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.