Why Evolution is True is a blog written by Jerry Coyne, centered on evolution and biology but also dealing with diverse topics like politics, culture, and cats.
[JAC: In lieu of “readers’ wildlife” today, we have “Mayer’s wildlife”: His disquisition on snake locomotion. Be sure to keep those photos coming in, and don’t worry if you haven’t seen yours yet, as I have them all.]
by Greg Mayer
Matthew sends the following tweet of a tiger snake making its way along a wire fence.
At first glance, two things struck me at about this, aside from its generalized coolness. The form of locomotion is a typical one for snakes called lateral undulation, in which waves of muscular contraction alternate down the sides of the body. You can see the snake is pushing first on one side of the wire, then the other, in waves down the body. This is not unusual for snakes. And there are many arboreal snakes (vine snakes, parrot snakes, etc.) that habitually move along very narrow surfaces, such as vines and branches. The novelty here to me is the length of the narrow surface– most vine snakes frequently encounter crosswise vines and branches, so they don’t move for any great distance in a perfectly straight line along a narrow surface, as this snake is doing.
Other climbers show a fabulous ability to throw their trunk into multiple, regular, and controlled bends of very short radius. The African file snakes (Mehelya) apparently can travel along telephone wires with alternate half-loops hanging respectively over the left and right sides of the wire.
The second thing that struck me was that a tiger snake is not a vine snake of any sort– they’re terrestrial. So, climbing along wires is not where I would expect to see them. But that’s book knowledge, and perhaps Australian readers can enlighten us from experience.
On reflection, I was also struck by this being an example of what Gans called “excessive construction”– the ability of structures (and in this case also behaviors) to be successfully used in circumstances that were not part of the historical evolutionary development of the structure. Gans thought, and I agree, that such circumstances can be the basis for adaptation (i.e. heritable changes in the structure/behavior) to the new circumstances. Again from Biomechanics (p. 14-15):
Gans provides a much more insightful view here of how functions change, and how new adaptations arise, than did Gould and Vrba in their largely unnecessary coining of the word and concept “exaptation“.
Gans, C. 1974 (1980). Biomechanics: An Approach to Vertebrate Biology. University of Michigan Press, Ann Arbor.
Gans, C. 1979. Momentarily excessive construction as the basis for protoadaptation. Evolution 33:227-233.
Gould, S.J. and E.S. Vrba. 1982. Exaptation- a missing term in the science of form. Paleobiology 8:4-15. pdf
Appropriately following upon Jerry’s monstrous, triffid-like seed pod, an attack by tiny monsters on an Australian teenager has been splashed across world media, including the BBC and the New York Times. The victim, Sam Kanizay was cooling off after a football match by wading in the sea near Melbourne. After a half hour in the water, he emerged bleeding profusely from the ankles, and the bleeding did not readily stop. He was taken to the hospital and should be just fine.
Sam Kanizay being treated in hospital. Photo by Jarrod Kanizay, via Australian Associated Press.
The interesting question from a biological point of view is “What did this to him?” We have a natural history whodunit, with two contenders, both crustaceans, and both quite small: isopods or amphipods. The BBC, citing Genefor Walker-Smith, said it was amphipods. The Times said the consensus was that it was isopods. Sam’s father Jarrod put some meat out in the water, and collected a host of critters, and posted a video of them to Youtube.
Most people in the north temperate zone are probably familiar with what we call in New York “cement bugs”, but are known by many other names: sow bugs, pill bugs, rollie pollies, etc. These are isopods, and there are marine ones called sea lice. Amphipods are less familiar in the Northern hemisphere, as they are aquatic and marine, and thus less commonly encountered. (There are terrestrial ones in the Gondwanan continents.) The ones that live at marine beaches are called sand fleas. One way to tell at least the usual ones apart is that isopods are dorso-ventrally compressed (‘squashed’ from above); while amphipods are laterally compressed (‘squashed’ from the side), and typically lay and move about lying on their sides. Both are said by the news reports to occasionally bite people.
In the video, you can clearly see that the critters are amphipods– pause the video, and enlarge on the screen if necessary, to see this. The Times quotes Alistair Poore, of the University of New South Wales, as also saying the critters are amphipods. However, although Jarrod trapped them by using meat as a bait, it’s not certain that what he trapped are the same things that bit Sam.
