Feather lice take a heavy toll on their bird hosts by reducing survival and mating success. But to do their damage, and to survive themselves, these parasites need to evade detection by the continued efforts of their grooming hosts. As shown in a remarkable new paper from Sarah Bush and her colleagues at the University of Utah, USA, one of the most effective ways that lice can save their skin is by blending in.
Bush and her team focused on the much-maligned rock pigeon, a feral species whose plumage ranges from black to white. Looking more closely, they observed a close correspondence between feather colour and lice colour: white birds have light lice and dark birds have darker lice. But what drives this association? One possibility was that lice are phenotypically plastic and can change colour by increasing or decreasing pigmentation if they find themselves on a mismatched host. This isn't apparently the case. When the team bred lice on a middle-of-the-road grey bird, they determined that baby lice are the same colour as their parents; colour is highly heritable. The second option for colour matching was natural selection imposed by the vigilance of preening pigeons.
The preening idea makes good sense. Our eyes, and those of pigeons, are very good at resolving colour contrasts – black stands out on a white background and vice versa. To test whether pigeons selectively preened mismatched lice, the team used a series of manipulations that were perfect for their simplicity and clarity. They painted lice: some white and some black. Then they loaded these lice onto black and white pigeons, half of which were fitted with little mouthguards that prevented them from fully closing their beaks, thus rendering their preening useless. As expected, preening works as a means of removing lice; birds with mouthguards had 10 times more lice than those without. More importantly, after only 48 h, birds that could preen normally were almost twice as likely as their impaired counterparts to have removed conspicuously mismatched lice. In other words, preening was highly selective.
Of course, natural lice aren't painted and lice live and reproduce on pigeons, which creates the potential for them to evolve increased camouflage through time. The team examined this possibility by infecting birds with a mixture of greyish lice. They then left them to do their thing (when two lice love each other very much…) for 4 years, representing about 60 lice generations, while periodically measuring lice coloration. Their results were unambiguous. Lice on dark birds evolved to become darker and lice on white birds evolved to become much lighter – indeed, as light as lice that have been living on another species of white pigeon for millions of years.
The most amazing thing about this study is the pace of the response. It reveals an incredible capacity for evolutionary change in these lice, and clarifies how effectively host defenses can drive and reinforce rapid adaptive changes in parasites. In addition, the work argues forcefully for the power of experimental evolution to measure adaptation in real time. Seeing, or not seeing in this case, is definitely believing.