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Chapter 1: What unique role does the immune system play in animal navigation?
Show me the science with Professor Luke O'Neill and News Talk Original Podcast.
Hello, Luke O'Neill here, and welcome to Show Me The Science, my weekly podcast. And this week, I'm overjoyed to tell you one about the immune system. Now, of course, many of mine are about the immune system and disease, but this is a wacky one. It's about how the immune system helps pigeons navigate.
So this is the science of how animals navigate and how they find their way home or they migrate or whatever it is. And it's been a fascinating subject in biology for a long time. And to everybody's surprise, It also involves the immune system. Who would have thought of that? This is especially for Aoife, who hates pigeons. Now, how do animals sense which way is north?
I mean, it's a simple question at one level. But we've known for a long time that certain animals, for example, pigeons, of course, turtles, sharks, dogs, even cattle. they think, can sense which way is north. And cattle will sometimes line themselves for some unknown reason, pointing in a certain direction. There's even some evidence that humans have what's called a vestigial magnetic sense.
And in biology, it's just a fascinating question how animals manage to do this. It's very important for migration, including fish and birds. They have to find their way home, don't they? And they know which way is north. We know this from pigeons. I mean, humans must have spotted that pigeons were going to do with this, you know, and began homing pigeons became a sport, didn't it?
You know, but it's been a fascinating biological question. What's in the brains? of animals that allow them to know which way is north or have a compass, if you like, or whichever direction that they're heading in. It's one of the senses. It's a bit like taste and smell, I suppose. Being able to sense direction seems to be a very important part of many different animals, you know.
And the big question is, how do they do it? And the breakthrough and very surprising... observation is it's the immune cells in the liver of all places that allow pigeons to find their way home. Now, it's a bit like having a little compass, I suppose, in the brain or on the liver, as I would explain. And they can sense the needle in the compass.
Now, it isn't the needle, obviously, the cells involved. But from that, they can then navigate their way home or wherever they want to go. And lo and behold, it is these macrophages, as I will explain, in the liver that seem to do this. Now, before this discovery, there was evidence of bits of magnet. Can you imagine? In different tissues. It's called magnetite.
Now, magnetite is a magnetic substance. And, I don't know, 30 years ago now, they find magnetite in...
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Chapter 2: How do animals, like pigeons, sense direction?
in different tissues, in different animals, and they realize, oh, maybe that magnetite is sensing the magnetic field, like a magnet would, like a needle in a compass. And then somehow that connects to the brain, and now the animal can have its direction and know where it's going. And that's true. There are these magnetite crystals. The second thing they found was the special proteins in the eye.
Now, get this. And I went to a talk, oh, about three or four years ago now, I suppose, about this. And a woman whose name escapes me or works on this, she said pigeons and birds can effectively see the colour of the north, strangely. So they move their head and the red area or whatever, maybe it's not red, but she said that there's a colour difference. towards the north.
And then the birds fly in that direction, right? And these are called cryptochromes and they react to magnetic fields as well. And they're in the retina, which senses light, obviously. And then somehow the pigeon can perceive which way is north and then begin to fly in a certain direction. And again, there's evidence for that. So there's magnetite, little magnets in the tissues.
There's these things called cryptochromes in the eye that can make you see which way is north through the magnetic field. And they're both there, right? But however, the The breakthrough is this business of macrophages. Now, before I do them, actually, there's one last thing they found.
There are currents in the ear, electrical currents from the nerves there that do change with the magnetic field. And they think that might be part of it, too. Somehow they stuck. It sounds a little bit cruel, I suppose. They sucked some pigeons into a magnetic field and changed it. And then again, the pigeons moved their heads. So they were sensing that magnetism somehow.
I think it was something to do with the ear. There was evidence of these special nerves in the ear. But what about this latest discovery, most unexpected? Well, there was a conference and an ornithologist was there. His name is Martin Wachelski. And that's his area of expertise. He studies animal behavior. And he bumped into an immunologist at the conference.
