More on swimming tortoises

May 8, 2015 • 1:30 pm

by Greg Mayer

Dennis Hansen, our Aldabra correspondent, sent Jerry a very nice video of a swimming Aldabra giant tortoise (Aldabrachelys gigantea). This immediately brought to mind what is, in my view, one of the most important recent papers in biogeography, “The first substantiated case of trans-oceanic tortoise dispersal” by Justin Gerlach, Catharine Muir, and Matthew Richmond.

In the paper, they report an Aldabra tortoise that came ashore on a beach in Kimbiji, Tanzania in 2004. After considering several possibilities, they conclude that the tortoise had floated in from Aldabra, over 700 km away across the Indian Ocean. The copious growth of large barnacles on the limbs and lower parts of the carapace certainly suggested that the tortoise had spent a considerable amount of time in the sea.

The Aldabra tortoise at Kimbiji, shortly after its discovery in December 2004. Photograph: C. Muir. Figure 1 of Gerlach et al. (2006).
The Aldabra tortoise at Kimbiji, shortly after its discovery in December 2004. Photograph: C. Muir. Figure 1 of Gerlach et al. (2006).

Dennis’s video shows that the tortoises enter water, and how they move about in the shallows. The Kimbiji tortoise, despite the ability to swim, could not have swum to the main, but was rather mostly carried by the currents, and presumably spent its time at sea keeping its head, and most of the dome of its carapace, above water. It was, however, walking ashore, apparently intentionally, when found.

So why is this important? It has long been supposed that animals and plants get to oceanic islands by what Darwin called “occasional means of transport“: carried along on logs, masses of vegetation, ice floes, attached to birds, or floating by themselves in the water. (Darwin carried out a series of experiments on the ability of various plant seeds to float in sea water, and their ability to germinate after varying periods of immersion.) Tortoises have usually been thought to float by themselves because of the difficulty they would have in clinging to vegetation, and also because the practice of mariners of earlier centuries of putting giant tortoises in the holds of their ships as a living food supply had shown that tortoises could live for many months without food or water.

Although Darwin and many subsequent zoogeographers (e.g., P.J. Darlington) invoked such occasional means of transport, there has always been a school of thought arguing that such crossings of the ocean by land animals were nigh impossible, and that the presence of non-flying land animals on an island implied a past land connection. In the first half of the 20th century, this school constructed speculative land bridges crisscrossing the oceans, in order for every island animal to have had a dry-shod passage to the island from its home of origin. In the later 20th century, with the development of plate tectonics, the land bridge builders were succeeded by drift enthusiasts, who thought drifting crustal plates could serve to bring oceanic islands into juxtaposition with continents, so that, again, animals might get to islands without having to cross water (or at least not much). (The drift enthusiasts had the advantage that continental drift actually does occur, even if not in the exact plate configurations they hoped for, whereas the land bridge builders’ long, thin isthmuses crossing abyssal oceans have not been borne out by geology.) So, the more or less direct observation of transoceanic crossing by an occasional means of transport provides a crucial link—a vera causa—in the argument for the occurrence and importance of such means in the colonization of islands.

Some younger biologists, raised (and properly so!) on plate tectonics, and perhaps lacking acquaintance with older literature and island organisms in the field, had taken the drift enthusiasts’ claims too much to heart, and seemed to be unaware of the importance of transoceanic dispersal. Now that molecular phylogenies often bolster the argument for the importance of occasional means of transport, they seem a bit surprised to find out that there indeed has been a lot of ocean crossing, not just to oceanic islands, but between continents and continental islands as well. We discussed one such case here on WEIT, the ratite birds, where what had seemed to actually be a good case for continental drift seems to actually involve a fair amount of oceanic crossing. The zoologist Alan de Queiroz has written a popular book on the biology of oceanic dispersal, and the sociology of its rediscovery by some biologists.

de Queiroz, A. 2014. The Monkey’s Voyage: How Improbable Journeys Shaped the History of Life. Basic Books, New York.

Gerlach, J, C. Muir and M.D. Richmond. 2006 The first substantiated case of trans-oceanic tortoise dispersal. Journal of Natural History 40(41–43): 2403–2408. pdf

50 thoughts on “More on swimming tortoises

  1. A tortoise walking a beach 700 miles from its island habitat, with barnacles growing all over its lower surfaces. You can’t get much more ‘smoking gun’ than that.

    1. Though it would seem to be extremely rare for a species like this tortoise to “take hold” since both sexes would have had to make the same journey. Either way, it is indubitable proof that it can happen.

