One interesting form of navigation is magnetoreception; the use of the Earth’s magnetic field to determine location of oneself and other objects. Unlike navigation by sight or smell, magnetoreception is a sense that humans can’t relate to, as they simply don’t have the ability to feel magnetic fields. However, birds (and other animals such as trout, but this post focuses specifically on birds) do have this sense, and have the ability to use it in amazing ways. In today’s blog post, we’ll take a closer look at the neurobiology of magnetoreception in birds, and how they use it to get where they’re going.
First, there are two types of magnetoreception, magnetic compass and magnetic map. A magnetic compass is the ability to orient movements with respect to the Earth’s magnetic field, while a magnetic map is the ability to sense where oneself is in respect to the target location. An animal with a magnetic map is able to tell where exactly it is by using its “map sense”. For example, if you took two birds, one with a magnetic compass and one with a magnetic map, that are getting ready to migrate 500 miles south and moved them 500 miles north, the one with a map would find its way to its migratory place, and fly 1000 miles south. The one with a compass would only be able to fly 500 miles south and end up back in its original location, as it is only able to tell which way is south, and knows it needs to fly 500 miles south to get to its migratory place. Obviously, a magnetic map is better than a magnetic compass in terms of accuracy.
Although the exact mechanism for how birds detect magnetic fields is still unknown (after over 40 years of research!), there are a few theories as to how it’s done. Structures made of magnetite crystals, which can be found in the upper beaks of birds like pigeons, are a strong candidate. Birds seem to lose their ability to detect differences in magnetic fields when nerves from their beak to their brain are cut. Magnetite, however, is quite difficult to study. The crystals are only 50 nanometers in diameter, making them extremely small and hard to locate even microscopically. Many fixative chemicals used for preserving actually end up dissolving the magnetite too. Due to these challenges, efforts to examine gene sequences involved in producing magnetite instead of the magnetite itself are underway.
It is also possible that cryptochromes, known for playing a part in circadian rhythms, in birds’ eyes also affect magnetoreception, as certain cryptochromes are only found in night-migratory birds. However the birds sense the magnetic field, this information must still be processed in the brain. In many night-migratory birds, a cluster of regions in the forebrain (called “cluster N” for short) is active only at night (when the birds should be migrating) and when the birds’ eyes are not covered. Like the cryptochromes above, this area is not found to be active in non-migratory birds. It is possible that this area is responsible for receiving information from either or both the magnetite in the birds’ beaks, or the cryptochromes in the birds’ eyes.
Although research in the field is still lacking, much progress has been made recently. Most of the discoveries in the field, such as the magnetite structures and cluster N, have been as recent as the mid-2000s. More research into the actual neurobiology is needed, as we only have a very vague idea of what cluster N may actually do. Hopefully, in the near future, we’ll have a better understanding of this sense, as well as the mechanisms behind it.
Sources:
Mouritsen, Henrik, and Thorsten Ritz. “Magnetoreception and Its Use in Bird Navigation.” Current Opinion in Neurobiology, vol. 15, no. 4, 2005, pp. 406–414., doi:10.1016/j.conb.2005.06.003.
Lohmann, Kenneth J, and Sönke Johnsen. “The Neurobiology of Magnetoreception in Vertebrate Animals.” Trends in Neurosciences, vol. 23, no. 4, 2000, pp. 153–159., doi:10.1016/s0166-2236(99)01542-8.
Johnsen, Sönke, and Kenneth J. Lohmann. “The Physics and Neurobiology of Magnetoreception.” Nature Reviews Neuroscience, vol. 6, no. 9, 2005, pp. 703–712., doi:10.1038/nrn1745.
As a birder who has seen lots of migratory birds come through, I always think about the mechanisms that allow them to accomplish this feat. In your reading, did you come across any information on how sensitive the birds’ magnetic sense is, as it relates to these possible neural mechanisms?