Now, the question is, what was an immunologist doing at a conference on ornithology? Maybe he was in the wrong room. But whatever happened, serendipitously they meet and they start talking about magnetism and magnetic sensing and so on. And the immunologist said to the ornithologist, oh, it's funny, he says, in the liver there are macrophages. Now, macrophages are a very important immune cell.
My lab works on them. They're called big eaters. These are the cells that engulf bacteria and eat them or eat viruses or they eat bits of damaged tissue. And it was well known for a long time that red blood cells in your circulation eventually wear out and the macrophage hoovers them up. So, if you like, it was a way to clear old red blood cells.
And this guy said, look, he says, it's interesting this, because the macrophage takes up the old red blood cell, breaks it down, and guess what's in red blood cells? Iron. And iron is part of haemoglobin, and you need the iron in the haemoglobin to carry oxygen.
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Chapter 3: What surprising discovery was made about pigeons and their navigation?
They couldn't find ferritin anywhere else. It wasn't in their brains. It's not like magnetite, you know, it's separate to that. So it seemed to be just in the macrophages, in the liver. They used electron microscope to examine this really good technology. And then they wondered, would that iron in the macrophage be required for the pigeon finding its way home? And did a very clever experiment.
There's a drug, which we've used from time to time, actually. It's called clodronate. Big word. Clodronate will kill macrophages. So it's like a poison. The macrophage eats it and then dies, right? And they gave the pigeons into their liver the clodronate and that depleted all the macrophages in the liver.
Then what they do is they let them loose and they track them to see if they could find their way home. It was a 19 kilometre route. Can you imagine these poor postdocs in a field releasing these pigeons who'd been given clodronate? And guess what? The ones who had their macrophages depleted in the liver could not find their way home. They flew randomly all over the place.
Now, that was very interesting, first of all. But if you're a good scientist, you question your data all the time.
Chapter 4: How do macrophages in the liver contribute to navigation in pigeons?
You wonder, was there an artifact, some other explanation? Maybe the clodronate made them sick and they got a bit dizzy or whatever, you know, and that might have been a possibility. So there was a control. They did this on a cloudy day and then the pigeons couldn't find their way home.
On a sunny day, you could deplete the macrophages and the pigeons could still find their way home because they could use the sun to navigate. And there's some evidence that the pigeons use the sun. Again, that's probably in the eye and the sunlight is now shining or the colours differently near the North Pole, you know, coming off the sun. So those pigeons were normal.
They could find their way home. Isn't that wonderful? So it can't be something like nonspecific, like making the pigeons sick or whatever. This was actually evidence that you can deplete the macrophages in the liver on a cloudy day and they can't find their way home. So it can't be something nonspecific is a good interpretation of this. Sunny day, no bother.
They could find their way home and that was important. And so therefore, they concluded that the macrophages in the liver were indeed important, especially on a cloudy day for the pigeons to find their way home. Now, what this means is sensing the direction and navigating, there's multiple ways you can do it. And this makes sense biologically, because maybe some days it is cloudy.
Maybe other days, you know, whatever it might be, there's other reasons why the magnetite isn't working or whatever through some disturbance. So a backup system kicks in. So it looks as if evolution has built in two or three ways for animals to navigate.
And evolution would be sensible there because navigation could be very important for survival, you know, to find your way back to where the food is or for mating or whatever it is. So evolution's built in two or three different systems to allow animals to navigate. And this particular one,
The macrophage with the iron in the liver seemed to be especially important when the other ones weren't working. You know, so it's very interesting that that was the case. And so the bottom line was that you need macrophages in your liver holding iron to figure their way north. And the iron then must be somehow aligning to the magnetic field. Now, of course, the next question.
And remember, science is an uneasy business to be in because you're always asking the next question. I guess these guys were happy for a day or maybe longer. They had a big publication out of this in Science, which is where I came across it. Then they wondered, well, how is this working? Like, how is the iron changing according to the magnetic field?
And maybe it is lining up differently and then somehow affecting the macrophage. They got evidence that when the magnetic field was applied, the iron does line up and it causes sort of stress fibres to change in the macrophage. So they think there's proteins in the macrophage that bend differently based on the iron accumulating in a certain direction.
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