Australia is a zoogeographer’s dream world—it’s the most spectacularly distinctive place on Earth, and we know why. Around 250 million years ago, most of the world’s continental plates amalgamated into a single super-continent—Pangaea. During the Mesozoic (the “Age of Reptiles”), Pangaea began breaking up, with many of today’s southern continents (South America, Africa, Antarctica, Australia, and India) pulling away to form the somewhat smaller super-continent of Gondwanaland.
The continental breakup continued, with the various parts of Gondwana separating from one another (hence the traditional rallying cry of irredentist geologists, “Reunite Gondwanaland!”). Africa, India, and, most recently, South America eventually bumped into the northern continental masses, making for interesting geology, and—of the greatest importance for zoogeography—allowing large land animals to move between the major land masses. Such animals are not, in general, susceptible to “occasional means of transport”, as Darwin called them, that allow birds, bats, and smaller animals to traverse greater or lesser expanses of the sea, and thus large land animals require a dryshod path to disperse.
But, unlike those other Gondwanan remnants, Australia has not—yet, anyway—bumped into the northern land masses, and thus has undergone a long and and continuing period of splendiferous isolation, during which time many unique endemic forms have arisen, and radiated into the great variety of ecological niches occupied by different lineages in the rest of the world. Most famous of these are the Australian marsupials, which have undergone a continental-scale adaptive radiation, which Jerry highlighted in chapter 4 of WEIT.
This radiation has brought marsupials into most of the ecological niches inhabited by placental mammals in the rest of the world—predators, herbivores, gnawers, burrowers, insectivores, gliders, etc. Two things are evident in the radiation of Australian marsupials. First, that convergence can lead to remarkable similarity when distantly related lineages adapt to similar environments—Jerry highlights this in the figure above; but, second, that sometimes forms inhabiting the same ecological niches can be quite different.
We can see both of these by thinking about which animals are the big, dominant, mammalian carnivores and herbivores. Everywhere on Earth but Australia, these animals are cats, dogs, cattle, and deer (taking cattle in the sense of the family Bovidae, including antelope, goats, etc.). In Australia, the dominant carnivores are the thylacine (or Tasmanian wolf or tiger), native cats (hence, placental cats in the post title, to make clear who I meant), and devils. The skull of the thylacine is remarkably wolf-like, showing a close convergence in shape and dentition to the placental wolf. Native cats have the name, but are less similar to cats; and devils are pretty much sui generis. The dominant big herbivores in Australia are kangaroos: instead of plains full of buffalo and antelope, Australia is full of kangaroos. Although they eat similar types of plants, the modes of locomotion are startlingly distinct, showing that close convergence is not inevitable. (There were some more ungulate-like marsupials in the past, but they are now extinct.)
All this was brought to mind by a new article in BMC Evolutionary Biology by Katrin Koch and colleagues on the placental cats of Australia. For some millennia now, in addition to Darwin’s occasional means of transport, another factor has allowed animals to cross the seas—human transport. And for thousands of years, man has broken Australia’s tens of millions of years of isolation by bringing in a diversity of placental mammals. Most famous is the dingo, the feral descendants of dogs brought from southeast Asia about 4,500 years ago. Since European settlement, the number of mammalian imports—both wild and domestic—has increased dramatically.
When zoogeographic regions collide: a feral cat in Queensland, eating a road killed kangaroo, by Joe Scanlan via the Daily Mail.
In a careful review of historical records (which includes this memorable sentence in his methods section: “Incidentally, I discovered that indexers of books rarely index ‘cat’.”), Ian Abbott (2002) showed that, despite the potential for cats to have been brought to Australia earlier by Aborigines, Malay trepangers (sea cucumber fishermen—it’s great that there’s a word for that!), or shipwrecks, the first cats seem to have been brought in by the earliest European settlers in the late 18th century. The map below shows places where cats were presumed to have been introduced (arrows) and dated records of cat appearance (dots):
Fig. 2 of Abbott (2002).
Koch and colleagues looked at microsatellite and mitochondrial DNA—both rapidly evolving parts of the genome, and thus good for studying infraspecific phylogeny—in over 200 feral cats from Western Australia and a number of islands round Australia, including the outlying territories of the Cocos (Keeling) Islands (an island group where cat control is an issue, due to their depredations on wildlife: Algar et al., 2003) and Christmas Island.
