Are Migrating Cancer Cells Like Rafting Monkeys?

Early in the morning, before getting sucked into the e-world, sometimes even before having coffee, I’ve been reading a book called Cancer: The Evolutionary Legacy by a British medical researcher named Mel Greaves. This probably sounds like an astoundingly bad way to start the day, but Greaves’ tone is pretty light, and he doesn’t get into the individual stories of cancer sufferers. It’s a concept book, and an incredibly fascinating one. I’m only making it through a few pages a day, because I’m constantly stopping to digest some point, and turning the margins into snarls of exclamation points, arrows, and scribbled thoughts.

Greaves obviously loves a good analogy, and when he describes metastasis—the establishment of cancer cells in new locations within the body—what comes out, strangely enough, is pure biogeography, and, in particular, long-distance colonization. The malignant cells in their original location (colon, breast, or wherever) are hemmed in by physical boundaries and barriers, like “an overpopulating and localized species of animal” confined to an island, with the surrounding sea cutting off escape. When a few cells manage to break out into the bloodstream or lymphatic circulation, “it is rather like a desperate migrant crossing the China Sea in a fragile vessel.” Most of these cells will not survive the journey, but “given the number that try and the time available, inevitably a few may succeed.”

In other words, the movement of cancer cells is a lot like monkeys crossing the Atlantic or other instances of chance, overseas dispersal, even extending to the idea that a small probability of success for any single attempt can translate to a much higher one given enough time. That quote about “the number that try and the time available” could have come from the paleontologist George Gaylord Simpson writing about overwater dispersal by Cenozoic mammals. Or, for that matter, from Darwin arguing for the natural colonization of oceanic islands.

Figure drawing: Edward Burne-Jones. Map: Ktrinko—own work, CC0, https://commons.wikimedia.org/w/index.php?curid=17169364

The analogy doesn’t stop there, though; in fact, the detailed parallels pile up almost to the point of absurdity. Thus, once a cancer clone makes it to a new location, it is more likely to thrive “in a similar environment or ecosystem to that from which it came—one part of the bladder to the other, skin to skin.” So the successfully colonizing cancer cell is like, say, a shrub from a tropical rainforest that, after dispersing, gains a foothold because it happens to find itself in another tropical rainforest.

Continuing on, some types of cancer cells are better migrants than others, just as ducks and rats are more likely overseas colonists than salamanders and kangaroos. Numbers at the source location are important in both instances—of cancer cells in the one case and of individual organisms in the other. Access to thoroughfares can be critical—cancer cells multiplying next to a large blood vessel are like monkeys living by a river that might wash them down to the sea. And lymph nodes can act as way stations for migrating cancer cells, the equivalent of stepping-stone islands for migrating species.

Greaves’ analogy makes it clear that metastasis is, at its core, a random process, but with the probabilities heavily influenced by various factors. The randomness of it—chance cell movement, like chance dispersal—gives some insight into the fact that two people could develop the same kind of cancer simultaneously, yet, in one, the disease might spread almost immediately whereas, in the other, it might take years to metastasize. Meanwhile, the influencing factors, such as access to many blood vessels and lymph nodes, make sense of why some kinds of cancer often persist for years without spreading, while others can be expected to metastasize very quickly.

Following up on these thoughts, I did a quick search in the Web of Science for mathematical models of  metastasis. None of the studies mentioned overseas dispersal or rafts of vegetation, but they did give me some reason to tweak the biogeographic analogy to more closely match the disease situation. It appears that, for at least some kinds of cancer, the initial and subsequent tumors might interact with each other beyond the original one-way colonizations. In particular, the tumors within a person seem to receive new “seeds”—colonizing cancer cells—from one another, and the re-seeding influences their growth. This makes me think that monkeys rafting across the Atlantic, which happened only once as far as anyone knows, is not the best analogy for these cases. The migrating cancer cells are more like Darwin’s finches colonizing and re-colonizing islands in the Galápagos.

Darwin’s evolutionary ideas have been fruitfully translated to all sorts of areas outside of their original domain, including such things as constructing genealogical trees of languages, describing the fates of memes, and creating computer algorithms that solve problems by mimicking natural selection. Close to the topic at hand, many people have recognized that the growth and spread of cancer within the body is a Darwinian process—selection occurring at the level of competition among cells—and that this knowledge might prove useful in developing new treatments.

To end on a vaguely practical note, I wonder if models of chance dispersal—the mathematical embodiment of ideas set forth by Darwin, Simpson, and others—might be applied to the process of metastasis. Could these biogeographic models give us some novel insight into that process, either by introducing new variables or a different kind of analysis? Could they provide something more medically useful than a metaphor?

Another Monkey’s Voyage: Panamacebus and the Central American Seaway

One of the great stories in biogeography is that, when the Isthmus of Panama emerged about 3 million years ago, a flood of species passed over this new corridor between the New World continents, some going north, others south. If you’ve seen opossums, armadillos, or porcupines in North America, jaguars (or, more likely, jaguar tracks), maned wolves, tapirs, or llamas in South America, you’ve encountered the descendants of animals that crossed over the isthmus. This narrative is so well known that it has both a name and an acronym that many biologists (and even others) instantly recognize; it’s called “The Great American Biotic Interchange,” GABI for short.

