What’s the News: One of the leading theories explaining why birds can travel thousands of miles each year without getting lost is that they are equipped with an internal compass on their beaks: iron crystals in cells there could be orienting to the Earth’s magnetic field, sending a message to their brains that helps them steer.
It turns out, though, that those beak cells, which had been previously identified as neurons capable of sending such a message, aren’t neurons at all. A new study shows convincingly that those iron-bearing cells are macrophages, immune cells that could never play that role. Birds’ magnetic navigation skills must be coming from somewhere else.
What’s the Context:
The magnetic beak neuron theory, proposed in 2007, had already been in doubt; a study in 2009 found that while certain pigments in birds’ eyes were required for them to respond to magnetic signals, screwing with the beak “neurons” didn’t seem to affect it. This team, though, has put the nail in the coffin with a series of detailed histological, or staining, dissections of the beaks of pigeons.
How the Heck:
While the 2007 study reported that the cells were distributed evenly on either side of the beak in a pattern that suggested they were neurons, this team found that they were not.
Moreover, different birds, even birds of the same type and sex, had wildly different numbers of the cells, a red flag that they probably weren’t involved in an ability that the birds had in common.
Stains that usually cling to neurons made no mark on the cells, while macrophage stains did, and when the team looked closely at their structure with an electron microscope, they noticed that they appeared to be engulfing neighbor cells, something macrophages do as part of their work but neurons don’t.
Just to hammer it home, they found cells that looked exactly the same throughout the pigeons’ bodies, in areas with no known connection to navigation. The cells even appeared to be responding to an infection in one pigeon, just as macrophages would.
The Future Holds: So where are birds getting their uncanny powers? The theory put forth by that 2009 study, that a chemical reaction between light and pigments in the eye is involved, is currently the best lead. Read more about it here.
Reference: Treiber, et al. Clusters of iron-rich cells in the upper beak of pigeons are macrophages not magnetosensitive neurons. Nature (2012). doi:10.1038/nature11046
Image courtesy of crossbone80 / flickr