Compass Confusion - The Pigeon's Magnetic Myth
Ben - Now, how do birds navigate very long distances? It's been known for a while that many different animal species are sensitive to Earth's magnetic field and they use this to find their way around. So when researchers spotted iron-rich cells in the beaks of homing pigeons, they concluded that they must be magnetically sensing nerve cells wired up to the bird's brain, like a sort of neurological compass. But it looks like this conclusion was actually a step in the wrong direction because now, new research obtained by scanning the heads of hundreds of pigeons has shown that these iron-rich cells are not neurons. They're actually immune cells and probably have nothing to do with navigation. David Keays from the Institute of Molecular Pathology in Vienna explains more.
David - The big question in this field is how do animals detect magnetic fields and for the last decade or so, it's been believed that pigeons detect magnetic fields by using neurons that contained crystals of iron that were located in the very front of their beak. We started working on this about 4 years ago and our goal really was just to replicate those studies that had previously been published, but unfortunately, we weren't able to do so.
Ben - What is the evidence that birds have this magnetic navigation at all? Could it just be that we've got the wrong idea entirely and so we're going off looking for a mechanism for something that doesn't actually happen?
David - So it's clear that when birds migrate, they use multiple cues. They use vision, smell, and the evidence is that they also use magnetic fields, and multiple groups have been able to show independently that birds can detect and respond to changes in the Earth's magnetic field.
Ben - So obviously, the question is still open, but what were you actually doing to look at what was thought to be these iron-rich neurons?
David - We took a pigeon and we sliced it up into wafer thin sections from the tip of the beak, all the way back and then we took these sections and we stained them with a very simple and basic chemical stain called Prussian blue - it makes iron-rich cells bright blue in colour. And then we wanted to map the location of these cells onto the pigeon beak so we used MRI and advanced imaging technologies, and this revealed a startling diversity in the distribution of the location of these iron-rich cells in the pigeon beak. In one of our pigeons, pigeon 203 (which is a reflection of how many pigeons we looked at), it had about 108,000 cells whereas pigeon 200, which was the same age and same sex, had just 200 of these iron-rich cells. So this kind of got us thinking that these blue cells probably were a bit of a red herring and weren't involved in the true magnetic sense.
Ben - You would assume that if it is so important for navigation that it would be quite well conserved between different pigeons and even between different species of bird. So clearly, that's an indicator that this isn't actually doing that job.
David - That's spot on. This is a big clue that these cells weren't the magnetic receptors and then we got another bit of luck really. Pigeon 199 had an inflammatory lesion in the front of its beak and it was surrounded by about 80,000 of these blue cells that kind of infiltrated this lesion, and that got us thinking that maybe these blue cells have something to do with the immune system. We confirmed this by using transmission electron microscopy to actually see inside the cells and we found these cells are packed full of ferritin granules which is kind of an iron storage protein and they have these long tentacles. And in some of their images, you can actually see them kind of engulfing other cells.
Ben - So that suggests that they are in fact macrophages, these big eater cells, part of the immune system that are responsible for getting rid of infections. Why would macrophages be so full of iron?
David - The other role that macrophages play is they recycle iron from red blood cells. So they build up all this iron and then they also play a role in iron homeostasis.
Ben - So we would expect macrophages to be sources of iron anyway and does this completely put to bed the idea that these are nerve cells and that they could be responsible for navigation?
David - We would say it does, but what it really does is just raise a whole lot of new questions. So, how do pigeons detect magnetic fields? How do other birds detect magnetic fields? It had previously been asserted that this was a magnetic sense system that was common to avian species. What we've shown is that macrophages are found in all avian species, so the jury's out as to how they do in fact detect magnetic fields.
Ben - When you were doing such high resolution imaging and looking very closely at this, did you get a clue from the distribution of what might be going on? Do we think that perhaps there's an interaction between the macrophages and nerves or are the macrophages purely doing their normal jobs and it's just a coincidence that we find them in the beak?
David - Well, we thought about this and I suppose it's hypothetically possible that a macrophage might be a magnetic receptor, but it really seems so unlikely. In the hunt for the true magnetic receptors, we're now looking at other regions of the pigeon head.
Ben - So where are we looking? Where do we think is the right place to start looking and, if they're not going to be these iron-containing cells that we originally thought, what are we now looking for?
David - At the moment, we're looking at the olfactory epithelium. So the olfactory rosette in rainbow trout has been implicated in magnetic reception and so, we're having a closer look at this region in the pigeon. And it's also clear that birds, particularly migratory birds, also use a light dependent mechanism that most probably resides in the retina and relies on a molecule, or is thought to rely on a molecule, called cryptochrome. And so, there's probably a magnetic compass in the eye of birds as well.
Ben - David Keays from the Institute of Molecular Pathology in Vienna and that work was published in the journal Nature this week and I think it's a good reminder that science is an ever changing field, and sometimes what we think we know turns out to be wrong.