  2. What I don’t understand is how there could be enough of the transplanted population of a species to breed, thrive and multiply. How many members of the group would you need in order to produce a thriving community in a new land (this will, I imagine, depend on the species)? And the contingency of all this, which de Quieroz’s blurb mentions, is so much greater, the more members of the species you need to initiate the expansion of the colony.

    Thanks a lot for this Greg. As you can tell, I’m not a scientist, but transoceanic dispersal, for some reason, floats my boat. x

    1. Hampton Carson, a mid-career(sabbatical) mentor/colleague proposed that isolated populations resulting from “founder” events can experience reduced selective constraint due to an “empty” habitat (relative to their specific niche), so the usual problems with inbreeding and small population size may be less critical. In such cases, a single founder individual might establish a viable population if it is a female in a species that stores sperm after mating (e.g., Drosophila, which Hamp studied) or with internal fertilization and some sort of gestation period (i.e., a lag between fertilization and birth or egg laying). Would tortoises fall in the latter category?

    2. In the case of tortoises, wouldn’t the minimum number be one pregnant female?

      1. And Joe’s comment above.

        [My browser caching seems off today, the comments comes in haphazardly. I didn’t need to comment at all!]

    3. If you think about a relatively long-lived species, then one arrival on the coast every half-generation (or so) may well be enough.
      Once you’ve got your initial (and probably severely inbred) population, then the intermittent arrival of one new package of genes can greatly change the status of a population, as the new genes spread like, umm, 2wild oats”?
      There have been some very interesting whole-group genetics studies of marginal populations like wolves in South Sweden, where the one self-propelled sperm sample can grandfather essentially every pup two generations down the line.
      It may be surprising, but the genetics are pretty clear.

  3. Interesting observation. That tortoise must have been in the water for a long time to get then many barnacles! I’ll bet it was ‘intentionally walking to shore’! how long would such a crossing take? it would seem that some organisms are more suited than others to long crossings.

    1. They sure are. The species that are native to volcanic islands provide a list of the kinds of critters that can make the journey. They are for the most part only those that can fly, swim, or drift over long distances of ocean. So we do see far flung islands populated with birds, reptiles, and insects. We do not see native species of freshwater fish, amphibians, or mammals (except bats) on such islands.
      I learned this from Jerry’s book (WEIT), long ago.

    2. In the paper, Gerlach et al. suggest it took 6-7 weeks based on the size of the barnacles. Based on currents, it would have taken less time, but winds and other events could have extended the time and impeded a direct crossing, giving the barnacles time to grow.


  4. Greg, good piece. Perhaps this is where sperm storage and delayed fertilization by the females (I guess turtles can do that?), coupled with temperature dependent sex determination, which is probably very common in turtles, could enhance a “founder effect,” especially if it had happened more than once for a long-lived species, such as seen in many turtle species.

    1. Aha, that makes a lot of sense. I know turtles store sperm (I tried to breed a couple sliders once and learned that from the research) so I imagine tortoises do too.

      1. Yes, and the temperature dependent sex determination would help in founding a new population with Tort-adam and Tort-eve.

    2. Yes, Jim, turtles definitely do store sperm, although I don’t know what the state of knowledge is for tortoises. I once had a Blanding’s turtle lay fertile eggs after overwintering in captivity without contact with a male.


  5. It was good to be reminded here that the evidence for ratite distribution now also favors more recent dispersal rather than ancient tectonic drift — an old idea I keep remembering and need to not remember so well.

  6. The mentioned book by de Quieroz is excellent, a story very well told. It was hard to put it down. There are a few weak arguments but most of it is convincing.

    1. I enjoyed Monkey’s Voyage as well, but during so an intellectual “temptress” occurred to me. Namely, while the possibility of “once in a 1000 years is sufficient for the phenomena to arise” explanations are no doubt true sometimes, one must be skeptical or else one will find oneself explaining everything in terms of low probability events.

  7. It’s funny how people debating the gritty details of evolution somehow feel that it is a damning critique for some event to be rare (“nigh impossible”), when evolution is to an extent about rare events smeared across long spans of time.

  8. > In the first half of the 20th century, this school constructed speculative land bridges crisscrossing the oceans, in order for every island animal to have had a dry-shod passage to the island from its home of origin.

    Actually, that school is slightly older, originating in the late 19. century. The founding father was Eduard Suess (1831-1914):

    The problem he and his followers tried to explain was even bigger than you state: There were not only island animals, but also fossils from continent dwellers living at times were land masses now separated through drift had been joined, like the famous glossopteris from Pangea:

    1. Yes, bridge building originated earlier, but to my Caribocentric mind it reached it’s apotheosis with Charles Schuchert’s 1935 book on the Historical Geology of Antillean-Caribbean Region. This was also the time when the herpetologist Thomas Barbour(brigdes) engaged in amusing debate with paleontologist W.D Matthew and entomologist P.J. Darlington (both over water dispersal)about island biogeography.