The 3-million-year age of the land bridge is widely accepted, so much so that many studies have used it to stamp a date on the evolutionary separation of marine “sister species” living in the Pacific and the Caribbean. (According to the simple version of the story, those species split from each other because of the rise of the isthmus.) But in the last few years, some geologists and biologists have questioned that age, and have claimed that the isthmus arose much earlier, at least 6 million years ago and perhaps as early as 23 million years ago. Those radical new claims have been attacked, maybe “slammed” would be a better word, but these earlier ages for the land bridge have found their way into scientific articles and popular news accounts.

With that background in mind, I read a paper that just came out in the journal Nature, “First North American fossil monkey and early Miocene tropical biotic interchange” by Jonathan Bloch and others. Based on seven fossil teeth, Bloch et al. describe an extinct monkey species from 21-million-year-old ash fall deposits that were exposed by dynamiting as part of an expansion of the Panama Canal. The location of this Miocene fossil, called Panamacebus, means that monkeys must have made it from South to North America millions of years earlier than anyone had thought.

So what are the implications of this new fossil discovery for putting an age on the formation of the Isthmus of Panama and the story of the Great American Biotic Interchange?

An immediate thought is that land mammals are not good at crossing sea barriers—Darwin said that in The Origin of Species—so the ancestors of Panamacebus probably moved by land to North America. The new fossil, therefore, is another argument for the early emergence of the isthmus.

I don’t think that’s the conclusion to be taken from this study though (and it isn’t a conclusion made by the study’s authors). For one thing, various kinds of evidence—biological, geological, and oceanographic—indicate the 3-million-year age is correct. Also, it’s not at all far-fetched to believe that monkeys could have rafted across the narrow Central American Seaway thought to have separated North and South America 21 million years ago. As I pointed out in The Monkey’s Voyage, there are many well-established cases of overwater dispersal by primates, including at least two—monkeys dispersing from Africa to South America over the Atlantic, and the ancestors of lemurs colonizing Madagascar from Africa—that involved much longer journeys than a crossing of the Miocene Central American Seaway. And, in fact, the monkeys that made the trans-Atlantic voyage were the ancestors of Panamacebus, while other South American monkeys probably rafted to Caribbean islands. In other words, even within this one branch of the primate evolutionary tree, there have been long overwater colonizations.

The story of the isthmus is, of course, partly about the major pulse of animals that moved over the new land bridge. But it’s also about a surprising number of groups that dispersed between the continents before the bridge emerged, including not only monkeys, but northward-moving sloths, frogs, flightless terror birds, and a capybara relative, and southward-moving snakes, toads, raccoon relatives, and rodents. Most surprising of all, a couple of groups of freshwater fishes somehow made the journey north across the seaway, perhaps using a plume of fresh water spreading from the mouth of a major river. (The dispersal of those fishes does give me a little pause about the age of the isthmus, much more so than the monkeys.)

Xenodon merremi, part of a group of some 300 species of South American snakes derived from a species that dispersed over water from North America. (Photo by Mateus S. Figueiredo—Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=42796144)

A final twist to the story is that the living monkeys in Central America are not the descendants of the ones that crossed the seaway. The monkeys that rafted north likely did not give rise to any modern species and, as near as I can figure from a quick look at evolutionary trees, the ancestors of all these living monkeys probably reached Central America after the Isthmus of Panama emerged. As much as I have argued for the importance of overseas dispersal, I have to think the most likely scenario for the squirrel, spider, howler, white-faced, and owl monkeys that now live in Panama and points north is that they used the land bridge, ambling or swinging their way to North America.

 

The House Crow’s Voyages: Oceanic Dispersal in the Age of Humans

My father-in-law spends a lot of time sailing—he’s made two solo trips across the Atlantic, among other things—and he says that land birds often hitch rides on his boat. One time, for instance, a small songbird sailed with him across the Adriatic, from Italy to Albania, and a Great Blue Heron rode with him for five hours on a trip from Cape Cod to Maine. (He even has a video of the heron, looking as uncomfortable as you’d expect a heron on a sailboat to look.)

Although I’m a birder, I had no idea that birds catching rides on boats was such a common thing. But apparently it is, and there’s even a semi-official term for the phenomenon: it’s called “ship-assisted dispersal.” For example, many songbirds from the Americas that show up in the Azores probably made some part of the voyage on cargo ships. And Great-tailed Grackles, New World birds that are not considered good overwater dispersers, have appeared in Hawaii and Spain, almost certainly having ridden rather than flown to those places, thousands of miles from their normal range.

These birds that hitch rides to distant lands are exciting for birders, but generally aren’t very significant in the long run; that is, they do not establish new populations “by ship.” They arrive, hang around for a while, and, almost always, die without ever reproducing.

Unless they’re House Crows.

The history of the House Crow (Corvus splendens) has become so entwined with humanity’s that I’ve begun thinking of it as the poster bird for the Age of Humans. It’s apparently the only wild bird species that not only does well around people, but actually can’t live without us; populations of these birds are only ever found around human settlements. (Even “trashy” city birds like the House Sparrow and Rock Pigeon can’t make that claim.) The native range of the House Crow is mostly within the Indian subcontinent, and it’s thought (or, at least, it’s reported on Wikipedia) that the human population explosion there produced an explosion of House Crows also. (On the other hand, should we drive ourselves to extinction, the House Crow will probably flame out with us.)

