Remember all the way to the "like" north, it wasn't that simple? In March 2020, 15 Asian elephants entered Pu'er City in Yunnan Province from Xishuangbanna and traveled north. Until August 2021, after continuous manual intervention and guidance, the elephant herd finally returned south to its suitable habitat.
Asian elephant northward migration route Image source: Xinmin Evening News
Different researchers have different views on this phenomenon. Ecologists believe they are collectively mobilized in search of food; Biophysicists believe that this is an awakening of animals that rely on the geomagnetic field, because the time when they began to move north coincided with the occurrence of solar storms and geomagnetic storms.
But, for whatever reason, it's not hard to find that the elephants show an excellent sense of direction along the way north.
Born with "GPS"
When it comes to direction, many animals seem to be born with GPS and are often able to make amazing long-distance migrations. And the idioms of flying pigeon books, wild geese flying south and old horses knowing the way all express a strong sense of direction in some animals.
Amazingly, in the Rocky Mountains there is a butterfly called the American monarch butterfly, which takes off from the southern United States every spring, travels all the way north, spends the summer in the northeastern United States, and flies south back to the starting point in the fall. The magic of this is that the entire migration process needs to be completed from generation to generation, and it is the fourth generation of butterflies who have returned to the beginning!
In addition, the record for the longest known migration route comes from Arctic terns, which can migrate back and forth between the north and south poles of the earth, migrating more than 70,000 kilometers per year, and in their twenty or thirty-year lifetime, the accumulated miles can be two or three times between the earth and the moon. They have few landmarks to refer to for long distances in the vast sea, but they still show a strong sense of direction during the migration of the north and south poles.
Who shows the way for creatures?
So, where does these animals' innate sense of direction come from? Could the ubiquitous Earth's magnetic field be a key clue to their location and navigation?
This conjecture was first proposed by Russian zoologist Middendorf, who recorded and mapped the migration routes of a variety of migratory birds, found that different migratory birds seemed to follow the northward trend during migration, and published relevant research results in the 50s of the 19th century, believing that migratory birds can use the earth's magnetic field for localization. However, due to the lack of experimental evidence, this view was not accepted at the time.
Later, some scientists found that after putting a small magnet on the head of the pigeon and releasing it, they would not come back nine times out of ten, but putting a small copper block of the same size was not affected. This simple experiment strongly shows that the magnetism of the magnet interferes with the pigeon's perception of the geomagnetic field, so the pigeon loses its sense of direction.
Schematic diagram of Emlen funnel experimental device Image source: "Mysterious Quantum Life"
In addition, there is a more classic experiment, the Emlen funnel experiment: the bird is made to stand on the underside of the black printing mud to stain its paws black, there is a barbed wire fence above to prevent the bird from flying away, and there is blotting paper on the funnel-like wall around it. Usually birds tend to take off in a specific direction (such as flying north), which leaves paw prints on the walls in the corresponding direction; But if an artificial magnetic field is applied to the periphery that is different from the direction of the geomagnetic field, the position of the paw print will change. This fully shows that the migration of birds is affected by magnetic fields. At this point, the fact that some animals can sense the geomagnetic field has slowly been accepted.
So, do humans have the ability to sense magnetism?
Do humans also have magnetic induction?
In fact, the first to pay attention to the sense of direction in the human subconscious was Darwin, who pointed out in a review in Nature that in Northern Siberia, local people can travel long distances in the vast snow and reach a certain destination, during which they will constantly adjust the direction of their travel, without relying on any known road signs, Darwin believes that this ability to recognize direction may come from a subconscious instinct.
In the 80s of the 20th century, Professor Robin Baker of the University of Manchester in the United Kingdom devised a simple behavioral experiment: several volunteers wore blindfolds containing bar magnets or brass, drove them 10 kilometers away, and then asked them for the direction of their starting point. Surprisingly, most of the volunteers wearing blindfolds containing brass could point in the right direction, while magnets interfered with the volunteers' correct identification of directions. The work was published in 1980 in the top academic journal Science. However, the experiment was later controversial and questioned because it could not be replicated in multiple laboratories, which also made people cautious about the study of human magnetic induction.
Human magnetic induction experiment (1980 behavioral experiment on the left, 2019 monitoring brain wave experiment on the right) Image source: References
With the development of technology, in 2019, a group of researchers at Caltech designed a magnetic induction experiment on humans: they had volunteers sit in a cage that could generate an artificial magnetic field while monitoring their brain waves in real time. The researchers found that changes in the magnetic field could alter their alpha brain waves, but the volunteers themselves did not have any special feelings. This leads us to speculate that our bodies may be able to sense magnetic fields, but we are not aware of it.
Hypotheses about bio-sensing magnetic fields
However, the magnetic field cannot be seen or touched, how do organisms perceive it?
There are currently three main hypotheses on this issue: the magnetite hypothesis, the chemical radical pair hypothesis, and the biological compass hypothesis.
