Imagine having a new sense - the ability to sense ambient magnetic fields. Imagine not just sensing it but actually seeing magnetic fields, especially in the dark when there are fewer visual cues. This may sound alien but it exists right here on earth. Research in the field of avian magnetoreception suggests that birds not only sense magnetic fields to help them migrate but actively see the fields.

Everyone knows that certain varieties of birds migrate vast distances. Unlike you or I, birds can’t hop on Google Maps or pull out a GPS. They can’t read maps yet somehow manage to align themselves for the long flight. Several mechanisms, such as using the sun as a guide or mountains as landmarks, have been proposed that allow birds to orient themselves and successfully complete such long journeys.

Migratory birds and homing pigeons can also orient themselves with magnetic fields and it is theorized this magnetic sensory ability plays a key role in migration. Two magnetoreceptor mechanisms have been proposed. One theory uses magnetite crystals while the other involves photoreceptor based chemistry.

More details after the jump.

Go on, stick a magnet up your nose

Magnetite is an iron oxide that is a natural magnet and found in many organisms, including birds. The upper beak of birds has been found to contain a large amount of magnetite. Furthermore, this structure is connected to the brain via a nerve fiber known as the trigeminal nerve. Experiments have shown that homing pigeons can detect strong magnetic fields in the form of intense pulses. Cutting the trigeminal nerve removed pigeon’s magnetic sensing ability to strong pulses. Conversely, cutting the olfactory nerve did not affect magnetoreception. This implicated the structure containing magnetite, not sense of smell, in the upper beak as the source of magnetoreception.

This isn’t a case-closed win for magnetite structures however. Magnetite synthesis has been shown to be a common way for multiple organisms to remove excess iron. The structure could be a result of removing a toxic substance from the birds body. In addition, the brain (and other parts of the body) is a very sensitive area with a lot of electrons floating around, many of which could be subject to a magnetic field. It is unknown what adverse affects a strong magnetic pulse could have on unrelated systems in the brain and body, thereby skewing experimental data.

Furthermore, birds would be required to sense much smaller magnetic fields than those used in the experiment (10 nanoTesla as compared to previously used 100 nanoTesla) for it to be useful in the real world. Further research is needed to determine what, if any, impact magnetite deposits have on migration and magnetoreception in general.

Chemical Photoreceptors are pretty radical (dude)

The other option that has been layed on the table is the use of chemical magnetoreceptors in the form of photoreceptors. It has been shown that migratory birds lose their magnetic compass ability when their right eye is covered up. Photoreceptors function in several different ways. Some change their molecular conformation when they absorb photons. Others use photons to transfer electrons. This electron transfer creates radical pairs which lead to further reactions in a cascading pathway. Magnetic fields have been shown to influence the functioning of radical pairs by modulating their speed of reaction or yield. It is theorized that magnetically sensitive photoreceptors in bird eyes may form a magnetoreceptor, piggybacking off the vision system.

Testing this theory, however, is a bit more difficult. One method is to subject birds to a large pulse of magnetism as used in the magnetite studies. This has the same disadvantages as before. A newer technique uses low radio frequency oscillations, which have been shown to affect radical pair reactions in photoreceptors. Birds that are subjected to a weak oscillating frequency lose their magnetic compass ability. Unlike strong pulses, these weak oscillations do not appear to interact or interfere with other processes, making them ideal for studying magnetic sensing photoreceptors.

Cryptochromes - Cooler than Kryptonite

Knowing that photoreceptors are involved is only half the battle, finding which ones is the next challenge. Current bets are on cryptochromes, a type of blue-green photoreceptors found in many types of organisms (including mice, humans and many birds). Cryptochromes have been shown to form radical pairs when stimulated with photons. Even better, these radical pairs have a long enough lifetime to be subject to magnetic interference, making them a likely candidate for magnetoreception.

Further evidence points at cryptochromes involvement. Cryptochromes are found highly expressed in migratory birds during the night, found in photoreceptors, ganglion cells and displaced ganglion cells in the basal optic root. In contrast, non-migratory birds express very little cryptochrome during the night. Interestingly, both migratory and non-migratory express cryptochrome during the day. This suggests that while all birds may use magnetic fields for orientation, only migratory birds need it expressed all the time, as many migratory birds fly through the night to their final destination.

Gangly Ganglions

The fact that cryptochromes are expressed in not only photoreceptors but also neurons suggests that magnetoreception might not only be reserved for the structures in the eye. It is important to note that any magnetic sensory information obtained from the photoreceptors must pass through ganglion cells before entering the brain. Ganglion cell activation was shown to be very high in migratory birds during the night, while non-migratory birds showed little activity.

This further suggests that cryptochromes are involved in magnetic sensory input. It is, however, unknown the extent that ganglion cells are involved. Perhaps all of the ganglion cells are involved in magnetic sensing, perhaps only a few. Further research is being done to ascertain the extent of their involvement. Mapping of brain birds has been done and a region, named “cluster N”, is active in migratory birds at night. When their eyes are covered, however, the activity in this region ceases. At first glance, one would assume this is because vision is withdrawn, hence the lack of visual input to a visual processing region. However, this behavior is not seen in non-migratory birds, suggesting it is correlated to magnetoreception and linked to photoreceptors.

On the cusp

The research in the past few years offer a tantalizing look at magnetoreception but much work needs to be done. It has been theorized that normal vision during the day overrides magnetic sensitivity, while magnetoreception masks normal vision at night. Alternatively, some believe that since cryptochromes are found in both migratory and non-migratory birds, magnetoreception is constantly used and modulates the visual field perceived by birds.

It should be noted that research into this often ignored sensory field is incredibly difficult. Commercial antibodies for migratory birds are not produced. No transgenic birds have been created and capture of wild birds is limited for ethical and legal reasons. Research, however, continues and progress is being made in this incredibly interesting field of study.

References

Mouritsen H, Ritz T. Magnetoreception and its use in bird navigation. Curr Opin Neurobiol. 2005; 15:406-14. DOI:10.1016/j.conb.2005.06.003