that might represent new species – a new river dolphin, a dwarf manatee, and a new giant otter. The new river dolphin [shown in adjacent photo] is unique to the Rio Aripuanã and almost certainly belongs to the genus Inia. The question, however, is whether it is conspecific with I. geoffrensis, the better known of the two Boto species*. While both types of dolphin have been seen in the same area, they stay apart, and the Rio Aripuanã form is reported to be more aggressive to people and to differ from the Boto in its timing and style of exhalation. It also looks different from the Boto in being smaller, and in having a less bulbous forehead and shorter jaws. It is never pink like the Boto.

      * The taxonomy of the boto has always been mildly controversial, with various people arguing at times that Inia includes two species (I. geoffrensis and I. boliviensis), while others argue that the two are conspecific. Several recent molecular studies (Banguera-Hinestroza et al. 2002, Martínez-Aguero et al 2006), and an unpublished PhD thesis on cranial morphometrics, have concluded that I. geoffrensis and I. boliviensis should be recognised as distinct species.

      Quite a few new cetacean species have been named within recent years, so Marc’s discovery of a new dwarf manatee, known to locals as the peixe-boi anão, is arguably more significant. Amazonia is already home to one recognised manatee, Trichechus inunguis: the dwarf species differs from it in reaching just 1.3 m and 60 kg (compared to 2.5-3 m and 350-500 kg) and in being deep black (instead of lead grey) [adjacent image shows adult male dwarf manatee. Yes, adult male. It's 1.3 m long]. The two are presumably close relatives (they certainly look more similar to each other than to either of the other two extant manatee species), and this has been confirmed by genetic analysis. A short technical paper on the manatee was submitted to a high-profile science journal, and news on its discovery has been discussed in the popular press. Like the dolphin, the dwarf manatee is restricted in distribution and has only been reported from one tributary of the Rio Aripuanã.

      Finally, the new otter – informally dubbed the Black giant otter – is both morphologically and behaviourally distinct from P. brasiliensis. It is not only darker than this species, but also somewhat smaller. All three of these mammals appear to represent dwarf endemic forms of larger, more widespread species.

  5. Robert says:

    The Orinoco is an example of stream capture in progress –

    It’s a process which happens reasonably often, and allows river-dwelling animals to get into new river systems through the temporary connection. Mostly, we only see the aftermath, but not with the Orinoco

  6. tomatetomate says:

    Wow. I also just found out about distributaries. Before, I was convinced that rivers were trees (in the mathematical sense of the word tree, given that real trees are not trees…)

  7. [...] Pink dolphins!  And manatees, too.  In the Amazon and Orinoco Rivers, there’s an interesting diversity of water-based mammals. [...]

  8. “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’. “

    I’ve heard this before. Apparently (one of the things I haven’t tested myself) is that certain parts of the anatomy of sea cows do enhance the illusion, at least somewhat.

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