  9. The Kimbiji tortoise, despite the ability to swim, could not have swum to the main,

    “mainland”, I suspect. Though the “bounding main” is a perfectly good if flowery description of the sea.
    I tried getting out into the Indian Ocean once from a coral reef. Actually I tried about four times before deciding that I really didn’t want to walk/ wade/ swim the couple of kilometres back to the island we were working on with a couple of broken limbs. Not to forget the sharks (we treated a lacerated local fisherman on our previous job there – “toothy” obviously wasn’t hungry that day) or the fairly strong tidal streams around that side of the island. The fetch on the Tanzanian coast is enough to make for some pretty chunky surf.
    Hmmm, Kimbiji … map view makes the coast looks pretty exposed. Photo view shows fringing reefs though. I’d guess the tortoise came up onto the fringing reef at high tide and walked in during lower tide. could have been pretty rough otherwise.
    I wonder if my Sea Beans have grown any while I’ve been away?

    1. Yes, ‘main’ means mainland; the shortened form may be more usually West Indian in English usage. The “Spanish Main” was Central America and northern South America, the mainland beyond the islands of the Antilles. Through a transference whose history I don’t know the details of, the “Spanish Main” also came to refer to the sea that had to be crossed to get from the islands to the main. Thus, in the West Indies at least, “main” came to have the meaning of “the continent”, but also its opposite, “the sea”!


  10. I think that the Fiji iguana is about the best example of a long-range trans-oceanic dispersal of a non-flying, non-aquatic tetrapod that we have record of.

    1. It used to be thought that the ancestor of the Fijian Brachylophus must have made a voyage across the Pacific from South America (home of the most similar-looking species such as Iguana iguana) via the equatorial current, in what’s been called Kon-Tiki dispersal. However, this has been questioned not only due to some individuals’ personal incredulity, but also the fact that the former (fossil) distribution of iguanian lineages does not show the clear regional patterns present today, raising the possibility that it’s a relict of former Old World iguanids. Another proposed ‘Kon-Tiki’ case was the Pacific Boas, Candoia, where trans-Pacific dispersal can be ruled out based on molecular phylogeny, but it is still considered possibly true for iguanas.
      (See e.g. this)

      1. Yes, the possibility of relictualism is there – definitely the case for the Madagascan iguanids Oplurus and Chalaradon, and the Madagascan boas Sanzinia and Acrantophis. The original scenario (I think that it dates back to Romer) was that iguanids were the older taxon, occupying most of the Old World, and were displaced there by the younger agamids.
        Thanks for the reference.

  11. It must suck to keep your little tortoise neck stretched out for days on end. Poor things probably had neck aches like Jerry’s back ache!

    1. They can actually keep their heads under water for a long time, too. When they sleep in a pond, I have seen them staying under for >1 hr at a time. Of course, in the ocean that does increase the risk of sharks taking a closer look at the tasty morsel…

  12. Did Darwin chuck a few tortoises into the ocean to see if the could swim/float, or am I just misremembering that? It sounds like the kind of thing he would have done, as he repeatedly threw iguanas into the ocean, and knocked a galapagos hawk of a branch with the barrel of his shotgun. He could have written a book on “How to annoy the hell out of animals in the name of science”.

    1. In Darwin’s final book, about worms, he talks about the worms’ reaction to hearing bassoon playing, so yup. 🙂

        1. I think you are thinking about saxophones, the most reprehensible instrument ever invented

          1. Saxophones are awesome. It’s the person who plays the triangle that puts me on edge.

      1. Yeah, I just finished that book! It was amazing the amount of time he spent just weighing and measuring worm crap. A bit tedious, but impressive.

  13. O.K., as the website’s official “not a biologist” (I’m not even remotely official, but it would be cool), I read the paper, and I have a problem with it. The authors not only don’t rule out, they don’t even address the possibility that this particular tortoise was transported by some cargo ship. A good number of cargo carriers are less than scrupulous in the maintenance of their vessels. A tortoise could have been living the bilge of one of these vessels easily for months without being discovered (which would have allowed for the same level of barnacle growth).

    I’m no biologist, but having spent a good portion of my adult life in the bowels of ships, I think possibility of a vessel-borne transport should have been more thoroughly investigated.

