Sometimes oceanic islands—islands formed de novo from beneath the sea, as with volcanic and coral islands—harbor endemic species that don’t seem like their ancestors could have gotten there. Birds, insects, and plants can easily disperse to distant islands from continents, but reptiles, amphibians, and mammals have a harder time, for they have no easy way to cross big expanses of salt water. The absence of the last three groups on oceanic islands, as compared to continental islands like Britain and Sri Lanka, was first noticed by Darwin, who used it as evidence for evolution in The Origin.
But sometimes you do find reptiles, amphibians, and mammals on isolated oceanic islands. The Galápagos Islands, for example, are famous for their marine and land iguanas, as well as other lizards that are found nowhere else. And although Madagascar was once connected to Africa, primates got there long after this separation had occurred, crossing the expanse of sea between the continent and the island. The geographic split occurred about 160 million years ago. But after that, about 50 mya, a primate made it to the island and radiated into the many species of lemurs found nowhere else. How did this primate (and it must have been either one pregnant female or two or more individuals of different sex) get there? The likely explanation is “rafting”, explained in Wikipedia:
Once part of the supercontinent Gondwana, the island of Madagascar has been isolated since it broke away from eastern Africa (~160 mya), Antarctica (~80–130 mya), and India (~80–90 mya). Since ancestral lemurs are thought to have originated in Africa around 62 to 65 mya, they must have crossed the Mozambique Channel, a deep channel between Africa and Madagascar with a minimum width of about 560 km (350 mi). In 1915, paleontologist William Diller Matthew noted that the mammalian biodiversity on Madagascar (including lemurs) can only be accounted for by random rafting events, where very small populations rafted from nearby Africa on tangled mats of vegetation, which get flushed out to sea from major rivers.
There can also be smaller rafts, like individual trees or small masses of plant material, and these can carry things like small amphibians or invertebrates. But the new PNAS paper below documents what is now the longest known rafting event among all terrestrial vertebrates: the dispersal of a land iguana from North America to Fiji. That’s a distance of over 8,000 km, or about 5,000 miles. Click the screenshot below to read the paper, and you can find the pdf here.
There are four species of the large iguana Brachylophus on the Pacific islands composing Fiji, where they’re endemic (Tonga also had a giant iguana that’s now extinct). Here is one of the species studied in this paper, Brachylophus bulabula (this is a male):
How did these reptiles get there and where did they come from? And when did this dispersal event take place? The first thing we need to know to answer this is what is the closest living (or fossil) relative to the Fijian species. It turns out that using DNA to gauge relationships also gives us an estimate of dispersal time using the calibrated “molecular clock,” in which DNA divergence, often calibrated with fossil data, can give us both genealogical relationships and divergence times.
The authors used more than 4,000 genes in each of 14 species of iguanas from eight of the nine known genera. It turned out, as the iguana family tree shows below, that the closest related genus to Brachylophus is the genus Dipsosaurus, which contains two living species, both found in North America:
- The Desert iguana Dipsosaurus dorsalis, found in Baja California and SW North America,
- Catalina desert iguana, Dipsosaurus catalinensis, found on Santa Catalina Island in the gulf between Mexico and Baja California.
Here’s the Desert Iguana:
And here’s the DNA-based family tree. The two genera at the top) are clearly more closely related than Brachylophus is to any other species, and they branched off from other genera of iguanas early during the divergence of the entire group (click all figures to enlarge them):
Note that both of the North American regions are dry and these iguanas are adapted to a hot, low-water ecosystem. The relationship between these two genera as sister taxa is very strong, and the divergence time between the two genera is estimated at about 34 million years. That fits nicely with the time that Fiji was created by volcanic activity—about 39 million years ago. It is likely, given this tree, that the Fiji iguanas came from a North American ancestor, and that would mean rafting 8000 km.
Could it have come from somewhere else? Other hypotheses are possible. Early biogeographers posited huge land bridges between Pacific islands and the continents, but there is no evidence that such bridges existed. They could have island-hopped from SE Asia or traveled from Gondwana before it broke up. Other models are possible, but these can be tested using various models, and also looking to see if there are fossil iguanas in other places that are more related to Brachylophus. Here’s a figure showing some of the models tested, but only one, with the lizard icon on it, was supported by the data. That’s a long trip, and given the size of these animals, it must have involved a fairly substantial raft.
But could an iguana really survive floating on a raft of vegetation over that immense distance? Well, for one thing there are currents that go that way, which would speed up the voyage, estimated by the authors to have taken between 80-120 days.
Can an iguana live that long without fresh water (there may have been food on the “raft”)? The answer is “probably,” because during cold weather many lizards undergo a period of metabolic and activity dormancy called brumation, during which they do not eat (though they need water). Here’s what the authors say:
Herbivorous iguanids forgo food for months at a time during brumation, and extant Dipsosaurus brumate from October–March. However, floating vegetation mats are a known substrate for oceanic dispersal, so iguanas rafting from North America to Fiji could have had a food source during their journey. Additionally, some iguanas have other traits that may augment their capacity to survive overwater dispersal, including resistance to heat and dehydration. For example, Dipsosaurus have the highest voluntary thermal maximum temperature among lizards and largely inhabit areas without permanent freshwater.
