A lousy paper

Well, it’s actually a good paper, but it’s about lice.  It’s also short and cute: a note by Bush et al. in the new American Naturalist.

Dale Clayton, a former student in my department, a professor at The University of Utah, and inventor of the famous LouseBuster™, has spent his career working on ectoparasites (parasites that live on the outside of their host).  His specialty, however, is feather lice in birds, which eat the feathers and dead skin of their hosts. (Like all lice, these ones are insects in the order Phthiraptera.) There are all kinds of nice evolutionary studies you can do on these beasts. For example, some species of birds harbor up seven species of feather lice, all specializing on different parts of the body.

This raises a phylogenetic (family-tree) question: can a louse form a new species on its host? This goes to the evolutionary controversy about whether a new species can form in a very small geographic area.  But when you make a molecular family tree of the lice on a single bird species, you never see “sister species” of louse (species that are each other’s closest relative) living on a single species of bird. That implies that it’s hard to form a new species on one’s host: the different species on different parts of a bird’s body apparently arise after cross-infection from others species of bird. (See our book Speciation for more details.)

Anyway, Clayton and his colleagues (Sarah Bush was the first author) asked an obvious question that nobody had raised before: can ectoparasites be camouflaged (“cryptic”) on their host?  You might expect this if hosts detect their parasites visually and groom them away.  This would give an obvious evolutionary advantage to those lice having mutations that made them harder to see. (There’s also an obvious advantage to the birds to remove lice, since a bad infestation can severely weaken a bird.)

Bush et al. took 26 pairs of closely related birds (each pair from a different family) that differed strongly in color—one light and one dark species. (Examples:  black swan versus mute swan, glossy ibis versus white ibis, European starling versus chestnut-tailed starling.)  These also harbored closely related species of feather lice—lice in the same genus.

The hypothesis was that if louse color evolved by selection to avoid being groomed to death, you’d find light-colored lice on light birds and dark-colored lice on dark birds.  As a control, the authors also looked at head lice, which are not detected visually since the birds can’t see them.  These are scratched off the head with the bird’s feet.  (The authors call the visually groomed lice “typical” lice.)

The upshot: there was a significant relationship between host color and louse color in typical lice—and it was in the expected direction.  The control showed no relationship between host color and louse color in head lice.  Here’s one of their pairs (two cockatoo species and their resident lice), showing the crypsis and the contrast when you put one host’s lice on the other:

Figure 1: Example of background matching in typical feather lice. The light-colored louse, Neopsittaconirums albus, parasitizes the sulfur-crested cockatoo (Cacatua galerita; A). The dark-colored louse, Neopsittaconirums borgiolii, parasitizes the yellow-tailed black cockatoo (Calyptorhynchus funereus; B). The hosts’ feathers are the natural background for these lice. Both species of lice were photographed on feathers from a sulfur-crested cockatoo (A, inset) and a yellow-tailed black cockatoo (B, inset). Cockatoo photos by Trevor Hampel (A) and Fir0002/Flagstaffotos (GFDL ver. 1.2; B).

You might have noticed one problem here: are the lice colored like the hosts simply because their color comes from eating feathers and skin that are either light or dark?  That is, do the differences in louse color represent evolved, hard-wired genetic differences, or are they simply environmentally induced traits—like the pink color of flamingos—that have the fortuitous benefit of protecting the lice?  If they’re not hard-wired, then the colors wouldn’t reflect natural selection acting on genetic mutations.  The authors consider this unlikely since the color of lice doesn’t reflect their gut contents, and because they’ve done experiments putting lice on differently colored rock pigeons without any effect on louse color.  Also, if color simply came from eating differently-colored skin and feathers, you’d expect to find head lice showing the same color correlation as “typical” lice—but you don’t.

Still, it would be nice to transfer louse species between the differently colored bird species to see if there’s an effect on louse color.  This would be hard, though, since lice often won’t feed well on host species to which they’re not adapted.

Conclusion: If the differences in louse color really are genetic, score one for natural selection.  If there hadn’t been a correlation, I suppose you could claim that color in lice isn’t important in hiding them in the feathers (maybe they’re really detected through touch), and of course the paper would not have been publishable! (People don’t find that kind of negative result very exciting). But the authors weren’t really testing the working of natural selection itself: they were testing whether the variation in color among louse species could be explained by the variation in color among their hosts.