    1. That is not at all a bad idea. But I do not see why they would keep a tortoise on board. Also, the barnacles grew and it seems to me, sitting here in Michigan, that so many barnacles would not get enough food from the bilgewater.

      1. Mark,

        Bilge water has an amazing amount of life in it. All the way down to a very interesting algae that breaks down drops of waste oil to live. As far as why they would keep a tortoise? Well, cruises get long and sailors get bored. Hell, we once had a couple of frogs (could have been toads I wasn’t concerned with species at the time) living in our bilges. We could hear them croaking, but we had tough time catching them. We eventually did and let them overboard in the Arabian Gulf. We may very well have introduced a invasive species into that region, it wasn’t our intention to do that, but we really didn’t know better. Turns out, it was just a couple of bored sailors that thought keeping a couple bilge pets would be fun (can’t say I blame them).

        There are also animals that get on board through no intentional actions of the crew. We have lampreys and invasive snails in the great lakes, as well as microbes like smallpox and syphilis that were all unintentionally brought across oceans.

        1. Frogs or toads in the bilge seems curious, as I would expect the bilge to be saline, and amphibians are notoriously intolerant of salt. Treefrogs, I suppose, could climb up on the sides out of the water, or maybe the ship had freshwater bilge? Tossing them overboard would not have introduced an invasive species, (they’d die in the salt water), unless they were pitched all the way to shore.

          Keeping a pet tortoise in the bilge also seems curious, since it would take quite a lot of effort to poach a tortoise off Aldabra, and it would seem an odd way to treat a valuable and difficult to obtain animal.

          That said, vessels have been an important source of moving animals around. Birds, especially, can be transported far distances, landing on ships and then staying with them for the voyage, and ornithologists are alive to that possibility when evaluating new distributional records. Alan de Queiroz has a piece on transport of house crows on ships at his blog.


          1. Greg,

            Thanks for the reply. In steam-driven ships (I can’t speak for diesel or gas turbine types) , the bilges aren’t nearly as salty as most people imagine. Yes, there is salinity from sea-water leakage from fire prevention and cooling systems. But the majority of the water comes from the steam plant itself (some intentional, some from leaks in the system). Having tasted an amount of it (never intentionally, but it went with the turf), I wouldn’t brew my coffee with it, but it’s far from straight sea water.

            As far as keeping a valuable and rare species, well, you kind have to get to know sailors. They are very dedicated and professional young men and women, but geographical biodiversity is just not going to show up in any training they received. Additionally, although I used the frog anecdote as an analogy, there are other transport possibilities (e.g. the tortoise latched on to a protruding structure on the ship). Finally, Great post! I do enjoy the pure biology posts on WEIT, but (to my gross discredit) I rarely comment.

  14. Aldabra correspondent? Oh dear. Must take it more seriously, then. Arrival of sealife-encrusted giant Aldabra tortoises on East African shores has actually been recorded at least three times in the last 30-40 years. Apart from the one in Gerlach et al.’s paper, two tortoises arrived on Kenya’s shores in the 1980s. They were both brought to the Rene Haller Park, where they probably still live today ( –this was also the home of the famous friendship between a giant Aldabra tortoise and a baby hippopotamus).

    Apart from potentially carrying eggs, tortoises arriving on a new island may also be carrying the seeds (literally) of their new ecosystem along. Gut retention time can be several months for some seeds.

    Much to learn still about the almost-lost dynamics of inter-island dispersal of tortoises in the Indian Ocean and elsewhere.

    1. I, and apparently Gerlach, had never heard of the Kenyan landings in the 1980s. The tortoises could conceivably have come from Changuu Island off Zanzibar, if that introduced population existed at the time. Even if the details of the toroises’ arrival and fate is not well known, this seems well worth publishing, and there are herpetological venues where a suggestive (as opposed to definitively documented) discussion would be welcome. Dennis, if you’re interested in pursuing this a bit (and I hope you are!), contact me and I can give further direction.


  15. OK, you still-working biologists, here is a great project! Step one would seem to be just how long a tortoise can float…?

    It would appear that Philip Darlington might have been more correct than some gave him credit for.

  16. Quick! Call the Discovery Institute!
    Now we have proof of how the animals from Noah’s Ark were able to disperse around the world. They hitched rides on the tortoises!

  17. I must read The Monkey’s Voyage: How Improbable Journeys Shaped the History of Life – I saw a copy last week but that was while buying about 4 other books…!

    Are Aldabran tortoises most closely related to Seychelle tortoises? I am thinking of the Seycelles/Gondwana plate… that the tortoises have spread in this case from remaining parts of a lost continental mass…?

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