We have known for a very long time that animals such as birds, turtles and butterflies use the Earth’s magnetic field to help them navigate on long journeys that span the globe. However, despite knowing that they can detect the magnetic field, we have no idea how they do it. We understand how the interaction of photoreceptors with light gives rise to vision, how receptors in our skin allow us to feel, how sapid molecules on our tongue allow us to taste and how olfactory sensory neurons in our noses give us our sense of smell. Beyond the human senses, we understand how some animals can perceive the polarisation of light and how others can detect electric fields. But how, or even which part of the body, bestows an animal with magnetoreception is currently a mystery. That is not to say there are no theories and, somewhat unexpectedly, some of them link magnetoreception with vision.
To explain further, it would be helpful to begin with a brief discussion of how we know that animals have magnetoreception. As I am sure you are aware, the Earth has a magnetic field generated by the movement of the planet’s molten metal core. For
thousands of years, human navigators have used magnetic compasses to locate north and find their way successfully across oceans and wide expanses of uncharted land. Animals have been doing the same probably before humans even evolved.
Birds are able to navigate over vast distances during their annual migration, as do butterflies and a surprising number of insects. Less well known, but no less impressive, are the oceanic migrations of sea creatures like turtles and lobsters. There is overwhelming evidence that, just like human navigators, these animals also use the Earth’s magnetic field to find their way. The first experiments showing this to be the case came about when scientists noticed that captive birds of species that migrate would get very restless at the time of year when, if not for captivity, they would be setting off on their long journey. Furthermore, they noticed these birds tended to hop or jump in a specific direction that was completely unrelated to any visual cues in their surroundings. Unsure what apparently invisible triggers these birds were responding to, they tried many different set-ups until eventually they surrounded the birds with a Faraday cage, which completely blocks the Earth’s magnetic field. They found that though the birds still hopped, they now did so in completely random directions. That directionality had disappeared. Their next experiment was to surround the birds with an electromagnetic rig, which gives rise to a controllable magnetic field. They found that, by changing the properties of their magnetic field such as the strength and direction, that they could change the direction the birds hopped in. Later, similar experiments were performed with migratory sea turtles. As juveniles, sea turtles perform vast migrations that can take them across oceans. Amazingly, despite there being no landmarks in the open ocean, once these turtles set out in a direction, they are able to keep their heading very precisely over huge distances. Scientists found that by exposing captive sea turtles (held in large circular swimming tanks) to the same type of artificial magnetic fields as used for the migratory birds, that they could get the turtles to turn and change their heading. Similar experiments have been performed with many other animals, giving a strong evidence base for magnetoreception being a widely used sense for navigation in the natural world. But how does magnetoreception link to vision?
Mysterious means
As mentioned previously, while it is clear animals can detect magnetic fields, how they do so is currently a mystery. There are a number of theories, with the leading hypothesis centring on a molecular phenomenon called radical pairs and their interaction with light. Radical pairs are magnetically sensitive chemical intermediates. To put it more simply, there are some types of chemical reactions where one molecule (the reactant) is
converted to a different one (the product) but require an intermediate step, which is where these radical pairs come in. To start a chemical reaction, there needs to be some form of energy input. This could be heat, or it could be the absorption of a photon of light. Once triggered, the reactant molecule undergoes a chemical transformation to form a radical pair. However, this intermediate step does not always result in the product molecule, sometimes the reaction can go back the other way and the
molecule remains as the reactant. Different factors affect whether this intermediate step successfully produces the reactant or not, one of which is the magnetic field direction. If the field is oriented in the ideal direction, it is likely that the complete reaction will occur and the reactant will be converted into the product molecule. Conversely, if the magnetic orientation is not ideal, the probability of successful reaction decreases and the molecule is more likely to remain in its original state. So this innate magnetic field preference, which governs whether or not the molecule is converted to its final product molecule, could potentially be used as a way of detecting the direction of the magnetic field, just like a compass. Lab experiments have confirmed that, even in the presence of the Earth’s quite weak magnetic field, that this radical pair compass could work.
Figure 2: Some chemical reactions, whereby one molecule (green) is converted into a different one (red) must undergo an intermediate step, which involves the production of radical pairs. This occurs when the original molecule absorbs a photon. However, the intermediate step of the reaction could go either way; producing the final molecule, or convert back to the original. For some reactions, the success of this intermediate step depends on the orientation of the magnetic field relative to the molecule. This may form the basis for the magnetic compass in animals such as birds. In (a) the bird is oriented such that the molecules are aligned with the preferred magnetic field orientation (north in this example), which results in many successful reactions. In (b) the bird is not aligned with north, so few reactions occur
But do molecules that follow this radical pair chemistry exist in biological systems? The protein cryptochrome is the most likely candidate in vertebrates like birds, although some forms of chlorophyll, found in plants, behave in the same way. Cryptochrome is a light-sensitive protein found in the retina and is thought to play a role in modulating an animal’s circadian rhythm. Without delving too deeply into organic chemistry, when a cryptochrome molecule absorbs a photon of light, it undergoes structural changes, first passing through a radical pair intermediary phase, which is magnetically sensitive. If the orientation of the magnetic field is favourable, the cryptochrome will complete its structural change, but if the direction is not favourable, it will remain in its original configuration. To make use of this property of cryptochrome in order to construct a compass, an animal would need a reservoir of cryptochrome molecules all oriented in a single fixed orientation and a way of distinguishing between the two forms of the cryptochrome molecule. The ratio of product (final form) to the reactant (original form) would be intrinsically linked to the orientation of the magnetic field, since the probability of the reactant turning to the product is dependent on the favourable field orientation. So if there is a lot of the reactant, the animal could determine that they were oriented with this preferred direction, while if there was no product, it is likely they are oriented at 90° to the preferred orientation.
