"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

October 26, 2014

Columbicola columbae

You would think that of all living things, parasites would have the least need to move around. After all, it is sitting in its ideal habitat and is already (in a way) surrounded by food. Why would it need to go anywhere else? But most parasites usually reside at a very specific part of the host's body - at some stage, it would have had to makes its way there somehow, even if it stays in one spot after that. Furthermore for some parasites, where they live on the host is not the same as where they eat, so they have to commute regularly in order to get their meal ticket.
Photo by Vince Smith at phthiraptera.info

One such parasite is the humble pigeon louse (Columbicola columbae), which is usually found hanging out on the wing feathers of pigeons. It has evolved a narrow body that allows it to fit snugly between the barbs of the flight feathers and safe from the preening beak of the host. But while wing feathers are a nice place to seek shelter, they do not make for such an appetising meal - they are far too tough for C. columbae to chew on. So when the pigeon louse gets hungry, it needs to make a move to the body region where the more palatable, downy feathers are found.

So how does C. columbae find its way from the wing to the body? It's not like it can just look up Google Pigeon or something like that and get directions. Well, based the study we are featuring today on this blog, they use temperature to find their way.

Like us, birds are homeotherms - which means they keep a consistent body temperature, regardless of the outside environment. But even for a homeothermic animal, the temperature is not consistent across the body. For example, the temperature at the wings and tail of a pigeon is about 32 °C (89.6 °F), whereas the body region temperature is approximately 36 °C (96.8 °F). So are the lice using temperature differentials across parts of the pigeon's body as a cue for navigation? To find out, a pair of researchers did a series of experiments to determine what temperature the lice preferred under different circumstances.

They did a choice experiment where they put some pigeon lice in a glass petri dish with one end resting on top of a heated metal block. They also did another experiment where they placed some lice on a piece of filter paper sit on a heating apparatus that they built to generate a radial temperature gradient. In both experiments, they recorded where the lice moved to and found while the lice did respond according to the temperature differences, it was also dependent on whether they were hungry or not.

Lice which had a full belly prefer to hang out at 32 °C (wing region temperature), but those that have been experimentally starved for 18-20 hours tend to move to where it is 36 °C (body region temperature). But if down feathers are so tasty, why don't they just hang out there all the time? While the pigeon's main body is covered in tastier feathers, it is also more exposed to the murderous beak of a preening host. Whereas on the wings, the skinny body of C. columbae allows it to tuck itself between the barbs of the pigeon's flight feathers, and stay safe and sound.

So some lice like it hot, but only if they are hungry.

Harbison, C. W., & Boughton, R. M. (2014). Thermo-orientation and the movement of feather-feeding lice on hosts. Journal of Parasitology 100: 433-441.

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