What they found was that the cats of mainland Western Australia, and the big island of Tasmania, were all genetically fairly similar (the red areas in the figure), while the smaller islands were more distinct genetically, including an island very close to the mainland (Dirk Hartog Island, DHI–green), as well as the two outlying islands of Cocos (yellow) and Christmas (blue). Curiously, two smaller islands off southeastern Australia grouped with Christmas. Small island populations can diverge due to random genetic effects (founder effect and genetic drift), historic phenomena (founded from different sources), and selective differences (distinct environments on islands). But it’s hard to tease these apart from these data alone.
Fig. 2 of Koch et al. (2015). Map of Australia, Southeast Asia and Europe with possible invasion routes. Possible invasion routes of cats shown on a map of Australia and Southeast Asia with Europe (EU) in the top left-hand corner. Arrows indicate invasion routes with highest support from the phylogeographic model selection approach (model 10 grey arrows; further details in Additional file 4: Figure S3). STRUCTURE plots showing ancestry (K = 4) inferred from microsatellite data for mainland Australia, Australian islands and Southeast Asia. Each individual cat is represented by a single vertical line in plots for each location.
This paper has made a minor splash in the media (see Jerry’s mention here about NY Times coverage), with most places proclaiming that it shows that Australian cats came from Europe as opposed to southeast Asia. Now, the paper does show that of the 63 mitochondrial haplotypes that they found, 25 are also present in Europe (the European data are from a paper by another group). But to distinguish sources of colonization, you need to have samples from all the potential sources, find out if the source populations have any diagnostic or characteristic alleles or mutations, and then see if these are found in the colonized (i.e., Australian) populations. But Koch et al. had only three non-Australian cats—apparently one from Sulawesi, and two from Borneo. (Curiously, they refer to these as “Malaysian”, and also use that term for 17th century Malay trepangers. But Malaysia is a 20th century political construct, and Sulawesi is in Indonesia, not Malaysia, so Malay would be a better term that covers the cultural/linguistic/geographic region.)
The three Malay cats group with Australian mainland cats. But on such a slim basis, we can make no conclusions about the relative importance of the two potential source areas. While concluding their data “indicate a mainly European origin of feral cats in Australia”, the authors do allow that, “However, caution is needed in inferring the involvement of Asian cats in the history of cat colonization in Australia due to the small number of Asian samples.” We can, in fact, be sure that a significant, if not the greatest, part of Australian cat ancestry is European, but that is because of the historical researches of Abbott (2002). The genetic work of Koch et al. lays a basis for further studies of genetic variation in Australian cats and their relation to cats from other regions, but it does not, on its own, really speak to the latter question.
Abbott, I. 2002. Origin and spread of the cat, Felis catus, on mainland Australia, with a discussion of the magnitude of its early impact on native fauna. Wildlife Research 29:51-74. abstract
Algar, D., G. J. Angus, R.I. Brazell, C. Gilbert and D.J. Tonkin. 2003. Feral cats in paradise: focus on Cocos. Atoll Research Bulletin 505. (Actually published in 2004.) pdf
K. Koch, K., D. Algar, J. B. Searle, M. Pfenninger and K. Schwenk. 2015. A voyage to Terra Australis: human-mediated dispersal of cats. BMC Evolutionary Biology 15 (262), 10 pp. pdf
Just in case you think rightwing homophobic nuttery is confined to a certain political party in a certain country south of Canada and north of Mexico, here’s heartening news. Or not.
Australia’s agricultural minister Barnaby Joyce is finally achieving global fame for claiming that legalising same-sex marriage could damage cattle exports.
“Where we live economically is south-east Asia, that’s where our cattle go” he argued.
“When we go there, there are judgments whether you like it or not that are made about us. They see us as decadent.”
He apparently previously opposed legislation allowing same-sex marriage on the basis that it would prevent his daughters from marrying men, so it is probably safe to say that he is not Australia’s finest example of a logical thinker.
So here’s a poll. Without knowing a thing about the man or his politics, which of the following statements do you believe to be most likely to be actual positions held by Joyce?
Because you are all psychic, you all correctly chose: (answers below the fold)