House Crows in Kolkata, West Bengal, India. Photo by J. M. Garg from http://commons.wikimedia.org/wiki/File:House_Crows_(Corvus_splendens)_grooming_in_Kolkata_I_IMG_4324.jpg

It turns out that House Crows have a special thing not just for our settlements (and our garbage), but also for ships. In particular, they often alight on ocean-going vessels, where they presumably live off the crew’s scraps, and can then be carried hundreds or even thousands of miles to distant ports. And, unlike most other birds that hitch rides on ships, House Crows often survive and thrive in their new outposts. Maybe that’s because several crows arrive at once from the same ship, or because port towns tend to be great habitats for these birds, or simply because there are so many of these crow voyages taking place—I don’t think anybody really knows the answer.

In any case, House Crows are now established in cities and towns all around the Arabian Peninsula and down the east coast of Africa. Some of these populations came from purposeful introductions, but most of them seem to be the result of birds catching rides on their own across parts of the Indian Ocean. They’re sort of the stowaway rats of the bird world, except that they ride in plain sight.

Distribution of the House Crow around the Indian Ocean. The introduced populations are thought to be mostly from ship-assisted dispersal, either directly from the native range or from earlier introduced populations. The map is redrawn from a worldwide map of the House Crow’s distribution by Colin Ryall (http://www.housecrow.com/?page_id=28). Used with permission.

Tangentially, the relationship between House Crows and people on boats has worked the other way around too, with sailors making use of the birds’ navigation skills. Specifically, ancient Indian and Singhalese (Sri Lankan) mariners apparently took House Crows along on their boats, and, if the mariners needed to find land, they would release a crow and follow its course, like Noah with the dove that led him to Mt. Ararat. (The Vikings used other kinds of crows in the same way, and these stories make me wonder whether Noah’s dove, if it has any connection to reality, was actually a crow.)

The long-distance colonizations of House Crows all seem to have taken place within the last 130 years or so, probably reflecting the invention of faster ships and the growth of shipping traffic, and, barring some concerted international effort against these birds, their ship-assisted expansion will no doubt continue. Anticipating this, a group of scientists recently made an ecological niche model which suggests that House Crows might thrive in significant parts of all continents except Antarctica. And, in fact, lone crows already have made it to ports on all of those continents, and small populations have become established in the Netherlands and in Florida (beginning in the 1990s and 2000s, respectively). The incredible spread of this species naturally reminds me of Lyanda Lynn Haupt’s book “Crow Planet.” Haupt was referring especially to the success of the American Crow, but maybe one day we’ll be talking, with more literal truth, about a “House Crow Planet.”

On one level, the history of the House Crow is discouragingly familiar, yet another illustration of the runaway train that is the Age of Humans, an age in which the Earth’s biota is increasingly and inexorably tied to what we are doing.

Less depressingly, though, I think of its story as an example of a wild bird species sort of turning the tables on us, using its very crow-ish adaptability to take advantage of our technological ingenuity, hopscotching from ship to port to ship. I don’t relish the idea of House Crows spreading around the world, but there’s something impressive about the fact that they might, without us even wanting them to.

For more information on the distribution, dispersal, and general biology of House Crows, see Colin Ryall’s House Crow Monitor website.

New study on early monkeys in the Americas

Since the centerpiece story in my book is the case of monkeys apparently crossing the Atlantic Ocean from Africa to South America, I now have a special (and vested) interest in studies about this example. So, when I heard about a new paper called “Eocene primates of South America and the African origins of New World monkeys,” I quickly devoured it, even skimming the supplementary material.

The paper, by Bond et al. in the journal Nature, reports fossil teeth of monkeys from Amazonian Peru, from what the authors believe to be Late Eocene deposits. Three findings from this study seem especially important: (1) If the estimated age of the specimens is correct, they greatly extend the fossil record of monkeys in the New World, from about 26 million years ago (the age of the previous oldest known New World monkey) to some 36 million years ago. (The older age, by the way, is still fully consistent with molecular clock estimates for the separation of New World and Old World monkeys.) (2) The teeth represent three different species, indicating a surprising diversity of early South American monkeys. (3) The new species, Perupithecus ucayaliensis, described from the best-preserved tooth, shows an apparent evolutionary connection to a specific extinct African monkey, Talahpithecus.

That last result is particularly relevant for validating the African origin of New World monkeys (although previous evidence from fossils also pointed to that scenario). However, the grouping of the South American Perupithecus with the African Talahpithecus, rather than with all other New World monkeys, also implies a more complex ocean-crossing scenario than previously imagined. Specifically, that relationship requires either two separate crossings from Africa to South America or a single such dispersal followed by another from South America back to Africa (see figure).

Two scenarios for the dispersal of monkeys to/from the New World. Left: two Africa to South America colonizations. Right: Africa to South America colonization followed by “back-dispersal” to Africa. Perupithecus is the newly described extinct species. The evolutionary tree is from the Bond et al. paper.

These more complex dispersal scenarios can’t be discounted; if monkeys crossed the Atlantic once, I suppose they could have done it twice (although a scenario involving a “back-dispersal” to Africa has, as far as I know, never been supported for a land vertebrate). But, if I had to bet on it, I would choose a different explanation: the grouping of Perupithecus with Talahpithecus is wrong. That wouldn’t be a shock considering that Perupithecus is known from just a single tooth and Talahpithecus from three teeth, two of them broken. Also, in the analysis by Bond et al., the pairing of these two taxa is very tenuous. (A tree that does not group these two taxa is just one step longer than the most parsimonious tree.) I wouldn’t be surprised if it turns out that Perupithecus groups with other New World monkeys, with Talahpithecus outside of this group. That result would bring us back to the simpler scenario of a single crossing of the Atlantic.