The only thing that concerns me with this hypothesis is this: where did the rafting iguanas get fresh water? The authors don’t really address this, but do mention iguanas’ resistance to dehydration. Also, there’s rain in the ocean, and any rain falling on a raft could be sucked up by the lizards aboard.
The best hypothesis, then, seems to be rafting, and the authors concatenate all the evidence supporting it:
The combination of evidence supporting oceanic rafting from North America to Fiji is 1) phylogenomic analyses that support a sister taxon relationship between Brachylophus and Dipsosaurus, 2) the distribution of fossil iguanids, extant Dipsosaurus, and most other extant iguanids in North America, 3) statistical biogeographic analyses that favor long-distance dispersal from North America over alternative hypotheses, including dispersal via Eurasia, South America, Antarctica, and/or Oceania, and 4) the late Paleogene divergence time between Brachylophus and Dipsosaurus.
Finally, just for fun, here’s are two bar graphs from the paper showing the greatest distances between islands harboring iguanids and the nearest mainland (first graph) and the same graph for diofferent groups of terrestrial vertebrates. The captions for the two graphs include this: “A) Distances between island and mainland for extant iguanid lizards and (B) distances for other proposed long-distance, overwater dispersal events in terrestrial vertebrates.”
Among iguanas, Brachylophus is The King, by far!:
Looking at all vertebrates, Brachylophuis still the king!
The asterisks in this graph indicate that stepping-stone dispersal is possible, with the distances for that scenario given by the white line across the two bars. The second longest dispersal, leaving out the asterisked animals involve Cadeidae, otherwise known as Cuban keel-headed worm lizards. They are found on Cuba but are said to have dispersed some 6000 km. This genus comprises two Cuban species and is enigmatic, but is thought to have rafted from the Mediterranean!
And so we have many instances of “founder-event speciation”: ancestors making it to distant islands and forming new species (in this case, four) after they land on islands or archipelagos. Note that this differs from the old and largely discredited theory of “founder-EFFECT speciation,” which posited that weird genetic stuff happens on small founding populations that speeds up formation of new groups. That theory was promoted by, among others, Stephen Jay Gould.
Rather than use your racist Western “science” to mansplain the world to us, we should be using the iguanas’ own precolonial Ways Of Knowing to understand their ability to travel the world.
But… but… eons of accumulated transgenerational trauma, much of it due to genus Homo, make them extremely averse to participating in any further exploitation. Free Free Iguanastan!!
Ah – refreshing, amazing to read.
I wonder if an unusual but particularly fortuitous series of weather events (perhaps lots of wind and regular short bursts of not-particularly- dangerous rain storms) contributed to the amazing iguana journey. Thor Heyerdahl could no doubt have made a series of tests using rafts made out of gummed up vegetation carefully thrown together from appropriate swamps.
It only had to happen once, like coincidences. We are amazed at apparently outlandish coincidences, but we notice only those that occur and remain ignorant of all the possible opportunities for them.
🎯
I have never seen creationists even try to address the question of the unusual flora and fauna of geologically old islands such as New Zealand and Madagascar or the limited fauna and flora of volcanic islands. I also never figured out why overgrown tortoises would dismount from the ark in Mt. Ararat in Turkey, make a march to South America, and somehow proceed from there to the Galápagos Islands.
You can be certain that they can concoct a solution, such as long distance dispersal during the Noachian flood.
Oh fallen creatures, how dare ye question the Almighty and xis Perfect Plan™.
Shel Silverstein explains re the unicorns that, their having missed the boat, “the [Flood] waters came down and sorta floated them away. . .” On rafts, presumably. This is a kinder explanation for children in the audience than that they drowned. Who knows where they might turn up?
Agreed. Of all the evidence for evolution, the one area where I have never seen creationists even try to explain the data is biogeography, particularly ocean island biogeography (I mention this in my book WEIT). I sure some creationist has made a game try, but it simply cannot be convincing. That is, unless, God did not WANT amphibians, freshwater fish, reptiles, and mammals on oceanic islands. It is not that these groups could not live on such islands, for when they are put there by humans (viz., the cane toad or mongoose on Hawaii), they can thrive. Darwin pointed that out, too, in The Origin (he was a smart guy).
IIRC what I read on Joel Duff’s site (Naturalis Historia, where religious myths are given an extraordinarily polite assessment) the most common suggestion in the young earth creationist (YEC with biblical flood to boot!) literature is that genetic preloaded proto beasts (or some such) traveled to their ideal habitats quickly (so no fossil trail). Rates of travel estimates are dismissed as irrelevant since these proto beasts no longer exist (nature always destroys in their view so the full potential of these proto beasts cannot be known).
