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Darwin Days: Tuco-Tucos Six Ways (Ctenomys spp.)

Posted on Feb 10, 2009 by in Rodents, Theme Weeks | 8 comments

Six species from the genus Ctenomys

Numbers 0232, 0233, 0234, 0235, 0236, and 0237

The day after tomorrow, February 12, 2009, is Charles Darwin’s 200th birthday. (It’s also Abraham Lincoln’s.) I’ve recently begun reading Darwin’s The Origin of Species, and I’ve decided to try to do a little something in celebration of Darwin’s immense contribution. I’m going to be highlighting a few mammals that Darwin discusses in The Origin of Species, and I’ll try to tell you something of what Darwin says about them. Since I haven’t read the whole book, I’m going to be reading the sections that mention these mammals out of order and a bit out of context, but I’ll do my best, and maybe my more Darwin-literate readers can help out where needed. I also am pretty clueless as to where Darwin’s ideas have been expanded or improved upon. Incidentally, the book is actually quite accessible, and I’m sorry I didn’t read it earlier. You can read it online or download a PDF at Google Books.

Chapter Five of The Origin of Species is titled “Laws of Variation.” In a section called “Effects of Use and Disuse,” Darwin posits that the shrunken state of the wings of some birds on islands where they have no predators results from disuse. There’s nothing to fly away from, so generations of birds stopped flying and used their legs more, and through natural selection their legs were strengthened and their wings, eventually, made useless. As another example, Darwin discusses animals that live in underground burrows and caves and have small, furred-over, or absent eyes, thanks to disuse. It’s dark down there, so eyes aren’t very helpful, and gradually, they are selected out, while things like longer whiskers and antennae, which are helpful underground, are favored.

The tuco-tucos, of which there are some 50 different species, live in burrows in South America. Although they spend a lot of time underground, they do occasionally peer out of their homes, and their small but useful eyes allow them to do that without necessarily having to actually exit the premises. It’s easy for them to duck back under if need be. (The amount of time they spend outside their burrows varies from species to species.)

Darwin says that he was told by someone who caught a lot of tuco-tucos that they are often blind. A tuco-tuco that Darwin kept in captivity was blind, he says, thanks to an inflammation of the eye. As Darwin points out, animals that live primarily underground don’t really need their eyes, and eyes are just another thing that can get infected, so why not do away with them? (Eventually. Over generations.)

Genetic analysis of one tuco-tuco species, the colonial tuco-tuco (C. sociabilis) yielded surprising results a few years back. For the past three millennia, the colonial tuco-tuco has had almost no genetic diversity. Usually, a lack of genetic variation means doom for a species, but the colonial t-t has survived pretty well. The reason, researchers think, is that it has evolved a very social way of life, unlike the other tuco-tuco species. If everyone is related, there’s no reason to compete; instead, you can help your cousins breed, since they’re passing on your genes, as well as their own. (In a practical sense, I think I would be more likely to get along with everyone if they were more or less my clones, too, but personality clashes are probably not a significant factor in the colonial t-t’s survival.)

According to Walker’s Mammals of the World, “Systematic understanding of this complex genus is in a state of flux, and it is likely that there will be much change in the number and relationships of species…” In this drawing of the goofy-looking guys, clockwise from the upper right are C. fulvus, C. sociabilis, C. flamarioni, C. talarum, C. peruanus, and in the middle, C. haigi.


  1. Thanks for such an interesting post.

  2. Yay tuco-tucos. I love the common names wikipedia has in the Tuco-tuco article: “Strong Tuco-tuco”, “Silky
    Tuco-tuco”, “Furtive Tuco-tuco”, “Tiny Tuco-tuco”, “Robust Tuco-tuco” I wonder how many of them are really in wide use.

    Darwin wrote, rather cryptically, about visual degeneration in Tuco-tucos in Chapter 3 of the Voyage of the Beagle published 20 years before The Origin:

    “In the tucu-tuco, which I believe never comes to the surface of the ground, the eye is rather larger, but often rendered blind and useless, though without apparently causing any inconvenience to the animal; no doubt Lamarck would have said that the tucu-tuco is now passing into the state of the Aspalax and Proteus.”

    That’s Darwin writing about evolution 20 years before publicly admitting that he believed in it!

  3. Hey, cool, Neil! Thanks! Proteus I know, but who/what is Aspalax? Do you know?

    I had the same thoughts about the common names. Walker’s lists no common names, but it is very conservative about using them, I think.

  4. Aspalax is an old name for Spalax the Eurasian blind mole-rats, not to be confused with the African naked mole rats!

  5. Oh. Well, now I have to ask, what’s Proteus in this context? I assumed it was the mythological figure who could change shape (

  6. That’s actually a very fortuitous convergence! Darwin was talking about the blind cave salamander known as the Olm. Ironically, olms are neotenous (they don’t transform fully to an adult form) so they are actually less “protean” than other salamanders. Go figure!

  7. Very nice article! I’m part of a research group that studies the morphological adaptations and the evolution of tucotucos.
    Our local species are C. talarum (which looks a lot like your depiction of it) and C. australis (superficially similar to C. flamarioni).
    Evolution is essential to our understanding of how tucotucos are similar and yet diverse, and convergent with other subterranean rodents around the world.
    Great comments by Neil too :-)

  8. Calimecita, thanks for stopping by! I’m relieved to hear my tuco-tuco drawing isn’t terribly inaccurate. I find the convergent evolution of various subterranean rodents very interesting, and then there are the moles that fit in the picture, too, and other critters.

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