In any case, Bond et al.’s study is a big step forward in deciphering the history of monkeys in the New World. I’m hoping that they or others will eventually find other critical fossils that will further flesh out the story of how and when monkeys reached the Americas. Maybe it’s too much to ask, but it would be fantastic to have a skull and some other bones from these early New World monkeys!

Thanks to Darren Lettinga for letting me know about this paper.

Central American squirrel monkey (Saimiri oerstedii). Photo by Manuel Antonio from Wikimedia Commons.

More on trees in the Santa Rosa Mts: what is and what could be

After writing the last post, I wondered whether anyone had done any research that would indicate whether there are conifer species in the general area that ought to be able to thrive in the Santa Rosa Mountains, yet don’t occur there. If such species exist, that would give more credence to the idea that some conifers are not in that range simply because they haven’t gotten there (or, at least, haven’t gotten there often enough to establish a population).

So I googled some conifer species names along with the phrase “niche model,” and one of the first things that came up was a page for the Faculty of Forestry at the University of British Columbia showing a map of the “modeled suitable climate niche for Douglas-fir.” The map showed in green the areas that the model said should be suitable for Douglas-fir populations (whether they actually exist in those places or not). And, sure enough, there was a small green patch for the Santa Rosa Mountains, including areas we visited. (The patch in question is just southwest of the three-way intersection of Oregon, Idaho, and Nevada.)

Digging a little more, I found a scientific article, by different researchers, that produced such maps for both Douglas-fir and ponderosa pine. These maps indicated that both of these species could live in the Santa Rosa Mountains.

Actually, Douglas-fir and ponderosa pine are already in the Santa Rosas—both species were planted there in small numbers. In his great 1996 reference book, Atlas of Nevada Conifers, David Charlet noted that, although these introduced trees had survived, they did not seem to be reproducing. However, I saw a Douglas-fir sapling in the area of Lye Creek campground, about a mile from the planted Douglas-firs. That’s another small piece of evidence that the area might be able to sustain a population of that species.

Finally, those maps of “suitable climate niches” for both Douglas-fir and ponderosa pine show that the Santa Rosa Mountains are surrounded by an “ocean” of environments where populations of those species cannot persist. As far as those trees are concerned, the Santa Rosas do seem to be a remote island that can only be reached by chance, long-distance dispersal, or by people carrying seeds or trees.

Biogeography in our backyard: The Santa Rosa Mountains, part 1

Once, when Tara and I were living in eastern Nevada, we went to an outdoor store in Salt Lake City, Utah, because I needed a pair of cross-country ski boots. The young woman who rang us up, finding that we were from Nevada, wondered where we could ski; she seemed to think the whole state—Utah’s neighboring state, I should add—was a giant, sweltering desert. We had to inform her, hopefully without sounding too condescending, that we lived in a valley at 6,500 feet, with mountains more than 10,000 feet high on both sides. At times, we could ski right out our back door.

No, Nevada is not all sweltering desert. Through most of the state, even the valleys are high and, in the winter, quite cold. More than that, though, Nevada is, by some measure, the most mountainous state in the country, with 314 named ranges (that’s 3…1…4, not a typo), 25 of them with peaks over 10,000 feet. (These mountains almost all run north-south, which led to a geologist describing the landscape as “an army of caterpillars marching toward Mexico.”) The high Nevada mountains can be very snowy, and the Snake Range, in the far eastern part of the state, even has a paltry but real glacier.

For Tara and I, one of the great pleasures of living in Nevada is visiting these different mountain ranges, all of them distinctive in one way or another. (Well, that’s an educated assumption—we’ve only been in several dozen ranges, a small fraction of the 314.) Having two small kids has cut way down on our pace of exploration, but this year we managed two trips to an especially intriguing and beautiful range that we’d never visited before, the Santa Rosa Mountains, in north-central Nevada, close to the Oregon border.

Granite Peak, the high point (9,732 ft) in the Santa Rosa Mountains. The rock of this peak formed in the same events that created the granitic plutons of the Sierra Nevada. Thanks to Alan Wallace for that bit of geological history.

One of the most striking things about the Santa Rosas is that, despite having multiple peaks over 9,000 feet high, the range has only two species of conifers. As you drive north up the road from Paradise Valley toward Hinckey Summit, you pass through sagebrush and bitterbrush and, where you might expect pinyon pines and junipers—the “PJ zone” so common in Nevada mountains—you find…more sagebrush, along with some mountain mahogany.

Farther along that road, at Hinckey Summit and points north, you run into dense stands of trees, but they’re not conifers, they’re aspens. My impression is that the stands are especially extensive here, which might be related to the absence of other trees (or, alternatively, could be a trick of the eye caused by that absence). Incidentally, on our trip last week, we hit the aspens at what I consider their peak—not the time of greatest brilliance, but when there’s a wild quilt of different colors, various shades of green, yellow, and orange. The “patches” in the quilt are groups of clones, with the aspens making up each clone sharing a common timing of the shift to fall colors and a common leaf hue.