This opens up a whole new line of research on camouflage in ectoparasites.  The authors cite previous work suggesting that parasitic flatworms in fish, for example, might be cryptically colored to hinder their detection by cleaner fish.

I suppose a creationist could explain the correlation found by Bush et al. by claiming that God made the lice to match the color of their hosts.  But that presumes that God has an inordinate fondness for lice, and likes to see them torture and kill birds.

UPDATE: I am informed by Professor Clayton that transferring lice between bird species in the same family should not impede their feeding behavior.


Bush, S. E., D. Kim, M. Reed, and D. H. Clayton. 2010.   Evolution of cryptic coloration in ectoparasites.  Amer. Natur. 176:529-535.

21 thoughts on “A lousy paper

  1. But “sister species” of lice do exist in humans. Body louse is apparently a slightly different version of head louse which evolved after we started to wear clothes.

    1. Good point—we talk about this case in our book. It’s not at all clear that head and body lice really are different species. The taxonomy is pretty messy, and they might not be “species” (that is, reproductively isolated entities) at all. This case is at present in the category “we don’t know.” Also—although this may be special pleading—some have suggested that head lice evolved in one population of humans, body lice in another, geographically isolated population, and then when those human populations came into contact, the lice simply hopped between people.

      1. This case is at present in the category “we don’t know.”

        Ah. A Mystery. The ontological status of the head louse and the body louse relative to each other points toward the transcendent, therefore God exists.

  2. Incidentally god does love lice, we know that, right?
    Just like he loves mushrooms. Why else would he create so many of them?

    1. Yeah, and how about beetles?

      I’d be interested in seeing what they found in pied birds, like the Black-billed Magpie, or multicolored birds, like the Painted Bunting. How about those species, like the Blue Grosbeak, where males and females are wildly differently colored, but where the juvenile males look like the females?

  3. What about lice on species like the Ptarmigan/willow grouse that change colour in winter? Also, all other selective pressures being equal, would there be an advantage to a bird having multicoloured feathers so lice showed up when moving from one colour to another? Finally, what happens in other frequencies of light – birds can see ultraviolet can’t they?

    1. Oh sorry – I see they cover that last point “Because feather lice are not brightly colored, per se, nor
      reflect much in the UV spectrum (Kim 2008), we simply scored “luminosity,” which is an index of the overall lightness
      or darkness of a subject.”

    2. Something else occurs that is interesting – a population of lice/parasites on an individual is isolated except where birds are in close proximity to each other. When the host dies the whole population on that ‘island’ dies with it. It is only by attaching to young birds every time the host breeds that they can hope to carry on. As it is a bird like an albatross (included in the study) is on its own for long periods – during that time the lice will I assume carry on breeding even though that host idividual may die before it gets to breed or come into close contact with others of its species.

  4. “If there hadn’t been a correlation, […] the paper would not have been publishable! (People don’t find that kind of negative result very exciting)”
    Wrongly, because negative results are still results 😀
    And they can be interesting enough, take for example this study: http://www.cell.com/current-biology/retrieve/pii/S0960982206002533
    The authors’ thought that “Ubx and abd-A Hox genes drove the evolution of body segment diversity in arthropods” (quoting from Biology, by Campbell and Reece – I read a few hours ago, that’s why it came to my mind) and to test it they examined a onycophoran, expecting that “Ubx and abd-A would not have been present in the common ancestor of arthropods and onychophorans, and hence onychophorans should not have these genes”.
    The results contradicted their hypothesis (the examined onycophoran does have those genes), and this negative result *is* an interesting result – it suggests that “the evolution of increased body segment diversity in arthropods must not have been related to the origin of new Hox genes”
    “I suppose a creationist could explain the correlation found by Bush et al. by claiming that God made the lice to match the color of their hosts. But that presumes that God has an inordinate fondness for lice, and likes to see them torture and kill birds”.
    This reminds me of that Darwin’s letter to Asa Gray: “I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of caterpillars or that a cat should play with mice…”

  5. I suppose a creationist could explain the correlation found by Bush et al. by claiming that God made the lice to match the color of their hosts.

    Nonsense. Everyone knows a good designer likes contrasting colors.

    Now you’ve got me all fired up for ‘Louse Week’.

  6. Creationists don’t generally tend to get too worked up about this kind of thing these days. They say it’s adaptation not evolution and are normally pretty happy to accept it. See the response to Hopi Hoekstra’s work on deer mice for the sort of reasoning they employ:


    They often go on to argue that this sort of evidence supports a creation model of rapid post flood adaptation – God has given creatures the tools to adapt or something.