Magnetite hypothesis
Magnetotactic bacteria moving with a changing magnetic field Image source: References
The hypothesis is mainly derived from the discovery of magnetotactic bacteria in 1975. Magnetotactic bacteria are a group of bacteria that can move with changes in the magnetic field. The researchers found that the bacteria all have some crystalline particles containing iron, and these particles are arranged in a chain-like structure, called magnetosomes (somewhat similar to our bar magnets), which are the key structures of magnetic sensing. Based on this, scientists have tried to find similar magnetite particles in other animals, such as the presence of iron particles in bee abdomen, rainbow trout noses and pigeon beaks.
Reported iron particles in different organisms (A is magnetotactic bacteria, B, C, D are iron particles) Image source: References
However, this hypothesis is also controversial, because it was later discovered that the beak of pigeons was actually macrophages, which were involved in the immune response and had nothing to do with magnetic induction; It has also been pointed out that what was found in rainbow trout was only iron contamination during the experiment, not iron in cells. But then again, science is always moving slowly through controversy.
Chemical radical pair hypothesis
In 2000, the team of German chemist Schulten proposed that a protein called cryptochrome (Cry) in birds' eyes is a key molecule in animal magnetic sensing.
Cryptoanthocyanin structure in pigeon eyes Image source: References
The protein is combined with a photosensitive pigment molecule FAD (flavin adenine dinucleotide), which can be excited by blue light and cause a series of electron transitions, eventually forming a free radical pair composed of two electrons, the transition between the different quantum states of the spin of the two electrons is affected by weak magnetic fields, especially magnetic inclination, thus forming a quantum compass that can sense the geomagnetic field. The researchers think birds may use such a quantum compass to navigate.
However, the limitation of this hypothesis is that it can only explain the perception of magnetic inclination in animals, because this quantum chemical reaction is not affected by the strength and direction of the magnetic field.
Biological compass hypothesis
In 2015, the research team of mainland scientist Xie Can proposed that Cry is only part of the magnetic receptors, not the whole, and they believe that there is also a protein that interacts with Cry to detect the direction and size of the magnetic field, and the two participate in the magnetic induction process.
Biological Compass Model Image source: References
Through layers of screening, Xie Can's research team finally found an iron-sulfur protein IscA 1 that met the expectations and named it the magnetic receptor MagR. They found that the two proteins were able to bind to each other and assemble to form a stick-like complex, similar to a small bar magnet. Among them, light-sensitive Cry proteins are wrapped in the periphery - as a light acceptor to sense light and magnetic inclination, while magnetically sensitive MagR proteins are linearly assembled into a stick-like structure inside - as magnetic acceptors to detect the strength and polarity of the geomagnetic field, and may be regulated by the peripheral Cry protein through electron transport, so that animals can use the two proteins to obtain complete geomagnetic field information through photomagnetic coupling. However, more experimental evidence is needed on the specific mechanism of optical and magnetic coupling between the two.
epilogue
It can be found that the principles of magnetic induction and biological navigation have always been a compelling unsolved mystery in the field of life sciences, and the solution to this basic biological question may lead to the proposal of new models of physics, the discovery of new biological mechanisms, and the development of new engineering technologies.
Therefore, when we really understand the magnetic induction mechanism behind the elephant herd going north and the wild geese flying south, perhaps we are not far from a new generation of bionic navigation technology, and by then, we may be no longer limited by satellite signals and truly achieve global positioning!
Finally, quietly tell you a little secret: according to research, paying attention to the "China Science Expo", the probability of not finding us next time you get lost will be greatly reduced~
Bibliography:
[1] Jim Al-Khalili, John Joe McFadden. Mysterious quantum life. Hangzhou: Zhejiang People's Publishing House, 2016.
[2] Baker, R.R. A sense of magnetism. New Scient,1980, 87, 844-847.
[3] Wang C X, Hilburn I A, Wu D A, et al. Transduction of the geomagnetic field as evidenced from alpha-band activity in the human brain. eNeuro,2019, 6: e0483-18.
[4] Monteil C L, Menguy N, Prévéral S, et al. Accumulation and dissolution of magnetite crystals in a magnetically responsive ciliate. Appl Environ Microbiol, 2018, 84(8): e02865-17.
[5] ZHANG Peng, XIE Can. Biomagnetic Induction: A Field of Research in Doubt and Hope. [J]. Chinese Science Bulletin, 2021, 66(21):2635-2648.
[6] Zoltowski B D, Chelliah Y, Wickramaratne A, et al. Chemical and structural analysis of a photoactive vertebrate cryptochrome from pigeon. Proc Natl Acad Sci USA, 2019, 116: 19449–19457.
[7] Qin S, Yin H, Yang C, et al. A magnetic protein biocompass. Nat Mater, 2016,15: 217–226.
Author: Zhang Peng
Author's institution: High Magnetic Field Science Center, Hefei Institute of Physical Sciences, Chinese Academy of Sciences