Patchwork of aspens at Lye Creek campground

An orange clone

On the ridge that runs from Hinckey Summit west to Granite Peak, at an elevation of 9,000 feet or so, you would expect to find three or four kinds of conifers if this were a “typical” Nevada mountain range (which I realize is an oxymoron). However, here you find only a few scattered limber pine, the one moderately common conifer in the Santa Rosas. (The other conifer is western juniper, which is rare.) Unfortunately I didn’t get up high enough to walk among the limber pines—I had to settle for looking at them through binoculars.

Dead limber pine on Granite Peak. Photo by Tara de Queiroz, who made it farther up the mountain than I did.

I was going to try to explain in some depth why there are so few conifers in these mountains, but, in looking into the topic, I realized this would involve a lot of speculation. However, it seems that at least part of the answer is pretty straightforward: Some of the species that might thrive in the Santa Rosas simply haven’t gotten here.

For instance, Douglas-fir and white fir, which are now found in eastern Nevada mountains, do not seem to have been in the Great Basin during most or all of the Pleistocene. The subspecies of these trees that are in eastern Nevada apparently colonized the Great Basin from the east or south, and, for that colonization process to reach the Santa Rosas, seeds would have to jump over numerous arid valleys, from one mountainous “sky island” to another. A similar argument might apply to singleleaf pinyon pine, which colonized the Great Basin from the south in the last 7,000 years, but hasn’t reached this far north (although it may be that the northward spread of this species was slowed or stopped by climate).

To the degree that this argument is right, you could say that the Santa Rosa Mountains (and other Great Basin ranges) are like a volcanic island that had to be populated from elsewhere by some kind of chance dispersal. But the analogy obviously isn’t perfect, because, unlike a newly-formed island, the Santa Rosas were not devoid of life before the Pleistocene, nor were they wiped clean of trees by Pleistocene glaciers. I assume that at some point there were conifer species other than limber pine and western juniper here. So an obvious question is “What happened to those Pleistocene or pre-Pleistocene conifers?” I’ve got little to say about that. I would love to hear if someone else has any insight or information on the subject.

In any case, visiting the Santa Rosas is a great reminder of the uniqueness of Great Basin mountain ranges and how their character is connected to deep biological and geological history. It’s also a great place to see the beauty of aspens, in a pure state, unadulterated by all those conifers!

Hawaiian ramblings: koa trees, broad-nosed weevils, and a Darwinian experiment

On bristletail collecting trips to Hawaii, we’ve often run across koa (Acacia koa), a native tree that’s common in so-called “mesic forest” (that is, not too dry and not too wet, by Hawaiian standards). Koa have sickle-shaped leaves that bring to mind eucalyptus, but it turns out that these leaves are actually modified leaf stalks, not true leaves. The real leaves can be seen on very young koa and are compound like those of typical Acacia species.

Compound leaves of a young koa tree

Modified leaf stalks (phyllodes) of an adult koa

The strange leaf development story of koa, and the fact that bristletails have been found on them, made me especially aware of these trees, and I’ve tended to notice them among the welter of mostly unknown Hawaiian plants (unknown to me, that is). That familiarity made it especially satisfying to hear about a new study by Le Roux et al. (also described here) indicating that koa dispersed naturally from Hawaii to Réunion, one of the Mascarene Islands, in the Indian Ocean, giving rise to a similar species on Réunion, Acacia heterophylla.

Yes, the Mascarenes are a long way from Hawaii, more than 10,000 miles, in fact. While writing The Monkey’s Voyage, I looked at a lot of maps, of course, but to get a good sense of this koa dispersal, I felt like I needed a more planetary perspective. So I pulled our kids’ globe off the shelf and, with my right index finger fixed on Hawaii, I spun the globe and put my left index finger on Réunion. My two fingers were not quite directly opposite each other on the globe, but they weren’t too far off. The implication was obvious: Hawaii to Réunion is a very, very long trip, even assuming the most direct route.

In addition to the sheer distance, several things make this case especially striking. One is that the area of origin is an island, which means that the source population would have been small compared to many continental ones, decreasing the already slim odds of a successful dispersal. Another is that koa grow in the mountains, not on the coast, so just getting to the ocean is a chance event for them. And, finally, the fruits and seeds of koa aren’t adapted for dispersal by wind, water or birds. For these reasons, it’s not a species you’d pick out as especially likely to make a tremendously long overwater journey.

Basically, the colonization of Réunion by Hawaiian koa must have been a major fluke, an extremely improbable event that ranks with monkeys rafting across the Atlantic or carnivorous sundew plants somehow dispersing from Australia to a tepui in northern South America.

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On koa trees at night, dark, broad-nosed weevils can be found munching on those sickle-shaped leaf stalks, something this group of insects has probably been doing for several million years. The weevils are large—some of the species can be 3 cm long, which is big for a weevil—and, probably for that reason, they’re coveted by beetle collectors. And, as it turns out, when it comes to unexpected long-distance dispersal, the koa trees have nothing on these weevils.

The koa-eating weevils are in the genus Rhyncogonus, a group that, like the koa itself, doesn’t seem like a great candidate for oceanic dispersal. In addition to being bulky for insects, Rhyncogonus are flightless, and their larvae live, not on plants, but in the soil. They seem like just the sort of insect that would not be good at dispersing long distances.

Rhyncogonus howarthi in the Waianae Range, Oahu. Photo by Karl Magnacca.