      1. Well, from their perspective they need a surprisingly large amount of evolution to account for post flood diversity. I think what happened was that gradually the old guard (Gish and the like) faded away and new people came on the scene who realised they really couldn’t go on denying all the evidence (just some of it). And, what better way to deal with needing to accept more evolution than to say ha, this actually supports our model of rapid diversification post flood! Here’s an example of them being happy with speciation, for example:


        There still doesn’t seem to be any consensus as to exactly how much evolution they’re prepared to allow (despite not knowing, they still confuse their followers with a micro/macroevolution distinction that they can’t possibly define – as they don’t know the limit to change). Here’s one such discussion:


        Indeed, some YECs have gone quite far in accepting evolution. The more sophisticated YECs accept the fossil record of horse evolution for example:


        Todd Wood, who wrote that article has a good blog that’s worth reading. An insight into the mind of the more modern sophisticated YEC. He spends quite a bit of time criticizing fellow YECs and writes posts such as “evolution is not a theory in crisis”:


        He’s a baraminologist. Interestingly, Phil Senter a dino palaeontologist recently wrote a paper in Evolution using baraminology methods to demonstrate that dino-bird evolution is likely:


        1. Amazing – they really do give their god a lot of work to do – thanks for the full explanation.

          If they say that the earth/universe is 6,014 years old & their ‘flood’ 2348 BC (Ussher) that means that 1,656 years comprises all the geological time & fossils up to that point!

  7. Damn you Darwin, had you not created natural selection the mor[m]ons would nary have a moment to spare for screwing the dead after separating the white lice from the black birdies prior to wetting.

    Damn you Darwin, damn you straight to heaven.

  8. Interesting study.
    One question:
    Are there albino mutants of the dark colored birds, and if so, what color are their lice?
    This should address the environmental issue of louse color.

  9. I’m a layman, so please forgive any stupidity on my part.
    I have read in several articles and books that DNA studies of body lice let scientists determine the date at which they evolved from head lice. That, in turn, allowed an estimate when people started wearing clothes.
    Here’s my question: Could a similar study of bird lice reveal when feathers evolved? I presume birdlike feathers first appeared on dinosaurs, so the 10-year-old kid in me would find that especially interesting. (Dinosaurs are way cool,)

    1. Interesting point – dinosaurs must surely have had ticks & mites as reptiles do – we would need to ask an ‘acarologist’ – but they are arachnids. Lice are insects – perhaps they originally flew onto hosts but lost their wings? Here is the lice page on the Tree of Life Project -http://tolweb.org/Phthiraptera/8237

  10. On a slightly related note, David Clark, in “Genes, Germs and Civilisation: How Epidemics Shaped Who We Are TodaY”, in the chapter on typhus noted:

    “It is difficult today to imagine just how lousy and verminous our predecessors were-even the upper classes. Archbishop Thomas a Becket was assassinated in December 1170, and his body lay overnight in Canterbury cathedral awaiting burial the next day. He had on “a large brown mantle; under it, a white surplice; below that, a lamb’s-wool coat; then another woollen coat; and a third woollen coat below this; under this, there was the black cowled robe of the Benedictine order; under this, a shirt; and next to the body a curious hair-cloth, covered with linen.” As the body grew cold, the inhabitants of these multiple layers began to evacuate. ‘The vermin boiled over like water in a simmering cauldron, and the onlookers burst into alternate weeping and laughter.'”

  11. What a satisfying study! Thanks!

    Free-associating on bird parasites…

    I’ve had 2 bald birds at my feeders this season, a cardinal and a redwing:


    The birding blogs say scientists think this may be due to head mites, but there are other possibilities…You’d think something this common would be pinned down better than that.

    Am also homing 6 rescue chickens ATM. They have a few lice; the lice are tannish, the birds reddish brown. But I’m sure this parasite/host color correlation wouldn’t apply to a species like a chicken that’s artificially bred to occur in so many colorful breeds! The lice are immediately noticeable when they crawl on one’s skin, though…I suppose feather eating lice have not had to evolve mechanisms to prevent the host from feeling them. Another area for research? Are lice that directly crawl on skin less noticeable to the host than those that normally stay on dead materials like feathers? Teensier tarsal claws?

    The nice thing about biology is that even these little husbandry problems have their fascinating side…:D

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