Once in their ideal habitats (Divinely/genetically and unerringly guided) the preloaded genetic potential was unleashed to create all species in the biological record.
Geologic problems are dismissed since a catastrophic global flood would certainly displace massive amounts of mud that could temporarily solidify to provide land bridges connecting all lands (no longer present due to erosion).
And that is just the YEC, for the intelligent design crowd (who seem indistinguishable from OEC to me) species manifested at or instinctively migrated to their ideal habitats. Genetic preloading provided genes to create all new species observed and other genetic info disappeared (since evolution always destroys).
And genetic relations are dismissed since an intelligent designer, who found a good design, would be stupid to not use that design repeatedly. So repeated genetic and morphological designs are further evidence the creator is intelligent!
(Full disclosure, I borrowed massively from Dr Duff who is a biology professor at Akron University and a Christian who wants to help other Christians accept science. He goes above and beyond to explain why young/old Earth creationism, intelligent design, and any idea involving a global flood, lack consistency and evidence. He criticizes the failure of creationists to address obvious counter evidence and to reconcile competing hypotheses. This post is a poor and extremely incomplete sketch of his scholarship)
I do not agree with any of the religious ideas I have conveyed in this post, I only submit that ideas must be known and understood before being proven wrong, much less dismissed.
Was the raft called Kon Tiki?
So there must have multiple lizards on one raft or there were multiple rafting events in a short period of time. I wonder if they could measure the size of the founding population from the genetic diversity of lizards today?
Or one inseminated female (if iguanas can store sperm for several months or have such a long gestation time).
My gut instinct is that water wouldn’t pose much of a problem. In order for the vegetal raft to have survived the long ocean voyage without being broken up by wave action, it must have been especially dense and thick (and probably quite large, too). A mat of that description would likely have at least a few semi-impermeable depressions where significant amounts of rainfall could collect and remain available for drinking for quite some time before draining away or evaporating. Also, animals could presumably access pockets of rainwater that percolated inside the mat by burrowing.
It also occurs to me that mariners must come across mats of vegetation out in the ocean every now and then. It would be interesting to study them for their ability to hold water and for how long they can last, on average.
Hi Jerry,
I’ve known about Brachylophus iguanas for a long time, and had the great pleasure of seeing a couple in Fiji in 2017 (one was a lodge pet, in residence along the stairway to the entrance). I had long assumed that the ancestor had gotten there by rafting, because the divergence from American Iguanidae was surely too recent for any continental drift scenario (the only alternative hypothesis). Nice to see this confirmed with relevant data, but not at all surprising.
Indeed, although perhaps it was not known then that the closest ancestor lived in North America rather than some closer large region.
Very nice. If you Google ‘pacific currents’, you do see a north equatorial current that runs pretty much along that route.
I suppose this is the simplest story, although I don’t know how one would rule out a migration that went the other way around, from the area of Fiji –> North America.
I assumed the point of origin was part of the utility of the fossils that were examined?
You would still need to explain how iguanas got to Fiji in the first place.
Thanks for this science post. Fascinating. I love the cats and news, but what makes this website special is the science.
Santa Catalina island is in Los Angeles county one of the channel islands off Los Angeles. In the Pacific Ocean.
I was talking about the (translated) Isla Santa Catalina in the Gulf of Mexico: https://en.wikipedia.org/wiki/Isla_Santa_Catalina
I have visited the American island off California some years ago to collect Drosophila.
Do these guys have to drink water or can they get enough by eating plant material? I would think a good rain would sustain the plant life on a large raft and supply plenty of liquid until the next rain.
Many desert reptiles never drink water. I raised desert iguanas and Arizona chuckwallas for more than 15 years in a xeric enclosure. Never did I offer them water in a dish or by misting. They thrived. The chuckwallas even bred twice. All the water they needed came from their food.
I can’t vouch for the water needs of the rafting ancestors of the iguanas.
Good to know. Thanks.
Love your science posts just to let you know.
Fascinating. I’m always learning new things from your excellent posts, Jerry. Thank you. Keep up the good work.
Very nice article. I think I mentioned this on the web site before, but before plate tectonics, great land bridges—often called “isthmian links”—were postulated to explain how organisms could get from place to place, either from one continent to another or from a continent to an island. Also postulated were hypothetical island chains that might allow island hopping, the (hypothetical) islands assumed to have eroded away leaving descendants and ancestors far apart.
The acceptance of plate tectonics changed all that by the 1970’s. The fact that the continents moved solved a lot of these problems but not all. In some cases, animals really did raft or island hop to get from place to place. Even humans made their way to North America via the Bering Land Bridge during the Pleistocene, the bridge itself having since melted away. This appears to be another rare example. Fascinating stuff!
Thank you for another wonderfully educational science post. I rarely comment, but enjoy these posts so much. A great beginning for the day, almost every day!