In fact, on one level, there are indications that Rhyncogonus are not good dispersers. In the Hawaiian chain and other islands where they’re found, species of Rhyncogonus are almost always restricted to single islands and, often, to small parts of those islands. For instance, of nearly 50 Hawaiian species, only one or two are known to occur on more than one island. That kind of geographic pattern suggests the weevils do not move freely between islands, even if the islands are quite close together. Once a lineage of Rhyncogonus establishes itself on an island, it essentially evolves on its own, rarely mixing its gene pool with populations on other islands.

However, counterintuitively, when it comes to extreme long-distance colonization, Rhyncogonus weevils have been ridiculously successful. Their ancestors probably came from Asia or western Pacific islands, and from there they spread all over the warm parts of the central and eastern Pacific; endemic Rhyncogonus are now found on Wake Island, Hawaii, the Line Islands, the Kermadecs, and the French Polynesian islands of the Marquesas, Societies, Australs, and Mangarevas, among other places. Defying intuition, these large, flightless beetles have somehow managed dozens of overwater colonizations, some of them involving voyages thousands of miles long.

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On a 2012 trip to Kauai, the bristletail collecting crew met up with Robin Rice, who runs the Kipu Ranch, one of the places where we wanted to look for the insects. One of Robin’s forebears, William Hyde Rice, who was the last governor of Kauai under the Hawaiian monarchy, apparently bought the ranch in the 1870s from Princess Ruth Ke‘elikolani, and the property has been in the family ever since.

I learned that history from Wikipedia, not from Robin; most of my conversations and email exchanges with him have been intensely biological and, especially, entomological. Studying insects is Robin’s hobby, but it’s more like a passion, one that’s resulted in significant discoveries in Hawaii and many other Pacific islands. I’m not really the person to judge, being an entomological neophyte, but my impression is that he knows the insects of Pacific islands like very few people do.

These days, Robin’s focus is on tree roaches, but at one point, back in the 1970s, he was especially interested in Rhyncogonus. And, like others who have studied the genus, he wondered how these bulky, flightless beetles had colonized islands scattered over a vast expanse of the Pacific.

Some people had suggested that Rhyncogonus got around attached to birds, but Robin doubted that. He had another notion about their dispersal, and thought he could shed some light on the subject by doing a quick-and-dirty experiment using Rhyncogonus simplex that he found feeding on native cotton plants on Oahu.

In an email last month, here’s how he described what he did:

I would take some simplex and drop them into vials of sea water, where they would immediately sink to the bottom and become immobile. Later I would take them out and put them on filter paper in a petri dish to see if they would revive. After one or two days completely submerged nearly all would revive. If I recall the most extreme experiment was 4 days on the bottom of a jar of seawater, whereupon about 2-3 of 15 weevils were walking around in the petri dish a few hours later.

Basically, Robin had performed an experiment along the lines of the famous seeds-in-seawater trials that Darwin had used to establish the plausibility of plants colonizing remote islands. Robin’s experiment (and Darwin’s) was what the philosopher of history William Dray has called a “how possibly” experiment. How possibly could weevils have survived a long rafting voyage in the ocean? Well, it would help if they could survive being inundated for days at a time. And, strangely enough, they can!

Of course, the Lazarus-like recovery of the weevils doesn’t prove or even strongly suggest that the main way that Rhyncogonus reached all those islands was by rafting. But it does push one toward thinking that answer might be the right one. The experiment helps to suspend one’s disbelief.

Darwin, a dispersalist, master of the “how possibly” argument, and, in his youth, an avid beetle collector, would have loved it.

A mania for jumping bristletails

Got a package a couple of days ago from Kauai. Not coffee or a sea turtle t-shirt or any of the other usual Hawaiian items, but something far more exciting—a small glass vial containing seven jumping bristletails preserved in ethanol! So, of course, I spent most of the rest of the day peering through a dissecting scope, trying to identify the things.

My friends/colleagues John Gatesy and Cheryl Hayashi and I have been studying these flightless insects for a number of years and can spout off various reasons why they’re fascinating from a “serious” scientific standpoint. But I just want to use these new Kauai specimens as an excuse to post a bunch of photos and give some sense of the attraction of these creatures for a geeky naturalist and taxonomist.

 

Mesomachilis nearctica in the Egan Range, Nevada.

Bristletails are extremely anatomically conservative—every bristletail pretty much looks like a bristletail. In a weird way, that’s part of their appeal; the diversity within the group is all about subtlety, not extravagant differences. And am I crazy or does this insect look kind of edible, like a tiny land shrimp?

 

Cheryl on Mt Moriah in the Northern Snake Range, Nevada.

The pale, carbonate rocks on Mt. Moriah might be 500 million years old, making them older, but not vastly older, than the bristletail lineage, which probably split 400 million years ago or so from the evolutionary branch that would become all other insects. Bristletails we collected here turned out to be a new, yet very abundant and widespread species.

 

Me, collecting on the Dana Plateau in the Sierra Nevada, just outside of Yosemite National Park. Photo by Seung-Chul Kim.

As John said on a recent collecting trip to Big Sur, bristletails seem to be found in really nice places (although one of the biggest concentrations of them I’ve ever seen was on an outhouse).

 

An especially handsome bristletail face—Petridiobius arcticus from Bellingham, Washington. Photo by Merrill Peterson.

Like other bristletails, this one looks like it’s wearing goggles and a gas mask. The elongate reddish brown blobs under the compound eyes are another pair of eyes, called lateral ocelli. Their shape is a key taxonomic character; looking at bristletails under a microscope, the first things I check out are the ocelli.

 

Neomachilis halophila from near Cambria, California.

