May 5, 2020
Written by: Rebecca Somach
How many senses do you have? You probably know that there are five senses. Sight, smell, vision, hearing and touch. Easy, right? Sorry to tell you, but your kindergarten teachers lied to you. If a ‘sense’ is the way that organisms process external information, there are more than five senses. Humans also have proprioception, the sense of space. You can tell exactly how high your hand is raised when you raise it, even if you can’t see or touch it. Humans can also sense their own balance using their vestibular sense. This is done with fluid filled channels in your ears, but it isn’t related to hearing at all. With this sense, you can stand on one leg and the body will adjust its muscles accordingly. There are also senses that humans don’t have, but certain animals do. One of these senses sounds more like a superpower than reality: magnet-sensing powers. Formally, this sense is called magnetoreception, or the ability to detect the earth’s magnetic field.
The magnetic field of the Earth is generated by a combination of the Earth’s spin and the movement of the molten metal in the core of the Earth. Based on where an animal is on the planet, and how far away they are from the Earth’s core, the magnetic field will have slight fluctuations. Humans also use this property to navigate, but in the form of compasses. While animals can’t read compasses, many different animals including some bacteria, birds, reptiles, bats, and sea turtles have structures to help them navigate their environment1. This sense might be strange to us, but scientists are sure it exists because they can change the behavior of animals using magnets1,2, though the mechanism for how animals sense magnetic forces remains elusive. Researchers have proposed several models that might explain how this magnetoreception sense works, though scientists aren’t sure which the method animals actually use.
One theory for how magnetoreception works is similar to how hearing works (Figure 1). To detect sound, the vibrations of air entering our ears cause a mechanical push and pull on the hair cells that actually transmit the sound signal to the brain. The signal is caused because of a physical change in the tissue. Similarly, it is thought that there is a magnetoreceptor that is connected to some type of magnetic molecule in the body. If the animal changes its position, the magnetic field of the earth will push and pull differently on anything magnetic in the body. That could physically pull open receptors and activate a cell, similar to how the physical push and pull of sound waves activates hair cells.
So are there magnetic materials in the body? In bacteria, the answer is yes3. Some bacteria contain magnetosomes, a small compartment in a cell that has iron particles inside of it. When these particles are close to each other, they can form a chain that will orient to a magnetic field—almost like a miniature compass inside of single cells. In bacteria, the whole organism can align itself with the magnetosome. Animals larger than bacteria could also be using magnetic particples to navigate, as pieces of iron minerals, notably a mineral called magnetite, have been found in the beaks of some birds4.
However, large animals and bacteria would likely use different mechanisms in their magnetoreception. In a bacteria, the whole bacteria organism can align itself with a crystal of magnetite. That isn’t true of a larger animal; they would need a receptor to help transmit signals to the brain. However, it has been very difficult to find the receptor that transmits the signals5. Additionally, the role of magnetite itself is still under debate. One study claimed that these particles are involved with the immune system6, not navigation. However, the theory still remains popular due to its simplicity and the fact that magnetite has been found in the bodies of animals that need to navigate, including homing pigeons, honeybees, and dolphins7.
Another main theory of how animals sense the Earth’s magnetic field is related to light. The evidence for this theory comes from experiments showing that some animals need certain wavelengths of light to navigate. An experiment found that bright yellow light disoriented birds but they oriented well with blue and green light2. How is light related to magnetism? To understand, let’s consider how another sense, vision, works. In the retina of the eye, there are cells that have a protein called rhodopsin that changes its shape when it interacts with light. This change of shape causes a chain reaction that ultimately causes a channel to open or close, sending the signal that the cell has been exposed to light or not. Because light has energy, it can influence the structure of certain molecules like rhodopsin. Light can also influence a molecule that’s possibly important for magentoreception called cryptochrome. Instead of changing the shape of cryptochrome, light and the earth’s magnetic field can combine to influence electrons in the molecule, which can ultimately activate the cell. Importantly, cryptochrome reacts specifically to blue light since the behavioral data in birds links blue light to magnetoreception. In fruit flies, flies without cryptochrome weren’t able to do a behavioral task that relies on a magnet8. However, there is also evidence that cryptochrome is involved in helping animals maintain their circadian rhythm, which is how an animal knows what time of day it is9. It is difficult to know if cryptochrome helps with both circadian rhythms and magnetoreception, or if there are separate cells. Those separate cell types have not been found yet so this theory still needs some more research to back it up. It is also unclear what an animal would do if it was trying to use this system to navigate at night. For these reasons, it is also difficult to say if cryptochrome is how animals sense magnetic fields.
Research on magnetoreception is difficult. These sensors could be anywhere in the body and it is possible that one cell in hundreds could be responsible for magnetoreception. Scientists also do not know where in the brain this information would be going, unlike other senses. It is also possible that multiple theories could be correct and the sense evolved multiple times in different animals. It is also worth noting that magnetoreception is difficult to study because humans don’t have it. Scientific experiments are done by people, and trying to understand something we can’t sense in any way is not an easy task. It could be similar to asking how one would ‘see’ in the 6th dimension; we simply don’t have the ability to understand what that means on an intuitive level. That doesn’t stop people from trying to learn about it, which is truly the most human superpower.
- Lohmann, K. J., Cain, S. D., Dodge, S. A. and and Lohmann, C. M. F. Regional magnetic fields as navigational markers for sea turtles. Science 294, 364-366. (2001) doi:10.1126/science.1064557
- Wiltschko W, Wiltschko R (2001) Light-dependent magnetoreception in birds: the behaviour of European robins, Erithacus rubecula, under monochromatic light of various wavelengths and intensities. J Exp Biol 204: 3295–3302. PMID: 11606603
- Uebe, R., Schüler, D. Magnetosome biogenesis in magnetotactic bacteria. Nat Rev Microbiol 14, 621–637 (2016). https://doi.org/10.1038/nrmicro.2016.99
- Solov’yov, Ilia A, and Walter Greiner. Theoretical analysis of an iron mineral-based magnetoreceptor model in birds. Biophysical journal vol. 93,5 (2007): 1493-509. doi:10.1529/biophysj.107.105098
- Nordmann, G. C., Hochstoeger, T., & Keays, D. A. (2017). Magnetoreception-A sense without a receptor. PLoS biology, 15(10), e2003234. https://doi.org/10.1371/journal.pbio.2003234
- Treiber, C., Salzer, M., Riegler, J. et al. Clusters of iron-rich cells in the upper beak of pigeons are macrophages not magnetosensitive neurons. Nature 484, 367–370 (2012). https://doi-org.proxy.library.upenn.edu/10.1038/nature11046
- Kirschvink JL, Gould JL. Biogenic magnetite as a basis for magnetic field detection in animals. Biosystems. 1981;13(3):181–201. doi:10.1016/0303-2647(81)90060-5
- Gegear, R. J., Casselman, A., Waddell, S., & Reppert, S. M. (2008). Cryptochrome mediates light-dependent magnetosensitivity in Drosophila. Nature, 454(7207), 1014–1018. https://doi.org/10.1038/nature07183
- Vitaterna, M H et al. “Differential regulation of mammalian period genes and circadian rhythmicity by cryptochromes 1 and 2.” Proceedings of the National Academy of Sciences of the United States of America vol. 96,21 (1999): 12114-9. doi:10.1073/pnas.96.21.12114
Cover image via Pixabay, https://pixabay.com/photos/compass-newspaper-finance-direction-2779371/
Figure 1 created in Google Slides