No time for anything new (working on a book chapter and putting the finishing touches to the Tet Zoo book), so here’s this, from the archives. NOT properly updated, so please be aware that it’s more than four years old…
There are three extant manatee species*: Trichechus inunguis of the Amazon Basin, T. manatus of the Gulf of Mexico, Caribbean and US Atlantic coast as far north as Virginia, and T. senegalensis of western Africa. So, how it is that they occur on opposite sides of the Atlantic? [West Indian manatee T. manatus shown here, from wikipedia. This one was photographed in Florida, not in the middle of the Atlantic].
* Since this article was written, a fourth living manatee species has been named: the alleged dwarf, Amazonian species Trichechus pygmaeus [type specimen shown here]. The article naming this species does not yet exist in print (it was published online by primatologist Marc van Roosmalen in 2008 after its rejection from a major journal), and at the moment it does not appear likely that T. pygmaeus will be accepted as valid by other mammalogists (Daryl Domning is on record as stating that it is based on juvenile specimens of T. inunguis: see also the Tet Zoo comment here).
A very old idea explains manatee distribution thusly: as the Americas and the Old World rifted apart in distant geological times, the ancestral manatee species got separated and, presto, a vicariance event resulted in speciation. However, I’m sure I don’t need to tell you how absurd it is, today, to suggest this. The North Atlantic opened something like 100 million years ago, yet manatees (well, those of the extant genus Trichechus anyway) are probably less than 10 my old (there being questionable Trichechus fossils from the Pliocene).
Granted, there are sirenian workers who have, indeed, suggested that Atlantic rifting might explain manatee distribution… but, those workers were publishing their papers in the early years of the 20th century (Arldt 1907). Dispersal is clearly the only option: that is, yes, manatees simply must have crossed the Atlantic at some stage, and a quick check of the literature on manatee evolution reveals many references to this hypothesis [image below shows all modern sirenian species, by the excellent Pieter Arend Folkens (best known to me for the art you see in marine mammal books, but well known to Hollywood as the designer of the whales in Star Trek IV and Free Willy). I can't believe I've never blogged about Steller's sea cow].
Based on a spurious idea about North Atlantic currents, Simpson (1932) thought that manatees migrated from east to west. However, the evidence clearly shows west to east to be more likely; the fossil trichechine phylogenetically closest to Trichechus (Mio-Pliocene Ribodon) is South American, and in fact all fossil trichechines are American; T. manatus and T. senegalensis are more like each other than either is to T. inunguis; the nematode parasites of T. senegalensis seem to be more specialised than the nematodes of T. manatus, and so on.
So, if manatees simply must have crossed the Atlantic to get to Africa, how did they do it? Well, they swam of course, and the really cool thing is that there are also reasons for thinking that this isn’t such a big deal: it is plausible, and in fact it’s supported by strange things that manatees have done in historical times.
Daryl Domning, world expert on sirenian evolution and history, published a paper on manatee evolution in Journal of Vertebrate Paleontology last year [note: I'm referring here to 2005]. In explaining the successful invasion of the African coast by American manatees, he brought in all of the arguments given above, but tied it together with data on Amazonian and Atlantic palaeocurrents. During the Pleistocene, the subtropical North Atlantic gyre was compressed, and a cold current ran along the Eurafrican coast as far south as the Gambia. Boekschoten & Best (1988) explained how this appears to have allowed Caribbean corals and certain molluscs to colonise the eastern Atlantic, and they speculated that manatees might also have used this route. Furthermore, it turns out that an ‘appreciable fraction’ of Amazon River water gets right across the Atlantic as far as Africa, so ‘manatees taking this route might even have access to relatively fresh water for a good part of the journey, if they rode in a large lens of Amazon water’ (Domning 2005, p. 699) [map below shows the distribution of the three living manatee species. In terms of distribution, the African species is the weird one. From wikipedia].
And the trump card? Domning (2005) suggested that purported manatee strandings made on the coasts of the North Atlantic in historical times may really have been genuine. Animals alleged to have been manatees have been reported from the shores of Greenland (1780), Scotland (1801 and 1837) and France (1782). Though we should remain sceptical about these accounts, it’s not implausible that they were genuine. Domning cites a radio-tracked Florida manatee that, in 1995, got as far as Rhode Island [UPDATE: in 2008, a manatee turned up in Sesuit Harbour, Cape Cod, Massachusetts (the individual - 'Dennis' - is shown below). It was captured and was going to be taken to Florida, but died during transit, perhaps because it was already too chilled. Some sources say that manatees are now turning up fairly regularly (every one or two years) off the Rhode Island or Massachusetts coasts].
Here’s another spin on this. A long-standing mystery in the cryptozoological literature has been the purported presence of manatees on St. Helena, in the South Atlantic (not too far south: St. Helena is at the same latitude as Bolivia, Angola, or northern Madagascar). Nobody’s ever really known what these animals were – were they really manatees, or were they actually seals of some kind? The several descriptions provided sound to me like those of pinnipeds: mostly sea lions, and indeed most authors have concluded that this is what the animals were. After reviewing the mystery, Shuker (1995) left the case open, however. While, previously, there were good reasons for doubting the idea that manatees might ever have gotten to St. Helena, our new understanding of manatee dispersal at least renders this idea a remote possibility: in other words, it probably is just about conceivable that manatees could have gotten to St. Helena after all. But I’m speculating to the extreme.
Oh – and if, on hearing about an Atlantic crossing made by manatees you thought of this…
… well: shame on you.
For previous Tet Zoo articles on sirenians see…
Refs – -
Arldt, T. 1907. Zur Atlantisfrage. Naturwissenschaftliche Wochenschift 22, 673-679.
Boekschoten, G. J. & Best, M. B. 1988. Fossil and recent shallow water corals from the Atlantic islands off western Africa. Zoologische Mededelingen 62, 99-112.
DOMNING, D. (2005). FOSSIL SIRENIA OF THE WEST ATLANTIC AND CARIBBEAN REGION. VII. PLEISTOCENE TRICHECHUS MANATUS LINNAEUS, 1758 Journal of Vertebrate Paleontology, 25 (3), 685-701 DOI: 10.1671/0272-4634(2005)025[0685:FSOTWA]2.0.CO;2
Simpson, G. S. 1932. Fossil Sirenia of Florida and the evolution of the Sirenia. American Museum of Natural History Bulletin 59, 419-503.
Shuker, K. P. N. 1995. The saga of the St. Helena sirenians. Animals & Men 4, 12-16.