When this bristletail was feeding, its compound eyes completely disappeared under the segment just behind them. (Compare the two photos. The white things are the lateral ocelli.) When I saw this, I got very excited and wrote to a couple of entomologist friends…and found out, to my slight disappointment, that having part of the head disappear like this happens in many kinds of insects. So it’s not unique, but it’s still kind of cool.

 

Neomachilis perkinsi from Kauai. Photo by Merrill Peterson.

At some point we learned that there are a few native bristletail species on the Hawaiian Islands and that they probably came from North America (which is weird—most Hawaiian arthropods, especially ones that don’t fly or drift on the air, came from other Pacific islands or from Asia). That led to several trips to Hawaii to collect specimens and verify the biogeographic story, still a work in progress at this point. The paired structures that are curved down sort of like fish hooks are the maxillary palps, which are used by the male to tap or grasp the female during mating, and are extremely thick in male perkinsi. This is the kind of thing that gets a bristletail taxonomist going.

 

One of the new specimens from Kauai.

This is a male, but it doesn’t have the thick maxillary palps, and, therefore, isn’t perkinsi. It’s probably Neomachilis insularis, a species never before identified from Kauai. Apart from the fact that these specimens may be a key for our biogeographic work, it’s just satisfying to be able to identify a species from a place where it’s never been found before.

 

John, on the island of Molokai, looking like he might try to flap himself airborne. The wings are actually “beat-sheets” used to collect arthropods.

Neomachilis halophila on driftwood on the Oregon coast. Is this how they got to Hawaii?

My kids on that same Oregon beach, presumably not thinking about taxonomy, long-distance dispersal, or the deep history of life on Earth.

Our son Eiji thinks bristletails are cool. But our daughter Hana keeps it real—“bristletails are dumb,” she says. (I think the subtext there is “When dad’s looking at those things under the microscope, he won’t play with me.”)

The surprises go both ways: flycatchers trapped in the Americas

A lot of animals that you’d never think would be able to cross oceans on their own have apparently done exactly that—for instance, monkeys and burrowing lizards almost certainly crossed the Atlantic, and iguanas probably crossed the Pacific. However, there are also animals that seem well-equipped to make such transoceanic colonizations, but don’t seem to have done it. I’m thinking especially of many kinds of birds.

Tropical forest birds are somewhat notorious for their lack of gumption when it comes to crossing water. For many of these species, narrow sea straits and large rivers are major barriers to movement, despite the fact that it might only take a few minutes for a bird to fly over them. I’ve just been looking at maps in a guide to the birds of Venezuela, and one thing that immediately jumps out is that many species are only found on one side of the Orinoco or the other. They’re certainly capable of flying across, but they don’t seem to do it, or at least not often enough to become established on the opposite side.

On a wider scale, there are some large taxonomic groups of birds that are entirely or almost entirely confined to contiguous landmasses, only being found in the Americas or only in Australia, for instance. A striking case is the group called the Tyrannides (the New World suboscines excluding the Sapayoa), which includes the New World flycatchers, antbirds, woodcreepers, cotingas, and manakins, among others. This group apparently evolved from a common ancestor that lived in South America some 50 million years ago, and today includes close to a thousand species. The suboscines are a huge part of the bird fauna of Central and South America, and many breed in temperate North America and migrate to the tropics or subtropics for the winter.

A few species in this group have colonized the Galápagos, about 600 miles west of South America, but none have become established on more distant Pacific islands. And, perhaps most surprising, none have colonized Africa or Europe. So, here’s a group that’s some 50 million years old, is very diverse, and is made up of birds that are strong fliers, many of them routinely migrating thousands of miles between breeding and wintering areas, but for some reason they haven’t managed any really long colonizations over expanses of ocean.

Vermilion Flycatcher in Colombia, part of an old and very diverse group that can’t seem to get out of the Americas. Photo by Julian Londono via Wikimedia Commons.

I imagine some people will take this example as a reason to doubt the whole modern approach to biogeography or even the theory of evolution. They might say that the conclusion that monkeys made a colonizing voyage across the Atlantic, but a large group of strong-flying birds hasn’t managed it just shows the absurdity of the whole Darwinian worldview. (That debate is something I’ll probably have to take up in a future post.)

Obviously, that’s not what I believe. Instead, I think our views about how easy/difficult it is for various organisms to make long ocean journeys and establish themselves in new lands are often too simplistic. In the case of birds, we tend to focus on the fact that they can fly, and imagine that this trumps everything. For the New World suboscines, though, there are presumably other factors that have held them back, keeping them a “provincial” American group. It may be relevant that some North American landbirds occasionally wind up in the Hawaiian Islands as what birdwatchers call “accidentals,” but, as far as I know, there are no such records in Hawaii for flycatchers or other New World suboscines. There’s probably a reason for that—maybe someone out there knows why—and it could help explain why this group has remained “stuck” in the Americas.

In my book I talked about scenarios for how the unexpected happened, like how bristletail insects might have made it from California to Hawaii as eggs stuck to driftwood or how crocodiles might have “surfed” on ocean currents across the Atlantic. The New World suboscine birds suggest there’s a lot of room for discussion of the inverse too, that is, why the expected often hasn’t happened.

Are small founding populations doomed? Not according to muskrats.

