Saturday, January 28, 2006

Those Transatlantic manatees

It was on the advice of Phil Budd, chairman of the Southampton Natural History Society, that, some weeks ago, I attended a talk on sirenians (manatees and dugongs) at the National Oceanographic Centre. And, without intending offence to Phil, I was not happy: like people who have seen Battlefield Earth, I left the lecture wanting my valuable time returned to me in full. While I most certainly do not want to get rude about the speaker or his talk, he unfortunately provided a review of sirenian biology that was unoriginal, boring and thoroughly outdated and inaccurate. Example? Well, here’s the best/worse bit….

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? The speaker explained it thus: 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. It was at this stage that I could only look on slack-jawed in disbelief. 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 theory.

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 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 (and I’ll return to that in a minute). Daryl Domning, world expert on sirenian evolution and history, published a paper on manatee evolution in Journal of Vertebrate Paleontology last year. 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 to as far south as the Gambia. Boekschoten and Best (1988) explained how this appears to have allowed Caribbean corals and certain molluscs to have colonised the eastern Atlantic, and they also speculated that manatees also 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).

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 – as Domning noted – it is not implausible that they were genuine, and Domning cites a radio-tracked Florida manatee that, in 1995, got as far as Rhode Island.

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 reviewers 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. To those of you that will be wondering, this explains my comments in my soon-to-appear review of Michael Newton’s Encyclopedia of Cryptozoology: A Global Guide.

At top, Will is holding a toy manatee in the bathroom sink. Best I could do without a real manatee to hand :)

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. P. 2005. Fossil Sirenia of the West Atlantic and Caribbean region. VII. Pleistocene Trichechus manatus Linnaeus, 1758. Journal of Vertebrate Paleontology 25, 685-701.

Simpson, G. S. 1932. Fossil Sirenia of the Floria 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.


Anonymous Anonymous said...

Thanks for this! I'm doing a research, erm, painting on the mangrove forests of Madagascar, and i found this both helpful and fascinating. I'm looking forward to the Newton review.

7:11 AM  

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