Scientists have known for more than 60 years that animals can sense the Earth’s magnetic field, and there is a wealth of behavioural evidence supporting their use of this cue as a navigation mechanism. But despite this evidence, there’s no consensus as to how animals actually sense the magnetic field.
Magnetic cues appear to be used by animals in all sorts of circumstances, not simply for long-distance navigation. Magnetoreception is used for orientation in invertebrates, fish, amphibians and reptiles. Mammals use it in behaviour such as nest building and hunting, as well as homing.
Yet there is no obvious sensory organ for this process, as there is with other sensory systems (think nose for smelling, eyes for seeing). This has led to some scientists calling it a “sense without a receptor”. However, this is not entirely accurate.
In fact, there may be as many as six kinds of magnetic mechanism across all animal species, after a recent study added a new potential option in homing pigeons. But we can’t be sure that any are actually the sensory receptor, as direct evidence is missing in every case.
It’s a situation that reminds me of the Hitchhiker’s Guide to the Galaxy’s description of currencies. “Monetary units: none. In fact, there are three freely convertible currencies in the Galaxy, but none of them count.”
Here are the six magnetic mechanisms that scientists have identified in animals to date:
1) A light-dependent mechanism in birds, insects and amphibians, thought to be based on a light dependent photoreceptor;
2) A receptor based thought to be based on iron particles in cells in the beak of birds;
3) A receptor also thought to be based on iron particles, but this time in the eye of mammals;
4) A receptor proposed to be based on the induction of ions (atoms with an electrical charge) in the semicircular canals of the ear, again in birds;
5) Another proposed to be based on the electroreceptors of some fish, again through the induction of ions;
6) And finally, hot off the press and possibly most surprising of all: the suggestion of a receptor in the liver of pigeons.
Disoriented homing pigeons
In the recent paper, published in Science, a team of Germany-based researchers treated homing pigeons with a chemical, clodronate, that removes macrophages (a type of white blood cell) from the liver, but not other areas of the body. They then released these birds 19km from their loft when the Sun was hidden by clouds, and tracked their path home.
The birds that had been treated with the clodronate were disoriented and did not return until the Sun came out. In contrast, pigeons that didn’t get the chemical, and pigeons with the clodronate treatment that had been released under sunny conditions, were unaffected.
How does this show that magnetoreception was affected? A previous study had shown that pigeons are disoriented under overcast weather if they have magnets attached to their heads, but not under sunny conditions. So, it has long been argued that homing pigeons use the Sun as a compass if it is available – but if not, revert to a magnetic compass.
The researchers in the new paper infer that removing macrophages from the liver stops the pigeons from detecting the magnetic field.
How can this work?
Macrophages are not sensory cells; they are part of the immune system. However, one of their roles is to remove degraded red blood cells from the liver – and what do red blood cells contain? Iron.
The paper suggests that as macrophages hoover up more iron, they begin developing magnetism – and thus start to be influenced by the Earth’s magnetic field. The researchers argue that they collect in an area of the liver that is rich in nerve cells, so this might be the way they communicate the magnetic field direction to the brain.
While this might seem far-fetched to some scientists, the idea is not so different to some of the other proposed mechanisms. But nor does it overwrite or reject them.
One of the other mechanisms is thought to be based on a blue light-sensitive molecule called cryptochrome, which is involved in animals’ biological clocks. Cryptochrome probably did not evolve to be a magnetoreceptor, but magnetic fields seem to influence chemical reactions requiring light. And so, by chance, cryptochrome may have become a magnetic sensor.
The German study is not the only one to suggest iron particles in cells may act as a magnetoreceptor. The beak’s trigeminal nerve, responsible for sensation and motor functions in the face, has been shown to be necessary for birds, including pigeons, to detect the magnetic field in some circumstances.
Nature’s many solutions
So it’s possible that nature has found not just one solution to magnetoreception, but many.
Calculations from quantum mechanics tell us that the light-dependent mechanism might not work in daylight, because the strong light puts the photoreceptor into a state that does not respond to the subtle effects of the magnetic field.
For a pigeon, which is navigating in daylight, another mechanism would be necessary – like the liver macrophages. For a night-migrating bird (remember night is not complete darkness), a light-dependent mechanism may actually be the best solution. So, perhaps magnetoreception depends on the species ecology or evolutionary history.
There’s still too much uncertainty to say for sure whether any of the mechanisms scientists have suggested definitely sense the magnetic field. However, we now have six strong contenders.
Or as the Hitchhiker’s Guide might put it: “Mechanisms of magnetoreception: none. In fact, there are six, but we don’t yet know if any of them count.”
Richard Holland, Professor of Animal Behaviour, Bangor University
This article is republished from The Conversation under a Creative Commons license. Read the original article.