As soon as I sent in the final corrections for The Monkey’s Voyage I started thinking about (and sometimes agonizing over) all the things I had left out. At various times I had made lists of points that I wanted to raise, but I probably also needed a list to remind myself where I had stashed all the earlier lists. No doubt some of this was just paranoia, and most of the things left out probably matter only to me, but some of these issues apparently have occurred to others too. In any case, one goal of this website/blog is to bring up some of these points that didn’t make it into the book.

Here is one, in the form of a question: Even if a group of organisms—monkeys, lizards, sundew plants, or whatever—managed to make it across an ocean, wouldn’t the small number of individuals doom the population because of the negative effects of inbreeding (matings between relatives) and/or a lack of sufficient genetic variation? The effects of inbreeding (“inbreeding depression”) would show up quickly in the form of individuals with genetic diseases and generally lowered fitness, while an overall lack of variation might make it difficult for the population to respond to changed conditions. Natural selection requires genetic variation to act upon; without much variation, the thought goes, a small founding group might be doomed.

How can we reconcile these ideas with the conclusion that small groups of organisms have often made chance, long-distance journeys and then established long-lived populations in their new homes?

One solution to this conundrum is that, in many cases, the groups of colonists might not have been so small. For instance: some birds could have flown to new areas in large flocks; insects and other small organisms might have crossed oceans on rafts containing many hundreds or thousands of individuals; and some organisms could have reached an area many times within a short period, giving rise to a fairly large founding population. A locust species that crossed the Atlantic to colonize the New World a few million years ago might represent an extreme example of a large founding population—this species might have made the crossing as a swarm of millions of individuals.

However, I admit that this argument is a stretch for monkeys and various other ocean-crossing land vertebrates. It seems very likely that the founding groups in such instances were typically very small, perhaps just a handful of individuals in many cases. How can we explain these examples?

Here, the answer seems to be that the negative effects of small population size have been overstated. For instance, both theoretical and empirical studies indicate that, if a small population quickly grows large, the loss of genetic diversity may not be great. (This is because, in a large population, the loss of genetic variation through the random process of genetic drift becomes much less significant and, at the same time, natural selection becomes more effective at maintaining beneficial variation.) And rapid population growth might be especially likely for organisms colonizing new areas—like monkeys reaching the New World—where these organisms’ usual competitors, predators, and parasites are absent. In addition, selection could rid the population of the deleterious recessive alleles that are a major cause of inbreeding depression. If the population survives such a genetic “purging,” it might end up relatively free of inbreeding effects.

The most compelling evidence that small founding populations aren’t necessarily doomed by genetics, however, is much more straightforward. It comes from the known histories of species introduced to new regions by humans. In particular, many of these introductions involved very few founding individuals, and yet these tiny groups have expanded to become vigorous, large (and often very destructive) populations.

Muskrat–a few were introduced to Europe, where they took off like rabbits in Australia. Photo by D. Gordon E. Robertson via Wikimedia Commons.

This is where muskrats (Ondatra zibethicus) come in. In 1905, three female and two male muskrats, a species native to North America, were released in Czechoslovakia. Within a decade, this tiny founding group had exploded into a population numbering in the millions. Muskrats are now found through most of Europe, and seem in no danger of imminent genetic collapse. In fact, far from being on the edge of extinction, they have become significant pests in many parts of Europe. (Incidentally, muskrats are one of those creatures that I feel compelled to watch every time I come across one, whether the animal is gliding on the surface of a pond or gnawing on some morsel on the bank of a river. I’m just glad I’m observing them on their native continent, where my pleasure in watching isn’t mixed with thoughts that they really need to be exterminated.)

A similar story holds for Indian mongooses (Herpestes palustris) which were introduced to Jamaica in 1872 to prey on rats that were damaging sugar cane plantations. As with the muskrats, the founding group was very small, consisting of just four males and five females. From this small band, the species came to occupy the entire island and, eventually, these Jamaican mongooses also became the source for populations on other islands of the West Indies and on Hawaii. (I’ve watched mongooses playfully rolling about on lawns on Maui, where my pleasure in seeing them was definitely compromised by thoughts of extermination.) An even more extreme example is that of a solitary bee species that was introduced to North America from Europe. In this case, a wide-ranging North American population apparently arose from a single introduced female.

Indian mongoose on Oahu. Photo by Daniel Ramirez via Wikimedia Commons.

The bottom line is that small founding groups often do give rise to large, persistent populations. In fact, if this were not the case, the worldwide havoc created by invasive species would be significantly less of a problem than it is. And it’s no leap at all to suggest that the same sort of colonization history has been followed by organisms that crossed oceans on their own.

None of this to suggest that the establishment of a population from a very small founding group is usually easy. Perhaps the genetic effects are, in fact, very strong for many species and contribute to failed colonizations. Even without those effects, many small dispersing groups probably disappear through chance events that have nothing to do with particular genetic shortcomings (“stochastic extinction”), and many must disappear simply because they never find environments suitable for them. But the point is that we should not assume that very small groups are always incapable of becoming established and producing vigorous populations in their new homes. The histories of introduced species show that such an assumption is clearly false.

References:

For the muskrat, mongoose, and solitary bee cases see Chapter Five in Invasive Species: What Everyone Needs to Know by Daniel Simberloff, 2013, Oxford Univ. Press.

For more details about the muskrat introduction see “The Muskrat, Ondatra zibethica (Linnaeus), in Europe” by Erna Mohr and Mabel P. Hollister, 1933, Journal of Mammalogy, vol. 14, pp. 58-63.