Biological compass
At this point, this is all still theoretical; cryptochrome molecules could be used by animals as a biological compass, but what experimental evidence is there? Truth be told, there is very little hard evidence for a cryptochrome-based compass. The most compelling evidence comes from a study involving transgenic fruit flies. It has become a matter of routine for scientists to produce genetically modified flies. Since their entire genome has been sequenced, scientists are able to modify specific aspects of the animal’s sensory system and observe the results of the modification. In this case, they created a mutant strain of fruit flies that completely lacked cryptochrome. They discovered that the flies could no longer use the magnetic field to navigate, strongly suggesting that cryptochrome plays a role in magnetoreception, in fruit flies at least. However, similar transgenic experiments in vertebrates cannot be performed, so it is not possible to use this same method to determine whether animals such as birds are truly using cryptochrome compasses or not. There is some circumstantial evidence, however. For the magneto-orientation experiments described earlier to work correctly, birds need light and they need that light to be a specific colour (indigo as it happens), the wavelength range that coincides with the absorption profile of cryptochrome. Recall that, in order to form the magnetically sensitive radical pair as the intermediate step in the chemical reaction, cryptochrome first needs to absorb a photon with a particular energy (and therefore wavelength). However, correlation does not imply causation, and just because both systems need the same coloured light to work, it does not mean they are one and the same.
Suppose, however, that cryptochrome molecules were responsible for magnetoreception. How would an animal perceive the magnetic field? Clearly, the answer to this question is going to involve some conjecture and not a little imagination. First, it is necessary to determine how the signal from the cryptochrome compass reaches the brain. In other words, suppose that the animal is oriented with the favourable magnetic field direction and all of the cryptochrome in the compass has been converted and this ratio of product to reactant is somehow measured. How does that signal reach the brain? It is possible that, over a certain threshold, the ratio of the two molecule formations changes the balance of ions in a cell, triggering an electrical impulse, which travels to the brain along axons. This is how biological nervous systems work, but this would require specific neural pathways for magnetoreception, which have so far not been identified. However, there is an alternate theory that involves the electrical pulse that occurs when the cryptochrome changes formation. This radical idea suggests that this electric pulse actually triggers the signal pathway governed by rhodopsin, the pathway that is responsible for vision. In other words, there is a theory that magnetoreception effectively hijacks the visual pathway.
How could this work and is there any evidence for this at all? There is no direct evidence for either the chemical signalling pathway or for the ‘visual hijack’ hypotheses. However, evidence from brain scans taken during tasks in which animals use their magnetoreception reveal that the areas of the brain that are activated are similar to those activated during visual tasks. The brain has sensory segmentation, where specific regions of the brain are responsible for specific senses. There is a region of the brain in birds called ‘Cluster N’ that is active during both vision and magnetoreception (though other regions are activated too). Scientists discovered that if this ‘Cluster N’ region were destroyed in songbirds, they were no longer able to use their magnetic compass, though they could still use their sun and star compasses, which rely purely on visual information. Again, this evidence is circumstantial only, but is suggestive.
Figure 3: There is no way to tell what magneto-vision would actually look like for a bird. Would north in the scene on the left appear as a dark spot (middle) or as a dark band (right)? We’ll probably never know
So given a potential mechanism and a potential pathway, how would birds perceive the magnetic field? If the hijacking of the visual pathway does occur, rather than perceiving the magnetic field separately from visual information in the way that touch or smell is separated from our sense of vision, birds may perceive it alongside vision, perhaps in the way that we can see colour alongside brightness. That is an incredibly intriguing thought; that birds can actually see the magnetic field alongside the ‘normal’ view of the world. In some senses, it makes sense, after all vision is incredibly useful for navigation, so it would seem sensible to incorporate magnetoreception with vision. But it raises many questions; does this magneto-vision interfere with true vision? Does magnetism appear like a colour, or would it appear as bright or dark regions? While we can speculate how it would look to a bird (if magneto-vision exists, of course), we will never really be able to tell for sure as perception is quite different from vision.
Although there is strong evidence for magneto-reception, the exact mechanism by which animals are able to detect the Earth’s magnetic field remains one of the great mysteries for sensory biology. To vision scientists, the idea that magnetosense is intrinsically linked to vision is intriguing, posing a huge range of questions.
Dr Ilse Daly is a research associate at the School of Biological Sciences at the University of Bristol.
